# Mohammad Zaman · Shabbir A. Shahid Lee Heng

# Guideline for Salinity Assessment, Mitigation and Adaptation Using Nuclear and Related Techniques

Guideline for Salinity Assessment, Mitigation and Adaptation Using Nuclear and Related Techniques

Mohammad Zaman • Shabbir A. Shahid Lee Heng

# Guideline for Salinity Assessment, Mitigation and Adaptation Using Nuclear and Related Techniques

Mohammad Zaman Soil and Water Management & Crop Nutrition Section, Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture, Department of Nuclear Sciences & Applications International Atomic Energy Agency (IAEA) Vienna, Austria

Lee Heng Soil and Water Management & Crop Nutrition Section, Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture, Department of Nuclear Sciences & Applications International Atomic Energy Agency (IAEA) Vienna, Austria

Shabbir A. Shahid Senior Salinity Management Expert, Freelancer Dubai, UAE

ISBN 978-3-319-96189-7 ISBN 978-3-319-96190-3 (eBook) https://doi.org/10.1007/978-3-319-96190-3

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Salt-tolerant grass growing under saline condition

# Foreword

Soil salinity is a major global issue owing to its adverse impact on agricultural productivity and sustainability. Salinity problems occur under all climatic conditions and can result from both natural and human-induced actions. Generally speaking, saline soils occur in arid and semi-arid regions where rainfall is insufficient to meet the water requirements of the crops, and leach mineral salts out of the root-zone. The association between humans and salinity has existed for centuries and historical records show that many civilizations have failed due to increases in the salinity of agricultural fields, the most known example being Mesopotamia (now Iraq). Soil salinity undermines the resource base by decreasing soil quality and can occur due to natural causes or from misuse and mismanagement to an extent which jeopardizes the integrity of soil's self-regulatory capacity.

Soil salinity is dynamic and spreading globally in over 100 countries; no continent is completely free from salinity. Soil salinization is projected to increase in future climate change scenarios due to sea level rise and impact on coastal areas, and the rise in temperature that will inevitably lead to increase evaporation and further salinization. There is a long list of countries where salt-induced land degradation occurs. Some well-known regions where salinization is extensively reported include the Aral Sea Basin (Amu-Darya and Syr-Darya River Basins) in Central Asia, the Indo-Gangetic Basin in India, the Indus Basin in Pakistan, the Yellow River Basin in China, the Euphrates Basin in Syria and Iraq, the Murray-Darling Basin in Australia, and the San Joaquin Valley in the United States.

The objective of this guideline is to develop protocols for salinity and sodicity assessment, and the role of isotopic nuclear and related techniques to develop mitigation and adaptation measures to use saline and sodic soils sustainably. We have focused on important issues related to salinity and sodicity and have described these in an easy and user friendly way. The information has been compiled from latest published literature and from authors' publications specific to the subject matter. This guideline is an outcome of a joint publication between the Soil and Water Management & Crop Nutrition Section, Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture, International Atomic Energy Agency (IAEA), Vienna, Austria, and a freelance senior salinity management expert based in the United Arab Emirates.

We hope that this guideline will be an excellent contribution to the science and enhance the knowledge of those seeking information to assess and diagnose salinity problem at the landscape and farm levels and the role of nuclear and isotopic techniques in developing strategies to use these marginal soils sustainably.

Soil and Water Management & Crop Nutrition Section, Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture Department of Nuclear Sciences & Applications International Atomic Energy Agency (IAEA) Vienna, Austria Mohammad Zaman Senior Salinity Management Expert, Freelancer, Dubai, UAE Shabbir A. Shahid Soil and Water Management & Crop Nutrition Section, Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture Department of Nuclear Sciences & Applications International Atomic Energy Agency (IAEA) Lee Heng

Vienna, Austria

# Acknowledgements

We are thankful to Prof. Pharis, R.P., Department of Biological Sciences, University of Calgary, Canada, and Dr. Shazia Zaman, University of Canterbury, for their critical review, feedback and editorial comments in the preparation of this book. We also thank Ms. Marlies Zaczek of the Soil and Water Management & Crop Nutrition Section, Joint FAO/IAEA Division, International Atomic Energy Agency (IAEA), Vienna, Austria, for her help in formatting this document and Ms. Petra Nabil Salame, PMO of Asia and the Pacific Section 2, Division for Asia and the Pacific Department of Technical Cooperation for her financial support.

# Contents

# About the Authors

Mohammad Zaman is working as Soil Scientist/Plant Nutritionist at the Soil and Water Management & Crop Nutrition (SWMCN) Section, Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture, International Atomic Energy Agency (IAEA), Vienna, Austria. Zaman's work is covering countries in Africa, Asia, Middle East, Europe, and Central and Latin America on developing climate smart agricultural practices for enhancing food production and environmental sustainability. Prior to joining the SWMCN Section, he worked for 19 years in integrated plant nutrient management of irrigated and dryland farms at different research, academic, commercial, and international organizations in both developing and developed countries.

Zaman completed a Ph.D. degree at Lincoln University, Canterbury, New Zealand, on soil nitrogen (N) mineralization and its relationship to soil microbial and enzyme activities in grasslands under different management practices. After completing his Ph.D., Zaman continued post-doctoral research at Lincoln University and Chiba University, Japan, in the areas of soil fertility, soil water quality, and greenhouse gases, using both conventional and stable isotopic techniques. After 2 years in Japan, Zaman went back to New Zealand to take up a position as a researcher in the National Institute of Water & Atmospheric Research (NIWA), Hamilton. Later, he moved to the farmers owned Cooperative Fertilizer Industry, firstly as a Senior Scientist and then as a Research Manager to increase the adaptive capabilities of soil/plant systems to climate change and to enhance nutrient use efficiency on farms. This research included mitigating greenhouse gas emissions in an integrated plant animal system, developing new decision support systems and tools, identifying novel fertilizer products to increase farm productivity and resource use efficiency, and minimizing nutrient losses to waterways and the atmosphere. Zaman is author and coauthor of over 65 research publications in refereed journals.

Shabbir Ahmad Shahid is Senior Salinity Management expert a freelancer based in the United Arab Emirates. He had served in different capacities in a number of organizations, including Senior Salinity Management Scientist in ICBA, Dubai, UAE, Associate Professor of Soil Science at the University of Agriculture, Faisalabad, Pakistan, Associate Research Scientist at the Kuwait Institute for Scientific Research, Kuwait, Manager of Soil Resources Department at the Environment Agency, Abu Dhabi, UAE. He has more than 36 years of experience in applied agricultural research in many countries and regions. Dr. Shabbir holds a Ph.D. in "soil micromorphology of salt-affected soils" from Bangor University, Wales, UK. He is author and coauthor of over 160 research publications, including five edited books published by Springer, principal author of two books and two manuals, 52 peer-reviewed journal papers, 31 book chapters, and 27 conference proceedings. He also authored and coauthored over 30 scientific reports. He is member of a number of scientific committees, international advisory boards of international conferences, and editorial boards of scientific journals. He is life member of World Association of Soil and Water Conservation (WASWAC). He is a recipient of Sir William Roberts and David A Jenkins awards. Shabbir is pioneer in introducing soil survey research in the UAE. He with his associates discovered anhydrite soil in UAE and formally added to 12 edition of US Keys to Soil Taxonomy. He also authored UAE Keys to Soil Taxonomy and chaired the Emirates Soil Museum Committee to establish a unique soil museum at ICBA. His research priorities include: soil surveys and salinity mapping, reclamation of salt-affected soils, integrated soil fertility management, agricultural intensification through soil health improvement using organic and inorganic amendments, conservation agriculture and climate smart agriculture, environmental impact assessment, land degradation, and carbon sequestration.

Lee Heng has a Ph.D. in soil science from Massey University, Palmerston North, New Zealand, and has more than 25 years' experience in soil-plant-water interactions, agricultural water management and water use efficiency, integrated nutrientwater interactions, and diffuse pollution control for sustainable agricultural production systems, at both national and international levels. Her work is covering countries in Africa, Asia, Europe, and Central and Latin America on sustainable land and water management for climate smart agriculture and the efficient use and conservation of agricultural resources for enhancing food production and environmental sustainability. For the past 18 years, Heng works at the Soil and Water Management & Crop Nutrition (SWMCN) Subprogram in the Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture. Currently, she is the Head of SWMCN Section, which assists scientists in Member States in the development, validation and dissemination of a range of soil, water, and crop management technology packages through the use of nuclear and nuclear-related techniques.

Prior to her current assignment with the Joint FAO/IAEA Division, Heng worked as a research associate in the Department of Agriculture and Forestry at the University of Melbourne, Australia. Her work involved measurement and modeling the transport of reactive solutes in soils, conducting field study on soil water and nitrogen dynamics under temperate pastures. She also worked with Landcare Research in New Zealand. Heng has won several awards for her professional achievements including the USDA-Agricultural Research Service (ARS) Outstanding Sustained Effort Technology Transfer Award in 2012 and several IAEA Merit Awards in different categories. Heng is author and coauthor of over 60 research publications in refereed journals.

# Acronyms and Abbreviations

# List of Figures

xxi

# List of Plates

xxiii

# List of Tables

# Chapter 1 Introduction to Soil Salinity, Sodicity and Diagnostics Techniques

Shabbir A. Shahid, Mohammad Zaman, and Lee Heng

Abstract It is widely recognized that soil salinity has increased over time. It is also triggered with the impact of climate change. For sustainable management of soil salinity, it is essential to diagnose it properly prior to take proper intervention measures. In this chapter soil salinity (dryland and secondary) and sodicity concepts have been introduced to make it easier for readers. A hypothetical soil salinity development cycle has been presented. Causes of soil salinization and its damages, socio-economic and environmental impacts, and visual indicators of soil salinization and sodicity have been reported. A new relationship between ECe (mS/cm) and total soluble salts (meq/l) established on UAE soils has been reported which is different to that established by US Salinity Laboratory Staff in the year 1954, suggesting the latter is specific to US soils, therefore, other countries should establish similar relationships based on their local conditions. Procedures for field assessment of soil salinity and sodicity are described and factors to convert EC of different soil: water (1:1, 1:2.5 & 1:5) suspensions to ECe from different regions are tabulated and hence providing useful information to those adopting such procedures. Diversified salinity assessment, mapping and monitoring methods, such as conventional (field and laboratory) and modern (electromagnetic-EM38, optical-thin section and electron microscopy, geostatistics-kriging, remote sensing and GIS, automatic dynamics salinity logging system) have been used and results are reported providing comprehensive information for selection of suitable methods by potential users. Globally accepted soil salinity classification systems such as US Salinity Lab Staff and FAO-UNESCO have been included.

 Soil salinity, the second major cause of land degradation after soil erosion, has been a cause of decline in agricultural societies for 10,000 years. Globally about 2000 ha

© International Atomic Energy Agency 2018

M. Zaman et al., Guideline for Salinity Assessment, Mitigation and Adaptation Using Nuclear and Related Techniques, https://doi.org/10.1007/978-3-319-96190-3\_1

of arable land is lost to production every day due to salinization. Salinization can cause yield decreases of 10–25% for many crops and may prevent cropping altogether when it is severe and lead to desertification. Addressing soil salinization through improved soil, water and crop management practices is important for achieving food security and to avoid desertification.

# 1.1 What Is Soil Salinity?

Soil salinity is a measure of the concentration of all the soluble salts in soil water, and is usually expressed as electrical conductivity (EC). The major soluble mineral salts are the cations: sodium (Na+ ), calcium (Ca2+), magnesium (Mg2+), potassium (K+ ) and the anions: chloride (Cl), sulfate (SO4 2), bicarbonate (HCO3 ), carbonate (CO3 2), and nitrate (NO3 ). Hyper-saline soil water may also contain boron (B), selenium (Se), strontium (Sr), lithium (Li), silica (Si), rubidium (Rb), fluorine (F), molybdenum (Mo), manganese (Mn), barium (Ba), and aluminum (Al), some of which can be toxic to plants and animals (Tanji 1990).

From the point of view of defining saline soils, when the electrical conductivity of a soil extract from a saturated paste (ECe) equals, or exceeds 4 deci Siemens per meter (dS m1 ) at 25 C, the soil is said to be saline (USSL Staff 1954), and this definition remains in the latest glossary of soil science in the USA.

#### 1.1.1 Units of Soil Salinity

Salinity is generally expressed as total dissolved solutes (TDS) in milli gram per liter (mg l1 ) or parts per million (ppm). It can also be expressed as total soluble salts (TSS) in milli equivalents per liter (meq l1 ).

The salinity (EC) was originally measured as milli mhos per cm (mmho cm1 ), an old unit which is now obsolete. Soil Science has now adopted the Systeme International d'Unites (known as SI units) in which mho has been replaced by Siemens (S). Currently used SI units for EC are:

The units can be presented as:

1 mmho cm1 ¼ 1 dS m1 ¼ 1 mS cm1 ¼ 1000 micro Siemens per cm (1000 μS cm1 )

No fixed relationship exists between TDS and EC, although a factor of 640 is commonly used to convert EC (dS m1 ) to approximate TDS. For highly concentrated solutions, a factor of 800 is used to account for the suppressed ionization effect on EC.

Similarly, no one relationship exists between ECe and total soluble salts (TSS), although a factor of 10 is used to convert ECe (dS m1 ) to TSS (expressed in meq l1 ) in the EC range of 0.1–5 dS m1 (USSL Staff 1954). One relationship between ECe and TSS is presented in the Agriculture Handbook 60 (USSL Staff 1954). This relationship was developed using USA soils and has been widely used (worldwide) for over six decades. No efforts have been made to validate this relationship in other soils, though recently Shahid et al. (2013) have published a similar relationship for sandy desert soils ranging from low salinity (desert sand) to hyper-saline soils (coastal lands) in the Abu Dhabi Emirate. This latter work established a relationship between ECe and TSS which differs significantly from that of USSL Staff (1954), thus, opening the way for other countries to develop country-specific relationships which will allow better prediction and management of their saline and salinesodic soils.

#### 1.1.2 Why Total Soluble Salts Versus ECe Relationship Is Required?

Laboratories in some developing countries do not generally have modern equipment, i.e. flame emission spectrophotometer (FES), atomic absorption spectrophotometer (AAS), or inductively coupled plasma (ICP) in order to analyze soil saturation extracts or water samples for soluble Na+ to determine sodicity (sodium adsorption ratio – SAR). In contrast, Ca2+ and Mg2+ are easy to measure using a titration method, one which does not require modern instruments. Currently, these laboratories in many developing countries determine soluble Na+ by calculating the difference between the total soluble salts (TSS) and the quantities of Ca2+ + Mg2+ in order to make the analyses affordable, as below:

$$\mathbf{Na}^+ = \left[ (\text{Total soluble salts}) \text{ minus } \left( \mathbf{Ca}^{2+} + \mathbf{Mg}^{2+} \right) \right]^\ast$$

The TSS are recorded from a graph [see Fig. 4, page 12 of the Agriculture Handbook 60 (USSL Staff 1954)] by using the ECe value (Fig. 1.1). The Na+ amount is then used to determine SAR so that exchangeable sodium percentage (ESP) can be calculated as:

$$\text{SAR} = \frac{\text{Na}^+}{\sqrt{\frac{1}{2}\left(\text{Ca}^{2+} + \text{Mg}^{2+}\right)}}$$

$$\text{ESP} = \frac{[100 \ (-0.0126 + 0.01475 \times \text{SAR})]}{[1 + (-0.0126 + 0.01475 \times \text{SAR})]}$$

Where, each of Na+ , Ca2+ + Mg2+ concentrations are expressed in milli equivalents per liter (meq l1 ) and SAR is expressed as (milli moles per liter)0.5 (mmoles l1 ) 0.5.

Fig. 1.1 Relationship between total soluble salts (TSS) on y-axis and ECe on x-axis. (Source: Fig. 4, page 12, Agriculture Handbook 60 (USSL Staff 1954))

In the above method of determining Na+ by calculating the difference between TSS and Ca2+ + Mg2+ , any K+ amounts present are added to the Na+ (which is thus overestimated). It should be noted that the TSS versus ECe curve developed by USSL Staff (1954) was developed for the Western North American soils and, thus, may or may not be representative of soils of other countries. Hence, using such a practice may lead one to overestimate the sodicity hazard in irrigation waters or in saturation extracts of soils. This could lead to incorrect predictions and the use of inappropriate management options.

The finding of Shahid et al. (2013) has revealed an appreciable difference between the straight line (TSS versus ECe) determined from USSL Staff (1954) relative to protocols established by Shahid et al. (2013), shown in Figs. 1.1 and 1.2.

The TSS/ECe ratio, thus, ranges between 10 (at ECe 1 dS m1 ) and 16 (at ECe 200 dS m1 ) based on USSL Staff (1954) relationship (Fig. 1.1). In contrast, use of the relationship obtained from the methods developed by Shahid et al. (2013), the TSS/ECe ratio ranged between 10 (at 1 dS m1 ), 11.38 (at ECe 200 dS m1 ) and 12 (at ECe 500 dS m1 ). A comparative representation is shown in Figs. 1.3 and 1.4.

Fig. 1.2 Relationship between TSS and ECe (from Shahid et al. 2013)

In order to test the above lines to determine soil sodicity, Shahid et al. (2013) gave three examples using one soil type, as below.

#### Example 1

Determination of sodium adsorption ratio (SAR) accomplished by analyzing soil saturation extract for ECe (using an EC meter), and soluble Na+ , Ca2+, Mg2+ determined by using an atomic absorption spectrophotometer.

Fig. 1.3 A comparison of the relationship between total soluble salts (TSS)/ECe established using the Agriculture Handbook 60 curve (USSL Staff 1954) for the soils of Abu Dhabi Emirate and the relationship established for the same soils by the ICBA/EAD curve (Shahid et al. 2013)

Fig. 1.4 Average lines showing the relationship between ECe and total soluble salts (TSS) from the USSL method (line from Fig. 1.1, above) for the soils of Abu Dhabi Emirate using the method developed by Shahid et al. (2013) using the average line adapted from Fig. 1.2, above

## Example 2

Determination of sodium adsorption ratio (SAR) accomplished by analyzing soil saturation extract for ECe (using an EC meter), soluble Ca2+ and Mg2+ (by titration procedure) and soluble Na+ estimated by calculating the difference between TSS and Ca2+ + Mg2+ using USSL Staff (1954) relationship (Fig. 1.1).

#### Example 3

Determination of sodium adsorption ratio (SAR) accomplished by analyzing soil saturation extract for ECe (using an EC meter), soluble Ca2+ and Mg2+ (by titration procedure) and soluble Na+ by calculating the difference using the relationship developed by Shahid et al. (2013) (Fig. 1.2).

The above three examples clearly show that the SAR (71.2) when determined by using the Shahid et al. (2013) relationship (Fig. 1.2) is very close to the SAR determined by analyzing the saturation extract by modern laboratory equipment (71.2 versus 72.4). However, when SAR was determined by using the USSL Staff (1954) relationship, it was appreciably higher (95.32) than the SAR values determined by other procedures.

This indicates that the Na+ value obtained by using the USSL Staff (1954) relationship can lead to higher SAR and can, thus, mislead the sodicity prediction. The above examples have confirmed that the relationship established for the soils of Abu Dhabi Emirate by Shahid et al. (2013) can be used reliably to determine soil sodicity (SAR and ESP). Thus, the analyses become rapid and affordable for the use in developing countries. Also of importance is the need for developing countries, e.g. the Gulf Cooperation Council (GCC) and ARASIA countries, who are relying on the USSL Staff (1954) curve to determine soluble sodium by calculating the difference between TSS and Ca2+ + Mg2+, to validate this relationship for local soils.

# 2 Causes of Soil Salinity

There can be many causes of salts in soils; the most common sources (Plate 1.1) are listed below:

Plate 1.1 Soil salinity development in agriculture and coastal fields. (a) Salinity in a furrow irrigated barley field, (b) Salinity in a sprinkler irrigated grass field, (c) Salinity due to sea water intrusion in coastal land

#### Box 1.1: Salt Loads in Soil Due to Irrigation

It is generally believed that irrigation with fresh water is safe for optimum crop production; this may be true for short duration. However, if this water is used over a long period without managing for salinity, a significant quantity of salts will be added into soil. A simple example is given below.

Assume that fresh water (EC 0.2 dS m1 ) is used for irrigating the crop and 8500 cubic meters per hectare (850 mm) of this water is used over the entire crop season. The water of EC 0.2 dS m1 contains approximately 128 mg l1 salts (0.2 640) which are equivalent to 0.128 kg per cubic meter of irrigation

(continued)

#### Box 1.1 (continued)

water. Over the crop season, 1088 kg of salts will, thus, be added to each hectare with the irrigation water. If we assume that the dry matter harvested from each hectare is about 15 metric tons, and there is 3.5% by weight of total salts in the harvested crop biomass, then the portion of salt harvested with the crop is 525 kg. This leaves 563 kg of salts in the soil directly or in the plant parts (belowground, stubbles, debris) which will be returned to the soil by cultivation and subsequent decay of the plant biomass left in the field. This example is a very conservative one. It is more likely that water of a higher salinity will be used for irrigation. Thus, a salinity management program needs to be implemented for virtually all irrigated agricultural crops, especially those growing in low natural rainfall areas.

Fig. 1.5 A hypothetical soil salinization cycle. (Adapted from Shahid et al. 2010)

# 3 Salinity Development in Soils – A Hypothetical Cycle

Recently Shahid et al. (2010) have hypothesized a soil salinity cycle in order to present various facets in the development of soil salinity (Fig. 1.5).

# 4 Types of Soil Salinity

# 4.1 Dryland Soil Salinity

Salinity in dryland soils develops through a rising water table and the subsequent evaporation of the soil water. There are many causes of the rising water table, e.g. restricted drainage due to an impermeable layer, and when deep-rooted trees are replaced with shallow-rooted annual crops. Under such conditions, the groundwater dissolves salts embedded in rocks in the soil, with the salty water eventually reaching the soil surface, and evaporating to cause salinity. Dryland salinity can also occur in un-irrigated landscapes. There are no quick fixes to dryland salinity, though modern technologies in assessment and monitoring can allow one to follow and better understand how salinity develops. The most important of these new technologies utilizes the interpretation of remote sensing imaging over a period of time.

The potential technologies to mitigate dryland salinity include the pumping of saline groundwater and its safe disposal or use, as well as the development of alternate crop plant production systems to maximize saline groundwater use, such as deep-rooted trees. Such a deep-rooted trees system utilizes groundwater and lowers water table, the process called biodrainage (or biological drainage). In Australia dryland salinity is a major problem, one which costs Australia over \$250 million a year from impacts on agriculture, water quality and the natural environment. Dryland salinity is an important problem which must be approached strategically using scientific diagnostics.

# 4.2 Secondary Soil Salinity

In contrast to dryland salinity, secondary salinity refers to the salinization of soil due to human activities such as irrigated agriculture.

Water scarcity in arid and desert environments necessitates the use of saline and brackish water to meet a part of the water requirement of crops. The improper use of such poor quality waters, especially with soils having a restricted drainage, results in the capillary rise and subsequent evaporation of the soil water. This causes the development of surface and subsurface salinity, thereby reducing the value of soil resource. Common ways of managing secondary salinity in irrigated agriculture are:

The management of secondary salinization in irrigated agriculture is discussed in more detail in Chap. 5.

# 5 Damage Caused by Soil Salinity

Some of the damages caused by increasing the soil salinity (Shahid 2013) are listed below:

# 6 Facts About Salinity and How It Affects Plant Growth

Franklin and Follett (1985) have described the effects of salinity on plant growth as listed below:

# 7 Visual Indicators of Soil Salinity

Once soil salinity develops in irrigated agriculture fields, it is easy to see the effects on soil properties and plant growth (Plate 1.2). Visual indicators of soil salinization (Shahid and Rahman 2011) include:

# 8 Field Assessment of Soil Salinity

Visual assessment of salinity only provides a qualitative indication; it does not give a quantitative measure of the level of soil salinity. That is only possible through EC measurement of the soil. In the field, collection of soil saturation extract from soil paste is not possible. Therefore, an alternate procedure is used, e.g. a soil:water suspension (1:1, 1:2.5, 1:5) for field salinity assessments.

Plate 1.2 Field diagnostics of soil salinity – visual indicators for quick guide. (a) Salt stains and poor growth, (b) Leaf burn grassy plot, (c) Patchy crop establishment, (d) Dead trees due to salt stress

	- 10 g soil +10 ml distilled water (1:1)
	- 10 g soil +25 ml distilled water (1:2.5)
	- 10 g soil +50 ml distilled water (1:5)

The EC values obtained for different soil:water ratio extracts can then be correlated to the EC of soil saturation extract (ECe) as explained below (Shahid 2013; Sonmez et al. 2008). It should be noted that EC values of 1:1, 1:2.5, 1:5 soil:water extracts are site- specific and, thus, can be used as a general guideline only. However, once correlations are established with ECe (EC of soil saturation extract) from the same soil samples, the derived ECe can be used reliably in salinity management and crop selection. Suitable conversion factors can be used based on soil type (Table 1.1).

Table 1.1 Conversion factor for deriving the ECe from the EC of extracts from different soil:water ratio suspensions

(continued)

Table 1.1 (continued)

# 9 Soil Sodicity and Its Diagnostics

Sodicity is a measure of sodium ions in soil water, relative to calcium and magnesium ions. It is expressed either as sodium adsorption ratio (SAR) or as the exchangeable sodium percentage (ESP). If the SAR of the soil equals or is greater than 13 (mmoles l1 ) 0.5, or the ESP equals or is greater than 15, the soil is termed sodic (USSL Staff 1954).

# 9.1 Visual Indicators of Soil Sodicity

Soil sodicity can be predicted visually in the field in the following ways

# 9.2 Field Testing of Soil Sodicity

Field assessment of relative level of soil sodicity can be determined through the use of a turbidity test on soil:water (1:5) suspensions, with ratings:

The relative sodicity can be further assessed by placing a white plastic spoon in these suspensions, as below.

# 9.3 Laboratory Assessment of Soil Sodicity

Accurate soil sodicity diagnostics can be made by analyzing soil samples in the laboratory. The standard presentation of soil sodicity is the exchangeable sodium percentage (ESP) derived through using sodium adsorption ratio (SAR). Alternately, ESP can be determined through measurement of exchangeable sodium (ES) and cation exchange capacity (CEC), as below.

$$\text{ESP} = \left(\frac{\text{ES}}{\text{CEC}}\right) \times 100$$

Where, ES and CEC are represented as meq100 g1 soil. An ESP of 15 is the threshold for designating soil as being sodic (USSL Staff 1954). At this ESP level, the soil structure starts degrading and negative effects on plant growth appear.

# 10 Sodicity and Soil Structure

A lack of sufficient volumes of fresh water for irrigation use in arid and semi-arid regions often results in the need to use water with a relatively high salinity and high sodium ion levels. It has, generally, been recognized that the sodicity affects soil permeability appreciably. The swelling and dispersion of soil clays ultimately destroys the original soil structure – likely the most important physical property affecting plant growth. The soil bulk density (the weight of soil in a given volume) and porosity (open spaces between sand, silt and clay particles in a soil) are mainly used as parameters for the soil structure. The hydraulic conductivity (the ease with which water can move through the soil pore spaces) is the net result of the effect of physical properties in the soil and is markedly affected by soil structure development. The effect of the sodicity of soil water on irrigated soils can be both a surface phenomenon, i.e. showing surface sealing, as well as a subsurface phenomenon (Box 1.2), one where subsurface sealing also occurs (Shahid et al. 1992). In surface sealing, the soil water sodicity causes a breakdown and slaking of soil aggregates due to wetting. When the soil surface dries, a surface crust is formed. In subsurface sealing, the clay particles in the soil are dispersed and translocate to subsurface layers, where they are then deposited on the surface of the voids, thereby reducing void volume and blocking the pores, thus restricting further water movement, e.g. yielding non-conducting pores.

The surface sealing and crusting due to either water sodicity, or through combined effects of sodicity and raindrop splash action, have both positive and negative effects.

#### Box 1.2: Effect of Saline-Sodic Waters on Soil Hydraulic Conductivity and Structure in a Simulated System

In a simulated system developed and used by Shahid and Jenkins (1992a,c) for quick screening of soils with regard to their salinity and sodicity, a laboratory experiment was conducted to investigate the effect of saline-sodic water on soil structure and hydraulic conductivity (Shahid 1993). In this system, glass columns were filled with non-saline and non-sodic soil (Typic camborthid) which contained both swelling (smectite and vermiculite) and non-swelling (mica, chlorite and kaolinite) minerals, and silty clay loam texture. Five irrigation waters having EC 0 (deionized water), 0.5, 1.0, 1.0 and 2.4 dS m1 , and SAR 0 (deionized water), 20, 25, 40 and 36 (mmoles l1 ) 0.5, respectively, were used in wetting and drying cycles. After 14 wetting and drying cycles, the soil columns were subjected to hydraulic conductivity measurement with the respective waters treatments (above), followed by simulated rain, application of a gypsum saturated solution, and a simulated subsoil application with a gypsum saturated solution.

#### Box 1.2 (continued)

# 10.1 Negative Effects of Surface Sealing

# 10.2 Positive Effects of Surface Sealing

# 11 Classification of Salt-Affected Soils

A soil which contains soluble salts in amounts in the root-zone which are sufficiently high enough to impair the growth of crop plants is defined as saline. However, because salt injury depends on species, variety, plant growth stage, environmental factors, and the nature of the salts, it is very difficult to define a saline soil precisely. That said, the most widely accepted definition of a saline soil is one that has ECe more than 4 dS m1 at 25 C.

# 11.1 US Salinity Laboratory Staff Classification

The term salt-affected soil is being used more commonly to include saline, salinesodic and sodic soils (USSL Staff, 1954), as summarized in Table 1.2.

#### 11.1.1 Saline Soils

Saline soils are defined as the soils which have pHs usually less than 8.5, ECe 4 dS m1 and exchangeable sodium percentage (ESP) < 15.

A high ECe with a low ESP tends to flocculate soil particles into aggregates. The soils are usually recognized by the presence of white salt crust during some part of the year. Permeability is either greater or equal to those of similar 'normal' soils.

#### 11.1.2 Saline-Sodic Soils

Saline-sodic soils contain sufficient soluble salts (ECe 4 dS m1 ) to interfere with the growth of most crop plants and sufficient ESP ( 15) to affect the soil properties and plant growth adversely, primarily by the degradation of soil structure. The pHs may be less or more than 8.5.

#### 11.1.3 Sodic Soils

Sodic soils exhibit an ESP 15 and show an ECe < 4 dS m1 . The pHs generally ranges between 8.5 and 10 and may be even as high as 11. The low ECe and high ESP tend to de-flocculate soil aggregates and, hence, lower their permeability to water.

#### 11.1.4 Classes of Soil Salinity and Plant Growth

Electrical conductivity of the soil saturation extract (ECe) is the standard measure of salinity. USSL Staff (1954) has described general relationship of ECe and plant growth, as below.

# 11.2 FAO/UNESCO Classification

Salt-affected soils (halomorphic soils) are also indicated on the soil map of the world (1:5,000,000) by FAO-UNESCO (1974) as solonchaks (saline) and solonetz (sodic). The origin of both terms, solonchaks and solonetz, is Russian.

## 11.2.1 Solonchaks (Saline)

Solonchaks (saline) are soils with high salinity (ECe >15 dSm1 ) within the top 125 cm of the soil.

The FAO-UNESCO (1974) divided solonchaks into four mapping units:

#### 11.2.2 Solonetz (Sodic)

Solonetz (sodic) is a sodium-rich soil that has an ESP > 15. The solonetz soils are subdivided into three mapping units:

Soils with lower ESP than a solonetz, lower than 15 but higher than 6, are mapped as a 'sodic phase' of other soil units.

# 12 Socioeconomic Impacts of Salinity

# 13 Environmental Impacts of Salinity

# 14 Soil Salinity Monitoring

Soil salinity is indirectly measured as electrical conductivity of the soil solution or of a soil saturation extract. Salinity is an important analytical measurement since it reflects the suitability of the soil for growing crops. On the basis of using a soil saturation extract, ECe values of 2 dS m1 (or mmhos cm1 ) are safe for all crops. Yields of very salt sensitive crops are negatively affected by ECe between 2 and 4 dS m1 . Yields of most crops are affected by ECe between 4 and 8 dS m1 . Only salt tolerant crops grow well above ECe 8 dS m1 .

Salinity is largely a concern in irrigated areas and in areas with saline soils, but generally is not important in rain-fed agriculture. As use of brackish irrigation water increases, there will be greater emphasis on the utilization of soil EC measurements in the future.

Many factors can contribute to the development of saline soil conditions. However, most soils become saline through the use of salt containing groundwater for irrigation. Salt concentrations in soil vary widely, both vertically and in the

Plate 1.3 Maximum soil salinity zone in drip irrigation system in a grass field – Alain, UAE

horizontal plane. The extent of the variability depends on conditions such as differences in soil texture, growing plants which transpire soil water and also absorb salts, quality of irrigation water, soil hydraulic conductivity and the type of irrigation system being used.

A salinity monitoring plan must be an integral part of any agricultural project which deals with irrigation water which has a salinity and/or sodicity constituent. An effective salinity monitoring plan must, thus, be developed so that salinity changes can be traced, especially for the root-zone soil.

# 15 Soil Sampling Frequency and Zone

A number of soil sampling techniques exist, and they should be used carefully based on aims of the study. Random soil samples can be taken from a number of representative sites to get composite sample. The duration over which soil sampling occurs for salinity monitoring is quite important and that duration should be decided based on the nature of the project and its aims.

The zone of soil sampling is an important criterion, particularly for drip irrigation where the maximum salinity builds up in the periphery of the wetting front (Plate 1.3). Salt accumulation occurs via two processes.

Solutes are, thus, redistributed in the soil, with the final buildup of salts in the soil resulting from the interaction of the above mentioned two processes throughout the crop growth period. Soil sampling from the middle zone (between two drip irrigation lines) will, however, present maximum salinity values, and may be misleading (Plate 1.3). Therefore, soil sample from within the root-zone can provide a better estimate of the soil's salinity status.

# 16 Current Approaches of Salinity Diagnostics – Assessment, Mapping and Monitoring

# 16.1 Salinity Assessment

Accurate and reliable measurements are essential to better understand soil salinity problems in order to provide better management, improve crop yield and maintain root-zone soil health. If soil salinity can be measured, it can likely be managed. The choice of the method for soil salinity assessment, however, depends on the objective, the size of the area, the depth of soil to be assessed, number and frequency of measurements, the degree of accuracy required and the resources available.

There are a number of soil salinity assessment tools. These include salinity monitoring maps, prepared over a period of time to assess present salinity problems, and to predict future salinity risks to the area. They also include the use of salinity indicators on the soil surface, vegetation indicators, conventional salinity tests (EC 1:1 or 1:5; ECe) and more modern methods (Geophysical – EM38; Salinity sensors).

#### 16.1.1 Routine Methods

The soil salinity measurements made using geo-referenced (using GPS) field sampling and laboratory analysis of extracts from saturated soil paste by an EC meter are accepted as the standard way of assessing soil salinity. The amount of water that a soil holds at saturation is related to soil texture, surface area, clay content, and cation exchange capacity. The lower soil:water ratios (1:1, 1:2.5, 1:5) are also used in many laboratories, though the results require calibration with ECe if they are going to be used to select salt tolerant crops.

# <IP_ADDRESS> Saturated Soil Paste – Justification for Its Use

The EC of the solution extracted from a saturated soil paste (which contains water content about twice the amount of water retained in the soil at field capacity) has been correlated with the growth or toxicity responses of a wide range of crop plants. This measure, known as electrical conductivity of the soil saturation extract (ECe), is now the generally accepted measure of soil salinity. The procedure is, however, time consuming and it requires vacuum filtration. It is important to note that EC measurements based on extracts obtained from saturated soil paste or suspensions of fixed soil:water ratios (commonly 1:1, 1:2.5 or 1:5) do not give reliable correlations. Such extracts, or extracts with a wider soil:water ratio are, however, more convenient where the ability to sample soils properly is limited. The lack of reliability is mainly due to the fact that the amount of water held at a given tension varies from soil to soil, depending on texture, the type of clay minerals present, and other factors.

## <IP_ADDRESS> Preparation of Saturated Soil Paste

## <IP_ADDRESS> Collection of Soil Saturation Extract and EC Measurement

Plate 1.4 Setup for collection of extract from saturated soil paste

# 16.2 Modern Methods of Soil Salinity Measurement

#### 16.2.1 Salinity Probe

An activity meter with a salinity probe is very convenient and gives instant apparent electrical conductivity (ECa) information, which is expressed in mS cm1 and g l1 . There are many models of equipment available to measure in-situ salinity. One is the German-made PNT3000 COMBI + model; it is commonly used in agriculture, horticulture and on landscape sites for rapid salinity assessment and monitoring. It provides an extended EC measuring range from 0 to 20 mS cm1 and from 20 to 200 mS cm1 . The unit includes a 250 mm long stainless steel electrode for direct soil salinity measurements; an EC-plastic probe with platinum plated ring sensors and a high quality aluminum carrying case. Its operation is convenient and simple; only one button makes the full operation possible. It is essential, however, to validate ECa values with ECe from same soil locations. In any case, the ECa must be correlated to ECe for use in assessing a crop's salt tolerance. Recently Shahid (2013) has established correlations between ECe and ECa measured by EC probe in a large number of saline fields.

#### ECa (Salinity Probe) Versus EC of Extracts (Varying Soil:Water Ratios)

For many reasons, laboratory analysis of the soil saturation extract is still the most common technique for assessing soil salinity. Salinity of the saturation extract (ECe) is considered to be the 'standard procedure' because the amount of water that a soil holds at saturation (the saturation percentage) is related to several important soil parameters, including texture, surface area, clay content, and cation exchange capacity (CEC).

Low soil to water ratios, for example 1:1; 1:2.5; 1:5, make extraction easier, but show a poorer relation to field moisture condition than the saturated paste. The choice of equipment or procedure depends upon several factors, including size of the area being evaluated, the depth of soil to be assessed, the number and frequency of the measurements, the accuracy required, and the availability of resources. The standard method of salinity monitoring involves collecting soil samples from the root-zone over a given period of time followed by their analysis in the laboratory as a soil saturation extract.

As a part of International Center for Biosaline Agriculture (ICBA) salinity monitoring program, the soil team collected a large number of soil samples from experimental plots which had been irrigated with water of varying salinity, up to a maximum of seawater (Plate 1.5). These samples were air-dried and processed in order to collect water extracts from soil:water suspensions of 1:1, 1:2.5, and 1:5, as well as soil extracts made from a saturated soil paste. The field conductivity (ECa) in mS cm1 using the salinity probe (field scout) was also measured. A simple statistical test was used to calculate the correlation and correlation coefficient (R2 ), and to derive factors from it in order to convert ECa (determined at several soil water contents by the use of the salinity probe) to ECe.

The correlations are developed for a fine sand (Soil Survey Division Staff 2017) textural class at ICBA experimental station (Table 1.3). The conversion factor for ECe determinations derived from the apparent EC (ECa) measured by the field scout in saline fields (Shahid 2013) was found to be: ECe ¼ ECa 3.81.

#### 16.2.2 Electromagnetic Induction (EMI)

Salinity assessment and management at the farm level must help farmers improve crop productivity. The conventional field sampling followed by laboratory analysis is a tedious, expensive and time consuming process. There are other modern

Plate 1.5 Salinity monitoring in a grass field using salinity probe. (Field scout PNT 3000 COMBI +)

Table 1.3 Correlations of ECa measured by using the field scout (EC probe) with EC of 1:1, 1:2.5 and 1:5 (soil:water suspensions) and ECe

methods which can be used quickly and effectively in field salinity mapping, e.g. electromagnetic induction (EMI) using the EM38. EM38 is the most commonly used instrument in agricultural surveys, and gives a rapid assessment of the soil's apparent electrical conductivity (ECa), expressed in mS m1 .

The EM38 has a transmitter coil and a receiving coil. The transmitter coil induces an electrical current into the soil and the receiving coil records the resultant electromagnetic field. The EM38 allows for a maximum of 150 cm or 75 cm depth of exploration in the vertical and horizontal dipole modes, respectively (Plate 1.6). EC mapping using EMI is one of the simplest and least expensive salinity measurement tools. Integration of GIS information with salinity data yields salinity maps which can help farmers interpret crop yield variations, and provide a better understanding of the subtle salinity differences across agricultural fields. These salinity maps may allow farmers to develop more precise management zones and ultimately obtain higher yields.

Plate 1.6 EC measurement with EM 38 in vertical mode (left: grassland) and horizontal mode (right: Atriplex field)

Technological advances such as EMI have, in the last 40 years, revolutionized soil salinity assessment. McNeill (1980) was among the first investigators to describe how the EMI method can be used in assessing soil salinity. He presented the theoretical basis for the use of EM ground conductivity meters to map lateral variation in subsurface electrical conductivity. The approach, and the instrumentation developed (GEONICS EM31, EM34-3, EM38), gained almost immediate acceptance as a replacement for the traditional use of the 4-electrode resistivity (galvanic) traversing techniques, especially when it was demonstrated that the two types of soil surveys produced very similar results (Cameron et al. 1981).

Accordingly, the spatial distribution of soil salinity on the individual field (Cameron et al. 1981), the agricultural farm (Norman et al. 1995a, b), as well as district (Vaughan et al. 1995) and the regional (Williams and Baker 1982) scales has been described. Baerends et al. (1990) used the EM38 for a detailed salinity survey in an experimental area of 37 ha. They measured electrical conductivity of the soil at more than 3600 locations, reporting significant difference between the salinity of barren, fallow and cropped fields. Thus, barren fields have a high salinity, fallow fields a moderate salinity, and the cropped fields showed the lowest salinity. Baerends et al. (1990) found good agreement between the EM38 survey and the results of the visual agronomic salinity survey, with EM38 survey yielding results with a better resolution. The EMI method is also more sensitive to salinity changes, and can be carried out at any time of the year.

The EM38 instrument has been mobilized (Rhoades 1992) by mounting it on the front wheels of a farm vehicle. The EM readings can be made with the sensor positioned at several different heights above the ground in both horizontal (EMH) and vertical (EMV) magnetic coil configurations. By this adjustment, each stop requires about 20–30 s, with multiple readings being needed to calculate ECa (and, in turn, salinity) within soil depths of 0–30, 30–60, 60–90, and 90–120 cm. The intensive data set of salinity by depth and location can also be used to assess the adequacy of past leaching/drainage practices (Rhoades 1992). For example, where salinity decreases with depth in the profile, the net flux of water (and salt) can be interpreted as being upward. This is reflective of inadequate leaching and/or poor drainage. Where salinity increases with depth in the soil profile, the net flux of water and salt can be inferred as being downward. This finding would be indicative of the adequacy of leaching/drainage. However, when salinity is low and relatively uniform with increasing depth, leaching is interpreted as excessive, probably contributing to waterlogging elsewhere, with a high salt loading of the receiving water supplies.

Rhoades and Ingvalson (1971) showed that soil salinity in the field could also be assessed with a conventional geo-electrical method. This method has been developed for the purposes of measuring the soil salinity of the entire root-zone (Nadler and Frenkel 1980; Rhoades and Oster 1986; Yadav et al. 1979). Elaborating the same technique, Rhoades and Van Schilfgaarde (1976) developed another electrical conductivity probe for measuring soil salinity distribution with increasing depth. With this probe, soil salinity is measured at specific depths in the root-zone. Thus, four electrode techniques can be successfully used for surveys of saline soils (Nadler 1981; Halvorson et al. 1977).

Williams and Baker (1982) first recognized the possibility of using EM meters for reconnaissance surveys of soil salinity variation. The high values of apparent electrical conductivity (ECa) measured by the EM meters were positively correlated with increased amounts of salts in the soil. The correlation led to empirical relationships (Rhoades et al. 1989; Cook et al. 1992; Acworth and Beasley 1998) that allow a prediction of soil salinity based on the measurement of the ECa. The correlation between salt content and the ECa is very high for zones of uniform soil material; though with zones showing different electrical conductivity, it is necessary to obtain separate values of ECa for each zone.

There is an electrical conductivity image method which provides an alternative approach. Here, the ECa function is sampled extensively in the vertical plane using a 4-electrode array. The method was described by Acworth and Griffiths (1985) and Griffiths and Baker (1993) but has not been widely used. There was difficulty in creating a starting model distribution of ECa values and extensive time consuming work was required in 'forward modeling' of the ECa data to achieve a match with the field data. Even so, the devices are used regularly for soil salinity surveys in different parts of the world (Boivin et al. 1988; Job et al. 1987; Williams and Hoey 1987). The main advantages of the EC image method are: i) measurements can be taken almost as fast as one can walk from one measurement location to another, and ii) the large volume of soil which is measured reduces the variability so that relatively fewer measurements yield a reliable estimate of the field's mean salinity.

In one study (Nettleton et al. 1994), the EM induction approach has been extended to identification of sodium-affected soils. This approach shows great promise as a measure of both salinity and sodicity. However, such studies on sodic soils have not gained much attraction due to constraints in measuring accurate sodicity levels.

## Factors Affecting EC Measurement of Soil by EM38

The conductance of electricity in soil takes place through the moisture filled pores that occur between individual soil particles. Therefore, the EC of soil is determined by the following soil properties (Doerge 1999).

Porosity The greater soil porosity, the more easily electricity is conducted. Soil with high clay content has higher porosity than sandy soil. Compaction of moist soils normally increases soil EC.

Soil Water Content Dry soil is much lower in electrical conductivity than moist soil.

Salinity Level Increasing concentration of electrolytes (salts) in the soil water will dramatically increase soil EC.

Cation Exchange Capacity Mineral soils which contain high levels of organic matter (humus) and/or 2:1 clay minerals, such as montmorillonite or vermiculite, have a much higher ability to retain positively charged ions (such as Ca2+, Mg2+, K+ , Na+ , NH4 + , or H+ ) than the soils lacking these constituents. The presence of these cations in moisture filled soil pores will enhance soil EC in the same way as salinity does.

Temperature As temperature decreases toward the freezing point of water, soil EC decreases slightly. Below freezing point, soil pores become increasingly isolated from each other and overall soil EC declines rapidly.

#### 16.2.3 Salinity Sensors and Data Logger

The most modern salinity data logging system is the Real Time Dynamic Automated Salinity Logging System (RTASLS). Here, ceramic salinity sensors are buried in the root-zone, each sensor being fitted with an external smart interface with a resolution of 16 Bits. This interface consists of an integrated microprocessor which contains all the required information to allow for autonomous operation of the sensor, including power requirements and logging interval. The smart interface is connected to a DataBus which leads to the Smart Data Logger which automatically identifies each of the salinity sensors and begins logging them at predetermined intervals. Instantaneous readings from sensors can be viewed in the field on the data logger's display. Data can also be accessed in the field with a memory stick or remotely using a smart mobile phone. Due to technology advancements, there are diversified sensors available. Such a real time data logging system (Plate 1.7) has been installed in the grass plots at the experimental station of ICBA, Dubai (Shahid et al. 2008).

A useful feature of the salinity data logging system is that it does not require knowledge of electronics or computer programing. For custom configuring the Smart Data Logger or the salinity sensors, a simple menu system can be accessed through HyperTerminal. This provides complete control over each individual sensor's setup. Data from the Smart Data Logger can be graphed using Excel.

Plate 1.7 Real time salinity logging system at ICBA experimental station (Shahid et al. 2008) (a) Sensor placement in the root-zone, (b) Buried sensors connected to the Smart Interface, (c) Smart Interface connected to the DataBus, (d) Instantaneous salinity read on Data Logger

## <IP_ADDRESS> System Installation and Operation – An Example

Salinity sensors are buried at 30 and 60 cm depths in a grass field (Distichlis spicata and Sporobolus virginicus) which is being irrigated with saline water of EC 10, 20 and 30 dS m1 (Plate 1.7). The dynamic changes of soil salinity within an irrigation cycle are showing the effect of salinity of the irrigation water on the salt concentration in the grass root-zone, and how the salt levels are constantly changing under irrigation (Fig. 1.6). The soil temperature can also give assistance with interpretation of soil-water movement as no soil moisture sensors were installed. Highlights of salinity monitoring for 25 days are presented in Fig. 1.6; note that days 15–19 are a period of rain.

## <IP_ADDRESS> Soil Salinity Monitoring

The soil salinity data recorded in the Distichlis spicata grass field show that:

• After initial installation, it takes about 10 days for the sensors to come to equilibrium with the soil-water solution. This is especially apparent for the 30 dS m1 irrigation water treatment.

Fig. 1.6 Soil salinity monitoring in a Distichlis spicata field at ICBA experimental station. Y-axis shows soil salinity fluctuations on different days. The experimental plots were irrigated with saline water of EC, dS m1 : 10 (A, C), 20 (E, G) and 30 (I, K)

# 16.3 Use of Remote Sensing (RS) and Geographical Information System (GIS) in Salinity Mapping and Monitoring

The RS and GIS have been used in many soil studies. Shahid (2013) has recently made efforts to compile this work in order to provide useful information to the potential users of these tools in soil salinity research. A comprehensive review is presented in the following section.

Salinity mapping can be accomplished by integrating RS and GIS techniques at both broad and small scales. The primary objective of reviewing publications dealing with these two techniques is to allow the prediction of sites vulnerable to the 'salinity menace'. GIS is a computer application that involves the storage, analysis, retrieval, and display of data that are described in terms of their geographic location. The most familiar type of spatial data is a map – GIS is really a way of storing map information electronically. A GIS map has a number of advantages over old-style maps, a primary advantage being the fact that because the data are stored electronically, they can be analyzed readily by computer. In the case of soil salinity, scientists can use data on rainfall, topography and soil type (indeed, any spatial information that is available electronically can be used) to determine the factors which make soils highly susceptible to salinization with the aim of being able to predict other (similar) regions that may be at risk.

RS imagery is well suited to map the surface expression of salinity (Spies and Woodgate 2005). For example, a poor cover of vegetation could be an indication of salinity, especially when combined with information on depth of soil to groundwater. The goal of such an exercise is to assess and map soil salinity in order to better understand the problem, and then provide information on how to take necessary actions to prevent increases in salinization of new areas over time. And finally, we need to know how to best manage salinity for sustainable use of land resources.

Salinized and cropped areas can be identified and assigned a salinity index based on greenness and brightness – one that indicates leaf moisture of plants being influenced by salinity. Here, classical false-color composites of separated bands can be used, or a computer-assisted land-surface classification can be developed (Vincent et al. 1996). A brightness index detects brightness as representing a high level of salinity. Satellite images can help in assessing the extent of saline areas and can monitor changes in real time. Saline fields are often identified by the presence of spotty white patches of precipitated salts. Such precipitates usually occur in elevated or un-vegetated areas, where evaporation has left salt residues. Such salt crusts, which can be detected on satellite images, are, however, not reliable evidence of high salinity in the root-zone. Another limitation in salinity mapping with multispectral imagery is where saline soils support productive plant growth such as areas of biosaline agriculture. There, plant cover obscures direct sensing of the soil because salt tolerant plants cannot be differentiated from other ground cover, unless extensive on-site ground investigations are made to corroborate the information.

However, RS can provide useful information for large areas with differing water and salt balances and can identify parameters such as evapotranspiration, rainfall distribution, interception losses, and crop types and crop intensities that can be used as indirect measures of salinity and waterlogging as evidence in the absence of direct estimates (Ahmad 2002).

# 16.4 Global Use of Remote Sensing in Salinity Mapping and Monitoring

Salinity mapping and monitoring by using RS and GIS are common in many countries and such procedures have recently been used in Kuwait and Abu Dhabi Emirate as part of the National Soil Inventories (KISR 1999a,b; Shahid et al. 2002; EAD 2009). At regional and national levels (Sukchani and Yamamoto 2005), RS and GIS was used for waterlogging and salinity monitoring (Asif and Ahmed 1999). RS technology was used for soil salinity mapping in the Middle East (Hussein 2001) and salt-affected soils using RS and GIS (Maher 1990).

The Thematic Mapper™ bands 5 and 7 are frequently used to detect soil salinity or drainage anomalies (Mulders and Epema 1986; Menenti et al. 1986; Zuluaga 1990; Vincent et al. 1996) and a broad scale monitoring of salinity using satellite remote sensing (Dutkiewics and Lewis 2008). Metternicht and Zinck (1997) have shown that Landsat TM (Thematic Mapper) and JERS-ISAR (Japan Earth Resources Satellite-Intelligent Synthetic Aperture Radar) data (visible and infrared regions) are the best ways to distinguish saline, alkaline, and non-saline soils. Landsat SMM (Solar Maximum Mission) and TM data for detailed mapping and monitoring of saline soils has been used in India for use as a reconnaissance soil map. Abdelfattah et al. (2009) developed a model that integrates remote sensing data with GIS techniques to assess, characterize, and map the state and behavior of soil salinity.

The coastal area of Abu Dhabi Emirate, where the issue of salinity is a major concern, has been used for a pilot study conducted in 2003. The development of a salinity model has been structured under four main phases: salinity detection using remote sensing data, site observations (on-site ground inspection), correlation and verification (a combination of a salinity map produced from visual interpretation of remotely sensed data and a salinity map produced from on-site observations), and model validation. Here, GIS was used to integrate the available data and information to design the model, and to create different maps. A geo-database was created and populated with data collected from observation points, melded together with laboratory analysis data. In this study, the correlation between the salinity maps developed from remote sensing data and on-site observations showed that 91.2% of the saline areas delineated using remote sensing data alone exhibit a good fit with saline areas delineated using on-site observations. Hence, the model can be adopted.

Remote sensing, thus, acquires information about the earth's surface without actually being in contact with it. The fundamentals of remote sensing in soil salinity assessment and examples of such studies from the Middle East, Kuwait, Abu Dhabi Emirate and Australia have been described recently by Shahid et al. (2010). The combination of salinity maps taken over period of time and Digital Elevation Model (DEM) help to predict salinity risk in the area.

# 16.5 Geo-Statistics

Geo-statistics is used for mapping of land surface features from limited sample data. It is widely used in fields where 'spatial' data is studied. Geo-statistical estimation is a two-stage process. First step is studying the gathered data to establish the predictability of values from place-to-place within the study area. This results in a graph known as a semi-variogram, which models the difference between a value at one location and the value at another location according to the distance and direction between them. The second step is estimating values at those locations which have not been sampled. This process is known as 'kriging'. The basic technique 'ordinary kriging' uses a weighted average of neighboring samples to estimate the 'unknown' value at a given location. Weights are optimized using the semi-variogram model, the locations of the samples, and all the relevant interrelationships between known and unknown values. The technique also provides a 'standard error' which may be used to calculate confidence levels. In mining, geo-statistics is extensively used in the field of mineral resource and reserve valuation, e.g. the estimation of grades and other parameters from a relatively small set of boreholes or other samples. Geo-statistics is now widely used in geological and geographical applications. However, the techniques are also used in such diverse fields as hydrology, groundwater, soil salinity mapping, and weather prediction. The application of geo-statistical techniques, such as ordinary- and co-kriging, have been applied to salinity survey data in an effort to represent more accurately the spatial distribution of soil salinity (Boivin et al. 1988; Vaughan et al. 1995).

In order to develop soil salinity maps, two approaches: Kriging and Inverse distance weighted (IDW) can be used, as explained below (Source: www.esri.com).

#### 16.5.1 Kriging

Kriging is an advanced geo-statistical procedure that generates an estimated surface from a scattered set of points with z-values (elevation) (Fig. 1.7). Kriging assumes that the distance or direction between sample points reflects a spatial correlation that

Fig. 1.7 Kriging method to generate salinity map

can be used to explain variation in the surface. The Kriging tool fits a mathematical function to a specified number of points, or to all points within a specified radius, in order to determine the output value for each location. As noted above, Kriging is a multistep process; it includes exploratory statistical analysis of the data, variogram modeling, creating the surface, and (optionally) exploring a variance surface. Kriging is most appropriate when you know there is a spatially correlated distance or directional bias in the data. It is often used in soil science and geology.

#### 16.5.2 Inverse Distance Weighted (IDW) Interpolation

Inverse distance weighted interpolation determines cell values using a linearly weighted combination of a set of sample points (Fig. 1.8). The weight is a function of inverse distance. The surface being interpolated should be that of a locationdependent variable. This method assumes that the variable being mapped decreases in influence with distance from its sampled location.

Fig. 1.8 Inverse distance weighted (IDW) method to generate salinity map

# 16.6 Morphological Methods

Morphological details of salt minerals can be obtained at four levels, macro-, meso-, micro- and sub-microscopic levels, as described below:

### 16.6.1 Macromorphology

Macromorphology can be established during field investigation on a broad scale (Plate 1.8 upper left), and features of interest can be captured with digital camera (still or video). Such investigations quickly provide useful information about the area of interest.

#### 16.6.2 Mesomorphology

Mesomorphological observations are made when the naked eye cannot resolve the details of the features of interest (Plate 1.8 upper right). The naked eye is then aided with hand lens or binocular microscope at lower magnifications. Such observations can be made in the field (hand lens) or in laboratory using binocular low magnification microscopes.

#### 16.6.3 Micromorphology

Micromorphology optical microscopy: When optical aid is needed for the naked eye to resolve details at higher magnifications, for example, in soil thin sections as studied with a polarizing microscope (Plate 1.8 bottom left); it is considered as an extension to field morphological studies. Soil thin sections are prepared by impregnating the saline soils with resin and hardened. After the resin has fully polymerized, the impregnated samples are cut into slices with diamond saw. The impregnated block is washed with petroleum spirit in an ultrasonic bath. One face of the impregnated soil block is then ground and polished successively with 6 and 3 μm diamond paste using Hyprez fluid as lubricant on polishing and lapping machine. The polished block is stuck to glass slide using resin and then ground to 25 μm thicknesses using lapping and polishing machine (Shahid 1988).

Micromorphology submicroscopy: Is the resolution of details at the submicroscopic level (Plate 1.8 bottom right) using an electron microscope (scanning and transmission). For submicroscopic investigation, the base of the sample and the surface of the stub are painted with a silver colloid suspension by a brush and then sample is mounted onto the stub using normal araldite. The prepared mounted samples are either carbon coated under vacuum or coated with a layer of goldpalladium (20–30 nm thick) to prevent charge buildup on the specimen, and to hold the surface of the sample and a constant electric potential. The prepared samples are then studied using the scanning electron microscopy.

The above details can be obtained as required. The microscopy techniques can also be combined. For example, Bisdom (1980) coupled optical microscopy with submicroscopic methods and later with contact microradiography (Drees and Wilding 1983). The electron microscopes can be supplemented with a micro-chemical analyzer (Energy Dispersive X Ray Analyses – EDXRA, Wavelength Dispersive X Ray Analyses – WDXRA), thereby allowing in-situ micro-chemical analyses (non-destructive). To study salt crusts at the submicroscopic level, soil samples need coating with gold-palladium (an alloy), or carbon applied to salt crusts is needed to make the sample a conductor. The use of submicroscopic techniques has widely been used in salt minerals studies, such as in Turkey (Vergouwen 1981) and Pakistan (Shahid 1988; Shahid et al. 1990; Shahid et al. 1992; Shahid and Jenkins 1994; Shahid 2013) (Plate 1.8).

Plate 1.8 Salt features at different observations scales (cf. Shahid 1988; 2013). (a) Macroscopic features of soil salinity – naked eye observation (salt crust) (Width 20 meters), (b) Mesomorphological features – binocular observation of nahcolite (NaHCO3) mineral (Width 1100 μm), (c) Micromorphological feature – optical microscopic observation of thenardite (Na2SO4) mineral in thin section (Width 1000 μm), (d) Micromorphological feature – submicroscopic observation of lath shaped glauberite [(Na2Ca (SO4)2] mineral – scanning electron microscopy (Width 64 μm)

# References

The opinions expressed in this chapter are those of the author(s) and do not necessarily reflect the views of the IAEA, its Board of Directors, or the countries they represent.

Open Access This chapter is licensed under the terms of the Creative Commons Attribution 3.0 IGO license (https://creativecommons.org/licenses/by/3.0/igo/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the IAEA, provide a link to the Creative Commons license and indicate if changes were made.

The use of the IAEA's name for any purpose other than for attribution, and the use of the IAEA's logo, shall be subject to a separate written license agreement between the IAEA and the user and is not authorized as part of this CC-IGO license. Note that the link provided above includes additional terms and conditions of the license.

The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

# Chapter 2 Soil Salinity: Historical Perspectives and a World Overview of the Problem

Shabbir A. Shahid, Mohammad Zaman, and Lee Heng

Abstract Soil salinity is not a recent phenomenon, it has been reported since centuries where humanity and salinity have lived one aside the other. A good example is from Mesopotamia where the early civilizations first flourished and then failed due to human-induced salinization. A publication 'Salt and silt in ancient Mesopotamian agriculture' highlights the history of salinization in Mesopotamia where three episodes (earliest and most serious one affected Southern Iraq from 2400 BC until at least 1700 BC, a milder episode in Central Iraq occurred between 1200 and 900 BC, and the east of Baghdad, became salinized after 1200 AD) have been reported. There are reports clearly revealing that 'many societies based on irrigated agriculture have failed', e.g. Mesopotamia and the Viru valley of Peru. The flooding, over-irrigation, seepage, silting, and a rising water table have been reported the main causes of soil salinization. Recent statistics of global extent of soil salinization do not exist, however, various scientists reported extent differently based on different data sources, such as there have been reports like, 10% of the total arable land as being affected by salinity and sodicity, one billion hectares are covered with saline and/or sodic soils, and between 25% and 30% of irrigated lands are salt-affected and essentially commercially unproductive, global distribution of salt-affected soils are 954 million ha, FAO in 1988 presented 932 million ha salt-affected soils, of almost 1500 million ha of dryland agriculture, 32 million ha are salt-affected. Precise information on the recent estimates of global extent of salt-affected soils do not exist, many countries have assessed their soils and soil salinization at the national level, such as Kuwait, United Arab Emirates, Middle East, and Australia etc. Considering the current extent of salt-affected soils the cost of salt-induced land degradation in 2013 was \$441 per hectare, a simple benefit transfer suggests the current annual economic losses could be \$27 billion.

Keywords Historical perspective · Mesopotamia · Iraq · Global extent · Economic losses · Viru valley

# 1 Introduction

Soil salinity is a major global issue owing to its adverse impact on agricultural productivity and sustainability. Salinity problems occur under all climatic conditions and can result from both natural and human-induced actions. Generally speaking, saline soils occur in arid and semi-arid regions where rainfall is insufficient to meet the water requirements of the crops, and leach mineral salts out of the root-zone. The association between humans and salinity has existed for centuries and historical records show that many civilizations have failed due to increases in the salinity of agricultural fields, the most known example being Mesopotamia (now Iraq). Soil salinity undermines the resource base by decreasing soil quality and can occur due to natural causes or from misuse and mismanagement to an extent which jeopardizes the integrity of soil's self-regulatory capacity.

Soil salinity is dynamic and spreading globally in over 100 countries; no continent is completely free from salinity (Fig. 2.1). Soil salinization is projected to increase in future climate change scenarios due to sea level rise and impact on coastal areas, and the rise in temperature that will inevitably lead to increase evaporation and further salinization. Salinization of soils can affect ecosystems to an extent where they no longer can provide 'environmental services' to their full potential. It is realized that recent estimates of the global extent of soil salinization do not exist. But it can be assumed that, since the earlier data gathering in the 1970s and 1980s, salinization has expanded as newly affected areas most probably exceed the areas restored through reclamation and rehabilitation. There is a long list of countries where salt-induced land degradation occurs. Some well-known regions where salinization is extensively reported include the Aral Sea Basin (Amu-Darya and Syr-Darya River Basins) in Central Asia, the Indo-Gangetic Basin in India, the

Fig. 2.1 World map representing countries with salinity problems. (https://www.researchgate.net/ publication/262495450)

Indus Basin in Pakistan, the Yellow River Basin in China, the Euphrates Basin in Syria and Iraq, the Murray-Darling Basin in Australia, and the San Joaquin Valley in the United States (Qadir et al. 2014).

# 2 Soil Salinity – A Historical and Contemporary Perspective

For centuries, humanity and salinity have lived one aside the other. There is good evidence for Mesopotamia that early civilizations flourished and then failed due to human-induced salinization. Jacobson and Adams (1958), in their publication, 'Salt and silt in ancient Mesopotamian agriculture' highlighted the history of salinization in Mesopotamia. Ancient records show three episodes of soil salinization in Iraq. The earliest and the most serious one affected Southern Iraq from 2400 BC until at least 1700 BC. A milder episode in Central Iraq occurred between1200 BC and 900 BC, and there is archeological evidence that the Nahrwan area, east of Baghdad, became salinized after 1200 AD. Flooding, over-irrigation, seepage, silting, and a rising water-table are considered to be the main reasons for these episodes of increased salinization (Gelburd 1985).

In southern Iraq in 3500 BC, both wheat and barley were equally important cultivated crops, though after 100 years wheat had slipped to one sixth, and by 2100 BC, its cultivation had become almost insignificant, dropping to only 2%. By 1700 BC, wheat cultivation was completely phased out. Historical records show that concurrent with the shift to barley cultivation, there was an appreciable and serious decline in soil fertility and gradual declines in barley yields, which for the most part can be attributed to salinization (Jacobson and Adams 1958). Thus, after almost 5000 years of successful irrigated agriculture, the Sumerian civilization failed. In the Indus plains of Pakistan and India, the practice of irrigation began about 2000 years ago during the Harapa civilization, but it is only recently that serious problems of salinity and sodicity are being encountered. In the Viru valley of Peru, irrigated agriculture began between 800 BC and 30 AD (Wiley 1953), and by 800 AD, the population was at a peak. Then from 1200 AD, the population declined appreciably and the residents of the once densely populated Viru valley bottom relocated to more narrow upper reaches of the valley. The historians partly attribute this relocation to increased salinity and a rising water- table, together with inadequate soil drainage (Armillas 1961). Tanji (1990) draws a historical perspective of irrigation-induced salinity in several regions. In a similar perspective, general remarks by Wiley (1953) clearly reveal that 'many societies based on irrigated agriculture have failed', e.g. Mesopotamia and the Viru valley of Peru.

# 3 An Overview of Salinity Problem

The earth's land surface is 13.2 109 ha, but only 7 109 ha of this is arable, of which only 1.5 109 ha is currently cultivated (Massoud 1981). Of the cultivated lands, about 0.34 109 ha (23%) are saline and 0.56 109 ha (37%) are sodic. Older estimates (Szabolcs 1989) described 10% of the total arable land as being affected by salinity and sodicity, with the effects extending to over 100 countries in all continents. One billion hectares of the 13.2 109 hectares of the land is, thus, covered with saline and/or sodic soils, and between 25% and 30% of irrigated lands are salt-affected and essentially commercially unproductive.

The countries affected by salinization are predominantly located in arid and semiarid regions, where continued irrigation with low quality groundwater has taken place. Low rainfall has also contributed to the expansion of salt-affected soils. The largest area of the world's saline soils occurs in the arid and semi-arid regions (Massoud 1974; Ponnamperuma 1984), where evapotranspiration exceeds precipitation. The rapid conversion into barren land through salinity/sodicity has negatively affected the environment and has substantially altered natural resources in a number of countries. Worldwide, some ten million hectares of irrigated land is abandoned annually because of salinization, sodication and waterlogging (Szabolcs 1989). These degraded soils occur principally in the hot arid and semi-arid regions, although they have also been recorded in Polar Regions (Buringh 1979).

Global statistics on salt-affected soils vary according to different data sources. Saline soils occupied more than 20% of the world's irrigated area by the mid-1990s (Ghassemi et al. 1995). Since then, the extent of salinity has likely increased and, in some countries, salt-affected soils occur on more than half of the irrigated lands (Metternicht and Zinck 2003). Kovda and Szabolcs (1979) reported global distribution of salt-affected soils as 954 million ha. Data summarized from Szabolcs (1974) for Europe and Massoud (1977) for the other continents, as reported by Abrol et al. (1988) in FAO Soils Bulletin 39, presents 932.2 million ha of salt-affected soils (Table 2.1). Of almost 1500 million ha of dryland agriculture, 32 million ha are saltaffected (FAO 2000). Although recent estimates of global extent of salt-affected soils do not exist, many countries have assessed their soils and soil salinization levels at the national level, such as Kuwait (Shahid et al. 2002), United Arab Emirates

Source: Abrol et al. (1988) in FAO Soil Bulletin 39; Summary of data for Europe (Szabolcs 1974) and for other continents (Massoud 1977)

Table 2.1 Worldwide distribution of salt-affected areas (Million ha)

(EAD 2009, 2012), Middle East (Hussein 2001; Shahid et al. 2010), and Australia (Oldeman et al. 1991).

In a comprehensive overview of the global identification of salinity problems and the global salinity status, Shahid (2013) reported that about 200 106 hectares of land is affected by salinity in Southwest USA and Mexico. In Spain, Portugal, Greece, and Italy, saltwater intrusion into aquifers is appreciable; in Spain more than 20% of the land area is desert, or is seriously degraded and, thus, nonproductive.

In the Middle East, 20 106 hectares are affected by increased soil salinity, the reasons being poor irrigation practices, high evaporation rates, growth of sabkhas (salt scalds), and an increase in groundwater salinity. In addition, productivity of the irrigated lands of the Euphrates basin (Syria, Iraq) is seriously constrained by salinity. In Iran, 14.2% of the total land area is salt-affected (Pazira 1999). In Egypt, 1 106 hectares of land which could be cultivated along the Nile is saltaffected. Salt accumulation in the Jordan River basin adversely affects agricultural production in Syria and Jordan. In Africa, 80 106 hectares is saline, sodic, or saline-sodic, of which the Sahel, in West Africa, is the most affected.

In Asia, 20% of India's cultivable land, mainly in Rajasthan, coastal Gujarat, and the Indo-Gangetic plains, is affected by salinity or sodicity. In Pakistan, 10 106 hectares is affected and about 5–10 hectares per hour is lost to salinity and/or waterlogging in coastal regions and in the irrigated Indus basin. In Bangladesh, 3 106 hectares is unproductive due to salinity. In Thailand, 3.58 106 hectares is salt-affected (3.0 106 hectares being inland and 0.58 106 hectares being coastal saline soils). In China, 26 106 hectares of their total land area is salt-affected (Inner Mongolia, the Yellow River basin and tidal coastal regions), while in Australia the extent of saline soils is 357 106 hectares.

The global extent and distribution of 76.6 million hectares of human-induced saltaffected soils (Oldeman et al. 1991) and a similar distribution for irrigated lands affected by secondary salinization (Ghassemi et al. 1995) are presented in Tables 2.2 and 2.3. These soils are distributed in desert and semi-desert regions, frequently occurring in fertile alluvial plains, river valleys, coastal areas and in irrigation districts. The countries where significant salinity problems exist include, but are not limited, to Australia, China, Egypt, India, Iran, Iraq, Mexico, Pakistan, the

Table 2.2 Global extent of human-induced salinization (Oldeman et al. 1991; Mashali 1995)

Table 2.3 Global estimates of secondary salinization in the world's irrigated lands. (Summarized from Ghassemi et al. 1995; Mashali 1995)

a Salt-affected soils within the irrigated area; values in parentheses are percentage

USSR, Syria, Turkey, and the United States. In Gulf States (Bahrain, Kuwait, Saudi Arabia, Qatar, Oman, and the United Arab Emirates), saline soils mainly occur in coastal lands (due to seawater intrusion), and also on agricultural farms irrigated with saline/brackish water.

Secondary salinization (i.e., soil salinization due to human activities such as irrigated agriculture) is predominantly located in the arid and semi-arid regions including Egypt, Iran, Iraq, India, China, Chile, Argentina, Commonwealth of Independent States, Spain, Thailand, Pakistan, Syria, Turkey, Algeria, Tunisia, Sudan and the Gulf States. About 76.6 million hectares (Table 2.2) of cultivated lands are salt-affected by human-induced processes (Oldeman et al. 1991; Mashali 1995; Ghassemi et al. 1995) and approximately 30 million ha can be attributed to secondary salinization of non-irrigated lands. However, according to Ghassemi et al. (1995), globally 20% or 45 million hectares out of a total 227 million hectares of irrigated land are salt-affected (Table 2.3).

# 4 Distribution of Salinity in Drylands in Different Continents of the World

As reported by UNEP (1992), the distribution of salt-affected soils in drylands in different continents is presented in Table 2.4. These soils are divided into two categories: saline (412 million hectares) and sodic (618 million hectares), totaling 1030 million hectares. Australasia has the widest distribution with 357.6 million hectares, followed by Africa with 209.6 million hectares.

Table 2.4 Salt-affected soils in drylands by continents (UNEP 1992; cf FAO-ITPS-GSP 2015)

# 5 Irrigation Practices and Soil Salinization

The practice of irrigation, if not planned and managed properly, can result in increased soil salinization. An estimate (Postel 1989) shows that about 25% of the world's irrigated lands are damaged by salinity, while Adams and Hughes (1990) have reported that up to 50% of irrigated lands are affected by salt. Szabolcs (1989) states that no continent is free from salt-affected soils and serious salt-related problems occur in at least 70 countries. Table 2.5 shows the area of irrigated land damaged by salinization for the five worst-affected countries (Postel 1989).

# 6 Regional Overview of Salinity Problem

More recent estimates of the regional distribution of saline soils do not exist. There is a need to update this information in order to better understand the extent of the problem and to develop soil use and management policies. Such estimates are essential given the continuing decline of soil resources for food production. An earlier search of the literature (Mashali 1995; FAO-Unesco Soil map of the world

Table 2.6 Regional distribution of salt-affected soils (mha). (cf. Mashali 1995)

1974) does, however, give an estimate of the extent of the regional distribution of salt-affected soils (Table 2.6). These estimates show the total extent to be 932 million hectares of salt-affected lands, with the maximum area occurring in the region of Australasia (357 million ha).

# 7 Extent of Soil Salinity in the Middle East

Information regarding the extent of salinization in the Middle East is very limited. However, some general information has been obtained through the use of Remote Sensing imagery and other methods. This information was used to develop a soil salinization map of the Middle East (Hussein 2001; Shahid et al. 2010). In this map, salinization was divided into four general categories: slight, moderate, severe and very severe, as shown in Table 2.7. Earlier, an estimated area of 209,000 hectares has been reported as being salinized in Kuwait (Hamdallah 1997), which is roughly 3% of the total Kuwait land area.

Table 2.7 shows an area of 11.2% of the Middle East being affected to varying levels by soil salinization. Realizing the soil salinity, a hazard to agriculture and to the ecosystem services, Shahid et al. (1998) described soil salinization as early warning of land degradation in Kuwait. Later, Shahid et al. (2002) interpreted the soil survey data (KISR 1999) using GIS and mapped soil salinity into different salinity zones, where area occupied by each zone is as: 4.1 – 10 dS m1 (0.685%), 10.1 – 25.0 dS m1 (4.37%), and more than 25 dS m1 (7.06%). This concludes an area of about 12.1% affected to varying degrees of salinity in the entire state of Kuwait. In the Abu Dhabi Emirate (EAD 2009), an area of 35.5% (2,034,000 ha) has been depicted to be affected to varying degrees of soil salinity. The highly saline soils on the soil salinity map are confined to the coastal land (King et al. 2013), the areas of deflation plains, and inland sabkha (salt scald) where the groundwater levels approach the surface, creating large areas of aquisalids at the great group level of US soil taxonomy (Soil Survey Staff 2014; Shahid et al. 2014).

# 8 Socioeconomic Aspects of Soil Salinization

A comprehensive review of published literature revealed very few publications dealing with socioeconomic aspects of salt-induced land degradation. On the global level, generation of such information requires appreciable resources and the commitment of properly trained staff to the project. However, Qadir et al. (2014) conclude that previous studies show a limited number of highly variable estimates of the costs of salt-induced land degradation. Even so, they have made simple extrapolations from these studies and the estimates show that the global annual cost of salt-induced land degradation in irrigated areas could be US\$ 27.3 billion in lost crop production. Based on these estimates, Qadir et al. (2014) recommended investing in the remediation of salt-affected lands and noted that remediation costs must be included in a broader national strategy for food security, and defined in national action plans.

Qadir et al. (2014) identified countries where such economic cost on salt-induced soil degradation has been reported, including but not necessarily limited to Australia, India, the United States, Iraq, Pakistan, Kazakhstan, Uzbekistan, and Spain. They further indicated that the valuation of the cost of salt-induced land degradation has been mainly based on estimates of crop production losses. However, it is unclear whether their comparisons are made with crop production values taken from land not affected by salinity.

Taking the above examples into account, Qadir et al. (2014) have concluded that, considering the current extent of global irrigated area 310 million hectares (FAO-AQUASTAT 2013) and 20% of this area as salt-affected (62 million hectares), and the inflation-adjusted cost of salt-induced land degradation in 2013 as US\$ 441 per hectare, a simple benefit transfer suggests the current annual economic losses could be US\$ 27.3 billion.

# References

The opinions expressed in this chapter are those of the author(s) and do not necessarily reflect the views of the IAEA, its Board of Directors, or the countries they represent.

Open Access This chapter is licensed under the terms of the Creative Commons Attribution 3.0 IGO license (https://creativecommons.org/licenses/by/3.0/igo/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the IAEA, provide a link to the Creative Commons license and indicate if changes were made.

The use of the IAEA's name for any purpose other than for attribution, and the use of the IAEA's logo, shall be subject to a separate written license agreement between the IAEA and the user and is not authorized as part of this CC-IGO license. Note that the link provided above includes additional terms and conditions of the license.

The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

# Chapter 3 Salinity and Sodicity Adaptation and Mitigation Options

#### Shabbir A. Shahid, Mohammad Zaman, and Lee Heng

Abstract Soil salinity and sodicity are twin constraints to agriculture production in many countries causing significant losses of crop production and land degradation. Once the salinity and sodicity problems are properly diagnosed, an integrated soil reclamation program may be formulated including combination of physical, chemical, hydrological and biological methods to rectify the twin problems. A combination of adaptation and mitigation technologies are to be adopted, for example adaptation allows the continued use of salt-affected soils by adjusting in response to the degree by which salinity and sodicity development has affected the soil, whereas, in contrast, mitigation refers to the technologies which are adopted to stop salinization to occur. It should be remembered that there is no single universal mitigation technology suitable for all soils, however, diagnostic based recommendations work satisfactorily for a specific site or location. Prior to setting up soil reclamation plan it is essential to review the available resources (farmer budget, availability and quality of water) and the objectives of reclamation and the reclamation plan established suiting the specific farmer needs. In this chapter, various soil reclamation methods such as; physical-leveling, subsoiling, mixing sand, seed bed preparation and salts scrapping); chemical (use of gypsum based on gypsum requirement, sulfur, acids etc.), hydrological-selection of suitable irrigation system-drip, sprinkler, bubbler, furrow, using the concept of leaching requiring/fraction to manage rootzone salinity, flushing, drainage, blending of water etc.; biological (use of organic amendments, green manuring, farm yard manures and selection of salttolerant crops) have be described. In addition, various methods of screening crops against salinity including hydroponics, field screening and serial biological concentration approach are described. Climate Smart Agriculture practices, integrated soil fertility management using 4 R nutrient stewardship are concisely reported. Procedures of salt-harvesting from saline lands and deep deposits and their commercial exploitation in industries are also introduced.

Keywords Salinity · Sodicity · Adaptation · Mitigation · Soil reclamation · Leaching requirement · Salt harvesting

# 1 Introduction

Soils affected by salinity and sodicity are not confined to just arid and semi-arid regions, where rainfall is insufficient to leach salts from the soil. Saline and sodic soils have been recorded in a wide range of environments under many different hydrological and physiographic conditions. Such a wide distribution tells us that 'no single adaptation or mitigation option or technique will be applicable to all land areas'. However, diagnostics-based, site-specific recommendations can be viable options. Most importantly, they suggest the formulation of an integrated reclamation and management plan, one which is based on the major constraints facing us – available resources and variable environmental conditions.

First, one needs an integrated system of soil reclamation and management techniques, and these can generally be grouped into four adaptation or mitigation approaches to deal with salt-affected soils. The four approaches are: (i) Hydrological, (ii) Physical, (iii) Chemical, and (iv) Biological, though under unique environmental conditions salt-affected soils may be used for other purposes.

# 2 Mitigation and Adaptation Options

Mitigation and adaptation are terms commonly used in the context of climate change and many scientists believe that mitigation is primarily concerned with emission of greenhouse gases (GHGs), while adaptation deals with water and agriculture. However, mitigation and adaptation also apply to how mankind must deal with saltaffected soils. In this chapter, we have defined both terms in the context of saltaffected soils and their reclamation and management.

Adaptation allows the continued use of salt-affected soils by making adjustments in response to the degree by which salinity and sodicity development has affected the soil. In contrast, mitigation refers to the technologies which are adopted to stop salinization to occur.

# 3 Diagnostics of the Soil Salinity Problem

Soil salinity in agricultural fields is increasing worldwide, mainly due to poor farm management practices and the increasing demand for intensification of agriculture for the short-term benefits of increased food production. This intensification ignores the long-term consequences on 'other services' provided by the soil. It is, therefore, very important to understand the salinity hazard, both spatially and temporally at the regional and national farm levels.

In agricultural fields, an effective measurement of salinity will identify the location and extent of root-zone salinity and ensure that root-zone salinity is kept below the threshold level for each crop. Soil salinity is dynamic and has a wide variation vertically, horizontally, and temporally. Many consider soil salinity a uniform feature in a soil profile. However, Shahid et al. (2009) showed that, for the saline-sodic soils of Pakistan, salinity was a layered feature as one moved down the profile.

At the regional (Middle East) and national (Kuwait, United Arab Emirates) levels, a salinity mapping program (Shahid et al. 2010) helped policy makers in taking necessary and timely actions to tackle the issue of increased soil salinity. Similar programs will help to avoid a further spread of soil salinity to new regions; and they, if prove successful, will prevent negative impacts on national economies through degrading a nation's soil resources.

# 4 Integrated Soil Reclamation Program (ISRP)

Salt-affected soils are distributed across a wide range of hydrological and physiographical conditions, soil types, rainfall and irrigation regimes, as well as different socioeconomic settings. This diversity makes one realize that there will be no single technique of soil reclamation applicable to all areas. The exploitation of these soils for agriculture will require an integrated reclamation and management plan based on a comprehensive investigation of soil characteristics, including water monitoring (rainfall, irrigation and soil water-table), a survey of crops and local conditions, including climate, the economic, social, political, and cultural environment, as well as the existing farming systems. Fortunately, several approaches can be combined into an integrated system of soil reclamation and management (Shahid and Alshankiti 2013).

# 4.1 Objectives of Salinity Reclamation

The main objectives of reclamation are to:

# 4.2 Prerequisite for Soil Reclamation

A soil reclamation plan can only be implemented if certain prerequisites are fulfilled. Some of these are essential for efficient, effective and long-term reclamation of saltaffected soils, as listed below.

Sustainable agriculture with salt-affected lands is most likely to be achieved through integrated soil reclamation program (ISRP) and natural resource management (NRM). These are approaches where the long-term condition of the resource is built in as a core consideration. Thus, crop production on salt-affected lands can only be successful if the soil is dealt with in a holistic manner, i.e. in an 'integrated approach' which includes all aspects of soil, water, plants and climatic conditions. There is, unfortunately, a misconception about Biosaline Agriculture, that this is a complete solution for using salt-affected lands and saline and saline-sodic waters. Rather, biosaline agriculture is just one of the components of ISRP, which also includes physical, chemical, hydrological and biological methods (Box. 3.1).

#### Box 3.1 – Integrated Soil Reclamation Program (ISRP)

It should be noted that no single approach can deliver a complete solution to fix/reclaim soil salinity problem. This means that we need to take holistic approach by using a combination of mitigation approaches, which could be site-specific, and should only be used in other areas where similar soils and environmental conditions exist. International Center for Biosaline Agriculture (ICBA), Dubai, UAE has sufficient expertise in the diagnostics of the problem, developing an integrated reclamation strategy and implementation of this strategy to transform marginal soils to good quality for crop production, using methods, such as physical (leveling, salt scraping, tillage, subsoiling and sanding); chemical (use of soil amendments such as elemental S, acids, gypsum, etc. based on gypsum requirements to rectify soil sodicity problems and to improve soil health); hydrological (irrigation systems: Surface, flood, basin, drip, sprinkler, subsurface irrigation, etc., and leaching and drainage), and biological (biosaline agriculture: Salt tolerant crops, and a serial biological concentration approach). (Adapted from Shahid and Rahman (2011) and Shahid et al. (2011))

It should be further noted that the exploitation of salt tolerant crops and saline waters, i.e. 'Biosaline Agriculture', without adopting other components of integrated soil reclamation, will ultimately degrade the soils further. These degraded soils will likely be unable to provide essential soil services, as such, but not limited to, agricultural production.

As discussed briefly above, recently established worldwide strategies for soil reclamation can be grouped into the following approaches which cover almost all aspects of soil reclamation and management.

# 4.3 Physical Methods of Soil Reclamation

There are several physical methods of soil reclamation, though not all of these methods are required for a given situation. Site-specific diagnostics can allow one to select the most suitable method(s). The most commonly used physical methods of soil reclamation are listed below.

#### 4.3.1 Leveling

Leveling of salt-affected lands prior to the implementation of a reclamation program is essential. This allows for a uniform water distribution, leading to effective leaching of salts. Unfortunately, the farmer usually accomplishes this task by plowing the field, followed by the use of a conventional planking tool. This practice usually leaves the land uneven, which means that when water is applied to the field in order to leach the salts, the water puddles in the depressions and heterogeneous conditions are formed. There exists a modern tool 'laser land leveling' to level the land in highly effective manner. It is, thus, suggested that each farmer should contact the extension services department in order to access this modern tool, which will facilitate an effective initiation of the farmer's soil reclamation program. The leveling process, however, may compact the soil due to the use of heavy machinery. If this occurs, the leveling process should be followed by subsoiling or chiseling.

#### 4.3.2 Subsoiling

The soils affected by sodicity are usually underlain by dense clay-sodic layer(s). These dense layers are created by the dispersion of clay particles in the highly sodic water. The dispersed clay particles move to the subsurface of the soil where they are lodged on the surfaces of the conducting soil pores, thus, blocking the pores and preventing further water movement. It is particularly important, then, to disrupt the dense layers deep in the soil in order to enhance permeability. This is especially important for reclaiming sodic soils after the addition of amendments such as gypsum, followed by watering the field. The addition of gypsum will enhance the removal of exchangeable sodium (which has already been exchanged by Ca2+) into lower layers of the soil prior to finally moving into the main drainage system. In addition to dense sodic layers, the soils may be underlain by a plow layer or by other hard pans occurring during soil formation. These hard pans must be disrupted and broken in order to enhance drainage capacity, and to facilitate the soil reclamation process.

#### 4.3.3 Sanding

If the soil to be reclaimed has heavy texture (i.e., a clay soil), the mixing of sand in an appropriate quantity can change the soil texture permanently; the soil becomes more permeable and is easier to reclaim. This practice also provides a favorable environment for plant growth compared to the original soil prior to sanding. Changing the soil texture is a difficult and costly task, though where sand is readily available, such as in a desert, this practice can be accomplished more easily.

A clay soil is considered to exist in an area when it has the percentages of primary soil particles, as: 10% sand, 20% silt, and 70% clay (Textural class: Clay). The clay soil is mixed with a known quantity of sand to develop the following percentages of soil particles: 60% sand, 15% silt, and 25% clay (Textural class: Sandy clay loam).

In this way, the original soil texture (Clay) is significantly changed to another soil texture (Sandy clay loam). Both soil textural classes can be located on the USDA soil textural triangle (Fig. 3.1).

The newly established texture, a sandy clay loam, has improved soil physical properties, e.g. an increased drainage capacity and infiltration rate. This leads to an enhanced soil reclamation process and results in a much better leaching of salts.

#### 4.3.4 Scraping

Salt accumulation in an irrigated field is common. Salts accumulate at certain zones based on the irrigation system in use and soil bed shape. The readers are referred to Chap. 4 of this book for more information in this respect. To clarify the scraping practice in order to remove salts, a furrow irrigation system is selected (Plate 3.1);

Fig. 3.1 USDA soil textural classes

Plate 3.1 Salts buildup in furrow irrigation system

both furrows are irrigated and the zone of salt accumulation appears in the center of the ridge. The salt crusts which accumulate at the surface, through capillary rise and subsequent evaporation, can be removed manually or through mechanical means. The mechanical removal is the simplest and the most economical way of reclaiming saline soils, if the area is small. This practice minimizes the salts temporarily; they will be accumulated again if there is a continuous feed of saline water to the soil surface. Therefore, the scraping method is considered a temporary solution.

#### 4.3.5 Seed Bed Preparation – Tillage

Tillage can improve the soil's physical condition, and can bring a saline layer to the surface. However, it can also create a plow layer through the continuous use of the plow. Care must, thus, be exercised in using tillage. By manipulating the soil surface into different shapes and the selection of a specific irrigation system, a zone of low salinity can be achieved. It is well recognized that salts tend to accumulate on the ridges away from the wet zone when furrow irrigation is adopted. Placing the seeds on the off-center slope (i.e., shoulder) of the single row will position the seed in a location with a minimum salinity and an optimum moisture condition. Under high salinity conditions, the alternate row should be left un-irrigated. This will ensure maximum accumulation of salts in the un-irrigated area, thereby leaving the irrigated furrows free of salts and fit for planting seeds. The readers are referred to Chap. 4 for a detailed account.

# 4.4 Chemical Methods of Soil Reclamation

It should be kept in mind that chemical methods of reclamation are commonly used to reclaim sodic or saline-sodic soils. Saline soils are unable to be reclaimed by chemical methods. Chemical reclamation includes the use of gypsum, elemental sulfur and acids (hydrochloric and sulfuric acids), and the methods used are based on the diagnostics of the problem. Sodic soil can be recognized through visual assessment in the field, or through analyzing soils in the laboratory for exchangeable sodium percentage (ESP). A soil with ESP > 15 is classed as a sodic soil (USSL Staff 1954). At this threshold ESP value, the soil will show effects on both soil physical properties (structural damage) and also on plant growth. In such soils, the objective is to bring the soil ESP below the threshold value. This can be achieved by adding suitable amendments to increase the concentration of calcium ions (Ca2+) in the soil.

#### 4.4.1 Use of Gypsum to Reclaim Sodic Soil

The most suitable method is to replace exchangeable sodium with calcium, and the subsequent use of organic matter to bind the soil and improve its structure. Gypsum (CaSO4.2H2O) and lime (CaO) can both add calcium and, thus, can overcome dispersion as the calcium causes the inter particle forces to more readily hold the particles together. The calcium causes particles to form clusters (flocculates), forming a very clear puddle of water. Gypsum usually gives an immediate response as it dissolves (although it has low solubility) in water, though it leaches sooner than lime. Amendments are those materials which supply Ca2+ for the replacement of exchangeable sodium and furnish calcium indirectly by dissolving calcite (CaCO3), which is naturally occurring in many arid zone soils. Gypsum is reported to reduce the levels of exchangeable sodium in the soil. It also improves both soil tilth and drainage, and achieves better crop production.

The addition of gypsum to a soil changes the soil chemistry in two ways: i) by increasing the amount of salt which is in solution, thereby avoiding the swelling and dispersion of the clay component. This is a short-term effect which occurs as the gypsum dissolves, and ii) the calcium from the gypsum replaces the exchangeable sodium which was adsorbed onto the clay at specific sites. This process changes sodic clay to a calcium-clay. The displaced sodium is then leached into lower soil zones, below the plant root-zone. Mined gypsum (less than 2 mm particle size) of commercial grade (~ 70% purity) is commonly used for the reclamation of sodic soils.

The following reaction occurs when gypsum is added to a Na-clay soil.

$$\begin{array}{rcl} \text{2Na}^+ - \text{Clay} + \text{CaSO}\_4.2\text{H}\_2\text{O} \rightarrow \begin{array}{c} \text{Ca}^{2+} - \text{Clay} + \text{Na}\_2\text{SO}\_4^- + 2\text{H}\_2\text{O} \\ \text{Nomal soil} \end{array} \end{array}$$

#### <IP_ADDRESS> Determination of Gypsum Requirement

Gypsum requirement (GR) is the quantity of gypsum needed per acre or per hectare of soil to lower the exchangeable sodium percentage (ESP) of the soil to a desired level.

#### <IP_ADDRESS> How to Determine the Weight of One-Hectare Soil?

Laboratory methods for measuring gypsum requirement of a soil are based on meq 100 g-1 of dry soil. Thus, one needs to convert the GR in meq 100 g-1 to metric tons (weight) of soil present in a hectare at either 15 or 30 cm depth. The area covered by a hectare of land is 10,000 m2 (100 m 100 m). The weight of one-hectare soil of varying depths (15 or 30 cm) can be determined by the following procedure.

Bulk density ¼ Mass of the soil gð Þ Volume of bulk soil collected cm3

Plate 3.2 Collection of a soil core to measure bulk density of the soil, and cleaning the core with a saw

Bulk volume is, thus, defined as the volume of soil occupied both by mineral matter of the soil and the space between the mineral matter particles.

#### Example

where π ¼ 3.143, r ¼ radius of the cylindrical core in cm; h ¼ height of the core in cm.

¼ 3.143 4 4 5 ¼ 251.44 cm3

## <IP_ADDRESS> Conversion of Gypsum Requirement – Lab Results to Field Application

Normally, the gypsum requirement (GR), determined through lab procedure, is in milli equivalent per 100 g of soil (meq 100 g-1 ), which needs to be calculated as

Plate 3.3 On-site diagnosis of a saline-sodic soil in Pakistan, and sharing the experience with farmers

metric tons per hectare for 15 or 30 cm soil depth for the purpose of field application. The factor for conversion of the gypsum requirement values from meq 100 g-1 to tons per hectare is explained here. Based on the bulk density (1.60 g cm-3 ) of the soil in the above example (UAE sandy soil average bulk density), 4.8 million kilograms weight of soil was determined to be present in one hectare area of soil to a 30 cm depth. Using these figures, a factor of 2.066 has been determined to convert the GR of 1 meq 100 g-1 soil to metric tons per hectare of soil to the 15 cm depth, and a factor of 4.132 to a 30 cm depth.

The USSL Staff (1954) has reported a factor of 0.86 to convert gypsum requirement (GR) from 1 meq 100 g-1 soil to tons per acre at a 15 cm depth. Since one hectare is equal to 2.471 acres, a factor of 2.125 is used in USDA Handbook 60 to convert GR from 1 meq 100 g-1 to tons per hectare at the 15 cm depth, and a factor of 4.250 for tons per hectare to a 30 cm depth.

#### <IP_ADDRESS> A Comparison of Gypsum Requirement Between USSL Staff (1954) and Sandy Soils of United Arab Emirates

Assume we have analyzed five soils (A, B, C, D and E) for their gypsum requirement. These five soils have shown a GR of 1, 2, 3, 4 and 5 meq 100 g-1 , respectively. Calculate the GR of these five soils in terms of tons per hectare to the 15 and 30 cm depths using the above conversion factors. The calculated GR for the five soils is presented in Table 3.1.

It should be noted that the use of the GR conversion factor from the USSL Staff (1954) publication will slightly overestimate the GR (e.g, 4.250 tons per hectare versus 4.132 tons per hectare to the 30 cm depth for soil A). Therefore, it is advisable that countries where the bulk density of the soils is very different to that of most US soils, a new factor should be determined for each of the local soils.

Table 3.1 Gypsum requirement of five soils determined in lab and calculated in tons per hectare using conversion factors

## <IP_ADDRESS> Gypsum Requirement

#### Method 1 (Schoonover 1952)

The procedure is described as follows.

	- 1000 ml of soil solution requires the GR ¼ x meq
	- 100 ml of soil solution requires GR ¼ (x/1000) 100 ¼ x/10 meq
	- Or, 5 g of soil requires GR ¼ x/10 meq
	- 100 g of soil requires GR ¼ x/10 1/5 100 ¼ 2x (Note: Factor of 2 only works when 5 g soil is used in 100 ml GSS, different factors exist for different soil quanity).
	- 1 equivalent of Na+ will require 86.06 g of gypsum.
	- 1 meq of Na+ will require 0.08606 g of gypsum.
	- Weight of one-hectare soil is 4,800,000 kilograms to the 30 cm depth, when bulk density is 1.60 g per cm3 .
	- The GR is, thus, 4.132 metric tons per hectare to the 30 cm depth or 2.066 tons per hectare to the 15 cm depth.
	- 1 hectare ¼ 2.471 acres

In the above procedure, an equivalent amount of soluble CO3 2 and HCO3 are also precipitated; therefore, this method accounts for exchangeable sodium and also for soluble CO3 2 and HCO3 in the soil.

The calculation, however, is based on 100% pure gypsum. Commercial grade gypsum purity is about 70%. Therefore, a factor based on purity must be calculated in order to correct the GR requirement for the commercial grade gypsum.

# Method 2 (USSL Staff 1954, Modified by Shahid and Muhammed 1980)

In this method, the gypsum requirement is calculated based on the exchangeable sodium and cation exchange capacity values taken from the laboratory analyses.

#### Example 3.1

A soil was analyzed in an accredited laboratory and following results were obtained.

Calculate the gypsum requirement (metric tons per hectare for the 30 cm soil depth), keeping in mind that the weight of one-hectare dry soil is 4.8 million kilograms.

From Example 3.1, it was determined that soil with a bulk density of 1.60 g per cm3 , a factor of 4.132 can be used to convert the GR from meq per 100 grams to metric tons per hectare for the 30 cm depth. Therefore, a GR based on the 2.5 meq per 100 grams will be equal to 10.330 metric tons of gypsum per hectare. However, the conversion factor for commercial grade gypsum should be applied to the 10.330 metric tons value.

#### Example 3.2

A soil was analyzed in an accredited laboratory and following results were obtained.

Calculate gypsum requirement (metric tons per hectare for the 30 cm soil depth), keeping in mind that the weight of one-hectare dry soil is 4.8 million kilograms.

From the example above, it was determined that for a soil with a bulk density of 1.60 g per cm3 , a factor of 4.132 can be used to calculate (convert) the GR from meq per 100 grams to metric tons per hectare for the 30 cm depth. Therefore, the GR based on adding the 1.25 meq per100 grams of gypsum will be equal to 5.165 metric tons per hectare.

From above examples, we can conclude that even if two soils have same Exchangeable Sodium Percentage (ESP), the gypsum requirement can be significantly different.

It should be noted that in this procedure of determining GR, an equivalent amount of soluble CO3 2 and HCO3 - (meq 100 g-1 ) must be added (taken into account) in order to properly calculate the total gypsum requirement. This is because a gypsum equivalent to CO3 2 and HCO3 will be precipitated. This modification to USSL Staff (1954) method was made by Shahid and Muhammed (1980).

#### 4.4.2 Use of Acids to Reclaim Calcareous-Sodic Soils

It should be noted that the use of acids is recommended for sodic soils which are also calcareous. The acids react with the calcium carbonates present in soil to mobilize calcium, which ultimately replaces exchangeable sodium in the soil, thereby reducing the ESP. The objectives of acids application are to:

Both sulfuric and hydrochloric acids react rapidly with soil lime, since they do not have to go through an oxidation process. However, they are highly corrosive, and dangerous to handle. Specialized equipment has recently become available to safely apply acid onto field soil, usually with irrigation water. The reaction of applied acids with naturally occurring soil CaCO3 and exchangeable Na+ is shown below.

$$\text{CaCO}\_3 + \text{H}\_2\text{SO}\_4 \rightarrow \text{CaSO}\_4 + \text{CO}\_2 + \text{H}\_2\text{O}$$

$$\text{2Na}^+ - \text{Clay} + \text{CaSO}\_4 \rightarrow \text{Ca}^{2+} - \text{Clay} + \text{Na}\_2\text{SO}\_4 \downarrow$$

$$\text{CaCO}\_3 + 2\text{HCl} \rightarrow \text{CaCl}\_2 + \text{CO}\_2 + \text{H}\_2\text{O}$$

$$\begin{array}{c} \text{2Na}^+ - \text{Clay} + \text{CaCl}\_2 \rightarrow \text{Ca}^{2+} - \text{Clay} + \text{ 2NaCl} \downarrow \\ \text{Sodium soil} \end{array}$$

#### 4.4.3 Use of Elemental Sulfur to Reclaim Calcareous-Sodic Soils

Elemental sulfur can also be used to reclaim calcareous-sodic soils with one condition – the sulfur must be completely oxidized. This occurs through biological

Table 3.2 Equivalent amount of various amendments for supplying Ca in terms of pure gypsum

Adapted from USSL Staff 1954 a

The quantities are based on the use of 100 % pure materials. If the material is impure, necessary corrections must be made. For example, if the gypsum is a 70 % agricultural grade, the equivalent quantity which must be applied will be 1.43 tons

b 100% oxidation is assumed though, in practice, it does not happen Partial source: Ayers and Westcot (1985)

oxidation of sulfur by Thiobacillus thiooxidans, although in sodic soils sulfur oxidation is a very slow process. The complete oxidation of sulfur results in formation of sulfuric acid.

$$\text{2S} + \text{3O}\_2 + \text{2H}\_2\text{O} \to \text{2H}\_2\text{SO}\_4$$

The H2SO4 formed through biological oxidation of sulfur reacts rapidly with lime (as shown above) and proceeds to reclaim the sodic soils.

There is a significant financial outlay required from farmers when using chemical amendments in soil reclamation. It is, therefore, recommended that the benefit of any amendment should be tested first in field trials with regard to its cost, safety in use, and effectiveness in improving (reducing) soil sodicity and increasing crop production. The theoretical amounts of various amendments to supply an amount of Ca equivalent to 1 ton of gypsum are shown in Table 3.2.

A good example is seen from an experiment with vertisol from India; Sharma and Gupta (1986) observed a similar change in ESP (Table 3.3) due to the application of either gypsum or H2SO4. However, they reported that there was a low hydraulic conductivity and higher amounts of water dispersible clay in the case of H2SO4 application. This was caused by the Ca2+ being a stronger flocculent than the H+ ion of Sulfuric acid.

Table 3.3 Effect of different amendments (applied @ 100% gypsum requirement) on physical and chemical properties of a sodic vertisol (Sharma and Gupta 1986)

a Saturated hydraulic conductivity

# 4.5 Hydrological Methods of Soil Reclamation

Hydrological methods generally include irrigation, leaching and flushing, and the drainage of the leached water. Blending of water to reduce its salinity and sodicity, and recycling the water can also be included under this topic.

The objectives of soil reclamation through hydrological methods are:

In irrigated agriculture, the salts in soil can be removed in two ways:

The salt content within the root-zone is likely to be increased if the net downward movement of salts is less than the salt input from irrigation water. Therefore, the soil salt balance must be kept under control. Control of the salt balance is, thus, a function of irrigation water quality, the quantity of dissolved salts in the water, and the success of the soil drainage system.

#### 4.5.1 Leaching

Soils rich in soluble salts can be reclaimed through dissolving of these salts and their successful leaching. This can be accomplished through flooding or ponding of water at the surface for saline soils. In general, the depth of soil leached is roughly equal to the depth of water infiltrated during leaching. In order to leach salts from a soil, an understanding about the leaching requirement (LR) concept is important. The LR is the calculated fraction (depth) or quantity of water that must pass through the root-zone in order to maintain the EC of the drainage water at or below a specified level. Some soil scientists are of the opinion that LR should be minimized to prevent raising the level of the groundwater table, and also to reduce the load placed on the drainage system (Mashali 1995). Recently established guidelines for successful and economic leaching methods are described hereunder.

## <IP_ADDRESS> Timing of Leaching Irrigation