Nutrient losses varied greatly depending on the tillage treatment, the element considered, the season in question and the type of loss that the element was subjected to (erosion, leaching or plant uptake). In general the highest losses were realised through plant uptake which is considered to be good since this is the only positive loss, when compared to the other two types of nutrient losses. There is, no doubt, a depletion in soil fertility, however, if only the harvested products are removed from the field and other crop residues are returned, in the absence of erosion and leaching, fertiliser requirements for the maintenance of soil fertility may be reduced. Erosion and leaching on the other hand are considered negative and should be minimised at all costs. Erosion losses were higher than those incurred with leaching for the conventional tillage, while the reverse was partly true for the conservation tillage systems.

A general overview of the nutrient losses showed that total nitrogen losses (through erosion, leaching and plant uptake) were least under mulch ripping with a range of 27 - 53 kg/ha/yr. followed by tied ridging (range of 25 - 64 kg/ha) and highest under conventional tillage (range of 44 - 87 kg/ha). The same trend was established for phosphorus loss although the amounts lost and the differences between the treatments were minimal (ranges of 7 - 10 kg/ha for CT; 6 - 8 kg/ha for MR and 6 - 10 kg/ha for TR). Potassium losses ranged from 29 - 92 kg/ha under CT; 30 - 43 kg/ha under MR and 22 - 57 kg/ha under TR. Table 8 shows the average losses of different elements found for the different treatments and years.

Table 8. Average total nutrient losses under different tillage treatments during three seasons at Makoholi Contill site

Generally, more nutrients were lost under conventional tillage as compared to the conservation tillage system. Only N losses gave significant treatment differences at P < 0.05. However, when the mean of the conservation tillage systems was compared to conventional tillage, the systems were significantly different at P < 0.01. Although quantitatively more P was lost under conventional tillage than under the conservation tillage systems, the treatment differences proved not to be significant. Overall there were no significant treatment differences for K loss, however conventional tillage differed significantly from the mean of the two conservation tillage treatments. The different years affected the nutrient losses more than tillage, resulting in all the elements differing significantly between the years (Table 8).

Nutrient losses varied greatly depending on the tillage treatment, the element considered, the season in question and the type of loss that the element was subjected to (erosion, leaching or plant uptake). In general the highest losses were realised through plant uptake which is considered to be good since this is the only positive loss, when compared to the other two types of nutrient losses. There is, no doubt, a depletion in soil fertility, however, if only the harvested products are removed from the field and other crop residues are returned, in the absence of erosion and leaching, fertiliser requirements for the maintenance of soil fertility may be reduced. Erosion and leaching on the other hand are considered negative and should be minimised at all costs. Erosion losses were higher than those incurred with leaching for the conventional tillage, while the reverse was partly true for the

A general overview of the nutrient losses showed that total nitrogen losses (through erosion, leaching and plant uptake) were least under mulch ripping with a range of 27 - 53 kg/ha/yr. followed by tied ridging (range of 25 - 64 kg/ha) and highest under conventional tillage (range of 44 - 87 kg/ha). The same trend was established for phosphorus loss although the amounts lost and the differences between the treatments were minimal (ranges of 7 - 10 kg/ha for CT; 6 - 8 kg/ha for MR and 6 - 10 kg/ha for TR). Potassium losses ranged from 29 - 92 kg/ha under CT; 30 - 43 kg/ha under MR and 22 - 57 kg/ha under TR. Table 8 shows the average losses of different elements found for the different treatments and years.

> Treatment N P K CT 63.54 8.42 60.70 MR 38.21 7.41 36.48 TR 49.85 7.83 39.61 Signif. level \* NS NS Year N P K 1 52.0 7.40 64.20 2 31.6 6.77 27.00 3 68.0 9.49 - Signif. level \*\* \* \*\*\*

Table 8. Average total nutrient losses under different tillage treatments during three seasons

Generally, more nutrients were lost under conventional tillage as compared to the conservation tillage system. Only N losses gave significant treatment differences at P < 0.05. However, when the mean of the conservation tillage systems was compared to conventional tillage, the systems were significantly different at P < 0.01. Although quantitatively more P was lost under conventional tillage than under the conservation tillage systems, the treatment differences proved not to be significant. Overall there were no significant treatment differences for K loss, however conventional tillage differed significantly from the mean of the two conservation tillage treatments. The different years affected the nutrient losses more than tillage, resulting in all the elements differing significantly between the

**3.4 Comparison of nutrient losses due to erosion, leaching and plant uptake**

conservation tillage systems.

years (Table 8).

at Makoholi Contill site

After the total nutrient losses were calculated and assessed, each type of nutrient loss was evaluated relative to the total nutrient loss. Plant uptake contributed between 38 and 99% of total nutrient loss, erosion losses were between 0.1 and 51%, while leaching accounted for 0 to 16% of total nutrient loss. This is the reason why different treatments did not show significant differences overall because plant uptake was very high and masked the effects of the other forms of nutrient loss. However, this general finding shows clearly that after plant uptake, erosion is the main factor contributing to nutrient loss in agricultural arable lands.

The different nutrients as influenced by tillage were then evaluated and gave the following results:

Nitrogen: the losses were highest due to plant uptake (Figure7), however the percentage of N taken up under conventional tillage (56 - 84%) was lower than under mulch ripping (88 - 95%) and tied ridging (85 - 92%). This finding implies therefore, that the contribution of the losses by leaching and erosion were lower for the conservation tillage systems. Nitrogen losses as a result of erosion were thus highest under conventional tillage, contributing to more than a quarter of total N loss (range 16 - 29%). The conservation tillage systems on the other hand realised minimum losses of less than one percent and highest losses of 11% (MR) and 8% (TR). The leaching losses were in all cases lower than the erosion losses and the highest percentage leaching loss of 15% was recorded under conventional tillage, while mulch ripping had a maximum of 7% and tied ridging of 10% of total N loss.

Phosphorus: Plant uptake contributed to most of the P loss even under conventional tillage (Figure 7). A range of between 82 and 97% was found under conventional tillage, while the conservation tillage systems had higher percentages of between 97 - 99.8% (MR) and 96 - 99.8% (TR). Erosion losses were high under conventional tillage (between 3 and 18%), while these were maintained at below 5% under both MR and TR. The results also prove that P is not very susceptible to leaching as all the treatments recorded less than 1 % of total P loss.

Potassium: The crop uptake of potassium had a low percentage under conventional tillage, where it ranged from 38 - 77% of total K loss. This was as a result of very high losses of K due to erosion, ranging from 23 - 46%. K leaching was almost as high as N leaching under this treatment (0 - 16%). The percentage of K uptake under the conservation tillage treatments was relatively higher than under conventional tillage (85 - 96% MR; 79 - 95% TR). This means that the contribution of erosion and leaching was also less than under conventional tillage. K loss as a result of erosion under mulch ripping remained very low, < 1 - 2%, while it was higher under tied ridging, 1 - 14% of total K loss. The contribution of leaching to total K loss was higher when compared to that of N and P. Between 3 and 13% of total K loss was lost through leaching under mulch ripping and 4 - 6% under tied ridging.

Total nutrient losses were significantly lower under conservation tillage treatments than under conventional tillage. Although nutrient uptake did not differ significantly among the different treatments, erosion and leaching losses accounted for the differences. After plant uptake, erosion presents higher nutrient losses than leaching. While N and K are mobile in the soil and vulnerable to leaching (Stoorvogel and Smaling, 1990), the N and K losses attributed to this type of nutrient loss were lower than the amounts lost with erosion and plant uptake, except for leached K under mulch ripping, where the losses were higher than erosion losses. This may be due to the high amount of K in solution, leached from the decomposing stover (Havis and Alberts, 1993). There could be an over supply of K in the soil and the high uptake may be because K is available, "luxury uptake" rather than due to demand. P loss with leaching was lowest and this was expected as P is immobile in the soil

The Significance of Soil Erosion on Soil Fertility Under Different Tillage Systems

MR 304% and TR 330%), see the negative balance on Table 9.

and Granitic Sandy Soils in Semi-Arid Zimbabwe: A Comparison of Nutrient Losses ... 235

mulch ripping. An equivalent of 99% of applied N was lost under tied ridging, indicating that the rate of N applied under tied ridging is literally just adequate to offset the nutrient losses, however, great care has to be exercised to ensure that the losses do not exceed the fertiliser applied. P losses under all the treatments were maintained well below the equivalent amount of P applied, an average of 70% under CT; 62% under MR and 75% under TR (Table 10). This implies that part of the applied fertiliser was retained in the soil, showing the extreme fixation of P under these conditions. Such low levels of P fertilisation may not show any yield response due to the high rate of fixation. The crop also took up relatively low amounts of P when compared to the other elements. Potassium on the other hand showed that much, more K was lost when compared the amount applied (CT 505%;

Fig. 7. Comparison of nutrient losses (N, P, K) through leaching, plant uptake and erosion

Nutrient budgets should be employed and fertiliser application should aim at replacing lost nutrients as well as meeting the crop requirements, for the following season. Conservation tillage is highly recommended as it reduces the danger of soil mining drastically, but the problem of soil mining may not be wholly solved through the introduction of soil and water conservation systems if inputs and outputs are not well balanced. While most of the nutrients may be conserved, even up to accumulation, other nutrients, especially those not applied as fertilisers may need replacement once in a while. Nitrogen is the only element that is significantly affected by the atmosphere, increasing its amount in the soil through biological fixation and natural deposition (Hach, 1979). Due to these factors, changes resulting from the N losses may not be easily quantifiable within a

under different tillage systems over three seasons at Makoholi Contill site

few seasons.

and many researchers have highlighted the affinity of P to fixation in the soil. This study has shown that P is not prone to leaching, but is susceptible to erosion, while N and K are mobile in the soil and are very susceptible to leaching and erosion. Although plant uptake contributes the highest percentage of total nutrient loss, the low percentages of 38, 49 and 56% under conventional tillage are a cause for concern as this means that leaching and erosion sometimes contribute up to > 50% of total nutrient loss.

#### **3.5 Soil nutrient balance**

The main purpose of this section is to try and balance the nutrients lost from the field with added inputs. The total nutrients from the field have been quantified in the previous section (erosion, leaching and plant uptake) and the inputs have only been limited to applied fertilisers. The results of this nutrient balance for the different treatments and elements, calculated using Equation 3, have been summed up in Table 9.

Where: IN1 = mineral fertilisers; OUT1 = plant uptake; OUT2 = soil erosion; OUT3 = leaching

Table 9. Nutrient balance assessment of the three tillage systems, average over three years at Makoholi Contill site

Under conventional tillage, there is evidence of soil mining (negative balance) for all elements except P. The conservation tillage systems only recorded negative nutrient budgets for K losses, due to very high losses as a result of crop uptake, which were generally very high to be compensated by the amount applied. Under conservation tillage soil mining was mainly as a result of crop harvests, while under conventional tillage substantial amount of nutrients were also lost as a result of soil erosion and leaching. Thus under this treatment, maintenance of soil fertility means to first of all replace nutrients lost with leaching and erosion before replacement of the amount taken up by the crop. The sum of N losses under conventional tillage is in excess of the amount of N applied annually (an equivalent of 125% of applied N) and some of the losses are directly from the soils' nutrient reserves (Table 10). Under mulch ripping an equivalent of 76% of applied N was also removed annually. The depletion rate was lower and was contained well below the annual N application rate. Soil fertility is thus better maintained under

and many researchers have highlighted the affinity of P to fixation in the soil. This study has shown that P is not prone to leaching, but is susceptible to erosion, while N and K are mobile in the soil and are very susceptible to leaching and erosion. Although plant uptake contributes the highest percentage of total nutrient loss, the low percentages of 38, 49 and 56% under conventional tillage are a cause for concern as this means that leaching and

The main purpose of this section is to try and balance the nutrients lost from the field with added inputs. The total nutrients from the field have been quantified in the previous section (erosion, leaching and plant uptake) and the inputs have only been limited to applied fertilisers. The results of this nutrient balance for the different treatments and elements,

> CT 51.5 44.63 15.81 3.10 - 12.04 MR 51.5 34.70 2.25 1.26 + 13.29 TR 51.5 44.38 2.70 2.77 + 1.65

> CT 12.0 7.65 0.75 0.01 + 3.59 MR 12.0 7.30 0.09 0.01 + 4.60 TR 12.0 7.65 0.17 0.02 + 4.16

> CT 12.0 34.05 24.50 9.31 - 55.86 MR 12.0 35.18 0.60 4.58 - 28.36 TR 12.0 40.67 4.30 2.99 - 35.96

Table 9. Nutrient balance assessment of the three tillage systems, average over three years at

Under conventional tillage, there is evidence of soil mining (negative balance) for all elements except P. The conservation tillage systems only recorded negative nutrient budgets for K losses, due to very high losses as a result of crop uptake, which were generally very high to be compensated by the amount applied. Under conservation tillage soil mining was mainly as a result of crop harvests, while under conventional tillage substantial amount of nutrients were also lost as a result of soil erosion and leaching. Thus under this treatment, maintenance of soil fertility means to first of all replace nutrients lost with leaching and erosion before replacement of the amount taken up by the crop. The sum of N losses under conventional tillage is in excess of the amount of N applied annually (an equivalent of 125% of applied N) and some of the losses are directly from the soils' nutrient reserves (Table 10). Under mulch ripping an equivalent of 76% of applied N was also removed annually. The depletion rate was lower and was contained well below the annual N application rate. Soil fertility is thus better maintained under

Where: IN1 = mineral fertilisers; OUT1 = plant uptake; OUT2 = soil erosion; OUT3 = leaching

OUT2 kg/ha

OUT3 kg/ha Balance kg/ha

OUT1 kg/ha

erosion sometimes contribute up to > 50% of total nutrient loss.

calculated using Equation 3, have been summed up in Table 9.

kg/ha

**3.5 Soil nutrient balance**

Nitrogen

Phosphorus

Potassium

Makoholi Contill site

Tillage IN1

mulch ripping. An equivalent of 99% of applied N was lost under tied ridging, indicating that the rate of N applied under tied ridging is literally just adequate to offset the nutrient losses, however, great care has to be exercised to ensure that the losses do not exceed the fertiliser applied. P losses under all the treatments were maintained well below the equivalent amount of P applied, an average of 70% under CT; 62% under MR and 75% under TR (Table 10). This implies that part of the applied fertiliser was retained in the soil, showing the extreme fixation of P under these conditions. Such low levels of P fertilisation may not show any yield response due to the high rate of fixation. The crop also took up relatively low amounts of P when compared to the other elements. Potassium on the other hand showed that much, more K was lost when compared the amount applied (CT 505%; MR 304% and TR 330%), see the negative balance on Table 9.

Fig. 7. Comparison of nutrient losses (N, P, K) through leaching, plant uptake and erosion under different tillage systems over three seasons at Makoholi Contill site

Nutrient budgets should be employed and fertiliser application should aim at replacing lost nutrients as well as meeting the crop requirements, for the following season. Conservation tillage is highly recommended as it reduces the danger of soil mining drastically, but the problem of soil mining may not be wholly solved through the introduction of soil and water conservation systems if inputs and outputs are not well balanced. While most of the nutrients may be conserved, even up to accumulation, other nutrients, especially those not applied as fertilisers may need replacement once in a while. Nitrogen is the only element that is significantly affected by the atmosphere, increasing its amount in the soil through biological fixation and natural deposition (Hach, 1979). Due to these factors, changes resulting from the N losses may not be easily quantifiable within a few seasons.

The Significance of Soil Erosion on Soil Fertility Under Different Tillage Systems

due to their ability to reduce soil erosion and leaching.

and input towards the success of this project.

*Science of America Proc,* 30, 110-114

*Policy intervention*. The World Bank, Washington D.C.

**5. Acknowledgements**

**6. References**

234-240.

133-137.

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and Granitic Sandy Soils in Semi-Arid Zimbabwe: A Comparison of Nutrient Losses ... 237

substantial amount of total loss, while under the conservation tillage systems most of the nutrients are taken up by the crop and a negligible amount is attributable to erosion and leaching losses. The nutrient balance of inputs versus outputs is negative for N and K under conventional tillage, while under conservation tillage systems, negative nutrient budgets are confined to K, which is subject to higher losses due to its high concentration in the soil. The optimal applications are adequate to offset all nutrient losses under conservation tillage treatments, while under conventional tillage these are inadequate and indicate soil mining. The conservation tillage treatments significantly reduce nutrient losses from agricultural lands

I would like to express my gratitude to GTZ for providing the much needed funding through CONTILL (Conservation Tillage), a collaborative project between GTZ and the Government of Zimbabwe (GoZ). Further acknowledgement goes to the GoZ, for providing me with the opportunity and research facilities. I would like to thank all the CONTILL members from both Domboshawa and especially Makoholi site for their relentless support

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Anon, (1969). *Guide to Makoholi Experiment Station*. Department of Research and Specialist

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Barisas, S.G.; Baker, J.L.; Johnson, H.P. & Laflen, J.M. (1978). Effect of tillage systems on run-

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Brady, N.C. (1984). *The nature and properties of soils*. Macmillan Publishing Co., New York. Braithwaite, P.G. (1976). Conservation tillage- Planting systems. *Rhodesian Farmer*, 10, 25-32.

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Table 10. Relationship between total nutrient loss and applied fertiliser over three seasons at Makoholi Contill site

Nutrient losses with plant uptake, erosion and leaching result in the depletion of nutrient status of the soil, i.e. "soil mining". Balancing the nutrients applied in a system with the outputs from the same system gave negative nutrient budgets, confirming the above hypothesis. What is important however, is that under conservation tillage, mainly the nutrients lost through plant uptake need to be replaced, while under conventional tillage, a significant amount of nutrients lost with erosion and leaching also need to be replaced. The maintenance of soil fertility under this system is more costly than under conservation tillage. Although the conservation tillage systems have proven without doubt that they conserve soil, water and nutrients, the nutrient budgets have also shown that it is of utmost importance to balance the inputs and the outputs. To maintain soil fertility and ensure sustained productivity of the soils, fertiliser rates have to address both the crop needs of the following season and replace the nutrients lost during the previous season.

#### **4. Conclusions**

The losses of nutrients with sheet erosion are primarily dependent on the content of each element in the soil, the amount of soil loss (determined by the type of tillage system) and the type of sediment fraction. Plant nutrient losses are higher when the nutrient status in the soil and/ or soil loss from a field is high. There is a very high association between nutrients and fine soil particles. This makes the suspended material the most detrimental sediment fraction with very high concentrations of nutrients. Drainage is dependent on rainfall and on tillage system. The bare fallows tend to have more drainage than cropped treatments, while the conservation tillage systems have more drainage than conventional tillage. Nutrient losses with leachate are highest under bare fallows compared to cropped treatments. Despite the higher drainage, the conservation tillage systems however, have lower nutrient losses compared to conventional tillage, due mainly to improved soil structure. N and K are mobile in the soil and are prone to leaching, while P is immobile and thus not susceptible to leaching. N and K are mobile in the soil and are prone to leaching, while P is immobile and thus not susceptible to leaching. Nutrient uptake by the crop is dependent on yield and nutrients are taken up in the following order: N > K > P. More nutrients are lost with plant uptake, followed by erosion and then leaching. For the conventional tillage system erosion and leaching contribute to a substantial amount of total loss, while under the conservation tillage systems most of the nutrients are taken up by the crop and a negligible amount is attributable to erosion and leaching losses. The nutrient balance of inputs versus outputs is negative for N and K under conventional tillage, while under conservation tillage systems, negative nutrient budgets are confined to K, which is subject to higher losses due to its high concentration in the soil. The optimal applications are adequate to offset all nutrient losses under conservation tillage treatments, while under conventional tillage these are inadequate and indicate soil mining. The conservation tillage treatments significantly reduce nutrient losses from agricultural lands due to their ability to reduce soil erosion and leaching.

#### **5.  Relationship between total nutrient loss and applied fertiliser over three seasons at

Nutrient losses with plant uptake, erosion and leaching result in the depletion of nutrient status of the soil, i.e. "soil mining". Balancing the nutrients applied in a system with the outputs from the same system gave negative nutrient budgets, confirming the above hypothesis. What is important however, is that under conservation tillage, mainly the nutrients lost through plant uptake need to be replaced, while under conventional tillage, a significant amount of nutrients lost with erosion and leaching also need to be replaced. The maintenance of soil fertility under this system is more costly than under conservation tillage. Although the conservation tillage systems have proven without doubt that they conserve soil, water and nutrients, the nutrient budgets have also shown that it is of utmost importance to balance the inputs and the outputs. To maintain soil fertility and ensure sustained productivity of the soils, fertiliser rates have to address both the crop needs of the

The losses of nutrients with sheet erosion are primarily dependent on the content of each element in the soil, the amount of soil loss (determined by the type of tillage system) and the type of sediment fraction. Plant nutrient losses are higher when the nutrient status in the soil and/ or soil loss from a field is high. There is a very high association between nutrients and fine soil particles. This makes the suspended material the most detrimental sediment fraction with very high concentrations of nutrients. Drainage is dependent on rainfall and on tillage system. The bare fallows tend to have more drainage than cropped treatments, while the conservation tillage systems have more drainage than conventional tillage. Nutrient losses with leachate are highest under bare fallows compared to cropped treatments. Despite the higher drainage, the conservation tillage systems however, have lower nutrient losses compared to conventional tillage, due mainly to improved soil structure. N and K are mobile in the soil and are prone to leaching, while P is immobile and thus not susceptible to leaching. N and K are mobile in the soil and are prone to leaching, while P is immobile and thus not susceptible to leaching. Nutrient uptake by the crop is dependent on yield and nutrients are taken up in the following order: N > K > P. More nutrients are lost with plant uptake, followed by erosion and then leaching. For the conventional tillage system erosion and leaching contribute to a

following season and replace the nutrients lost during the previous season.

Makoholi Contill site

**4. Conclusions**

Total loss % Total loss % Total loss % %

I would like to express my gratitude to GTZ for providing the much needed funding through CONTILL (Conservation Tillage), a collaborative project between GTZ and the Government of Zimbabwe (GoZ). Further acknowledgement goes to the GoZ, for providing me with the opportunity and research facilities. I would like to thank all the CONTILL members from both Domboshawa and especially Makoholi site for their relentless support and input towards the success of this project.

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57, Stichting Mondiaal Alternatief, Armsterdam.

Hudson, N. W. (1992). *Land Husbandry.* Batsford, London.

*America,* 57, 797-802.

*Southern Africa*, 9, 157-164.

¾, 27-31.

*244-256.*

Rhodesia. *Science Forum*, 2, 5-20.

*America, Special Publication 19.*

*Research Bulletin*, 2249, 1-3.

**10**

 *Uganda*

**Effect of Slope Position and Land-Use Changes to Bio- Physical Soil Properties in Nakasongola**

Land degradation, which includes degradation of vegetation cover, soil degradation and nutrient depletion, is a major ecological problem in Uganda. It is estimated that fertile top soil is lost at a rate of one billion cubic meters per year, resulting in massive environmental degradation and constituting a serious threat to sustainable agriculture and forestry (Yost

Forests and the benefits they provide in the form of wood, food, income, and watershed protection have an important and critical role in enabling people to secure a stable and adequate food supply. Deforestation and land degradation, however, are impairing the capacity of forests and the land to contribute to food security, and to provide other benefits, such as fuel wood and fodder in Uganda (Siriri *et al*., 2000). Ugandan environmental issues are causing a great concern because the expansion rate of such major problems as drought,

In Nakasongola district, the presence of steep slopes subject to cultivation since many years, has led to serious soil erosion. The lag in agricultural productivity advancement behind population growth has caused intense land use conflicts, particularly between the agricultural and the forestry sectors. To compensate for the low agricultural productivity, deforestation for arable land expansion has been the principal land use conversion employed in Uganda and in particular in Nakasongola for centuries. There are several repercussions of such land use conversion, the most important in Uganda's and in particular in Nakasongola context being accelerated soil erosion and deterioration of soil nutrient status (FAO, 1986). However, Nakasongola is known not only for the severity of land degradation, but also, since the last decades, for the concentrated efforts taking place to redress these problems including construction of stone terraces and soil bunds, protected

The protected areas, which are a type of land management implemented on degraded, generally open access land, are a mechanism for environmental rehabilitation with a clear biophysical impact on large parts of the formerly degraded commons. In places where protected areas are established, particularly in the western part of the country, they

desertification, water pollution, is reaching an alarming stage (NEMA, 2001).

areas and afforestation (Osiru and Hahn, 1994).

**1. Introduction**

and Eswaran, 1990).

**Pastoral Rangeland Areas, Central Uganda**

*2Department of Geography, Kyambogo University, KYAMBOGO*

M. Buyinza1 and M. Nabalegwa2

*Makerere University, Kampala*

*1Faculty of Forestry and Nature Conservation,*

## **Effect of Slope Position and Land-Use Changes to Bio- Physical Soil Properties in Nakasongola Pastoral Rangeland Areas, Central Uganda**

M. Buyinza1 and M. Nabalegwa2

*1Faculty of Forestry and Nature Conservation, Makerere University, Kampala 2Department of Geography, Kyambogo University, KYAMBOGO Uganda*

#### **1. Introduction**

240 Soil Erosion Issues in Agriculture

Salinas-Garcia, J.R.; Hons, F.M. & Matocha, J.e. (1997). Long-term effects of tillage and

Scoones, I. &Toulmin, C. (1999). *Policies for soil fertility management in Africa*. A Report prepared for the Department for International Development, Nottingham,UK. Singer, M.j> & Munns, D.N. (19987). *Soils, an introduction*. Macmillan Publishing Co. New

Stevenson, F.J. (1985). *Cycles of soil: Carbon, nitrogen, phosphorus, sulphur, micro-nutrients*.

Stocking, M.A. (1984). Soil potentials: an evaluation of a rating method in Zimbabwe.

Stoorvogel, J.J. & Smaling, E.M.A. (1990). *Assessment of soil nutrient depletion in Sub-Saharan*

Tanaka D.L., 1995. Spring-wheat straw production and composition as influenced by top

Thompson, J.G. & Purves, W.D. (1978). *A guide to the soils of Rhodesia,* 1, Department of

Thompson, J.G. & Purves, W.D. (1981). *A guide to the soils of Zimbabwe,* 3, Department of

Thompson, J.G. (1967). *Report on the soils of the Makoholi Experiment Station,* Department of

Tiesen, H.E.; Cuevas, E. & Salcedo, I.H. (1998). Towards sustainable land use, furthering

Troeh, F.R.; Hobbs, J.A. & Donahue, R.L. (1980). *Soil and water conservation for productivity and environmental protection*. Prentice-Hall Inc. Englewood Cliffs, New Jersey. van Reuler, H. & Prins, W.H. (1993).*Plant nutrients and food production. The role of plant*

Vogel, H. (1993). An evaluation of five tillage systems from small-holder agriculture in

Wendelaar, F.E. & Purkis, A.N. (1979). Recording soil loss and run-off from 300m2 erosion

Wrigley, G. (1992). Tropical agriculture: The development and production. Macmillan

Xu, F. & Prato, T. (1995). On-site erosion damages in Missouri corn production. *Journal of Soil*

Cooperation between people and institutions. *Proceedings of the 13th International Soil*

*nutrients for sustainable food crop production in Sub-Saharan Africa*. VKP

soil removal. *Journal of Soil Science Society of America*, 59, 3, 649-653.

*Africa1983-2000. Report* 28, Wageningen, The Netherlands

*Conservation Organisation,* Bonn, Germany, August 1996.

research field plots. *Research Bulletin 24*, Conex, Harare.

Schroeder, D. (1984). *Soils. Facts and concepts*. Berne, Switzerland.

Discussion Paper 172, UEA, Norwich

Research and Specialist Services, Harare

Research and Specialist Services, Harare.

Research and Specialist Services, Salisbury.

Leidschendam, The Netherlands.

Publishing Co. New York.

Zimbabwe. *Der Tropen Landwirt,* 94, 21-36.

*and Water Conservation,* 50, 1, 312-316.

61, 152-159.

York.

USA.

fertilization on soil organic matter dynamics. *Journal of Soil Science Society of America*

Land degradation, which includes degradation of vegetation cover, soil degradation and nutrient depletion, is a major ecological problem in Uganda. It is estimated that fertile top soil is lost at a rate of one billion cubic meters per year, resulting in massive environmental degradation and constituting a serious threat to sustainable agriculture and forestry (Yost and Eswaran, 1990).

Forests and the benefits they provide in the form of wood, food, income, and watershed protection have an important and critical role in enabling people to secure a stable and adequate food supply. Deforestation and land degradation, however, are impairing the capacity of forests and the land to contribute to food security, and to provide other benefits, such as fuel wood and fodder in Uganda (Siriri *et al*., 2000). Ugandan environmental issues are causing a great concern because the expansion rate of such major problems as drought, desertification, water pollution, is reaching an alarming stage (NEMA, 2001).

In Nakasongola district, the presence of steep slopes subject to cultivation since many years, has led to serious soil erosion. The lag in agricultural productivity advancement behind population growth has caused intense land use conflicts, particularly between the agricultural and the forestry sectors. To compensate for the low agricultural productivity, deforestation for arable land expansion has been the principal land use conversion employed in Uganda and in particular in Nakasongola for centuries. There are several repercussions of such land use conversion, the most important in Uganda's and in particular in Nakasongola context being accelerated soil erosion and deterioration of soil nutrient status (FAO, 1986). However, Nakasongola is known not only for the severity of land degradation, but also, since the last decades, for the concentrated efforts taking place to redress these problems including construction of stone terraces and soil bunds, protected areas and afforestation (Osiru and Hahn, 1994).

The protected areas, which are a type of land management implemented on degraded, generally open access land, are a mechanism for environmental rehabilitation with a clear biophysical impact on large parts of the formerly degraded commons. In places where protected areas are established, particularly in the western part of the country, they

Effect of Slope Position and Land-Use Changes to Bio- Physical Soil

Properties in Nakasongola Pastoral Rangeland Areas, Central Uganda 243

N †

Fig. 1. Location of the study district and specific study site at Kakooge sub-county

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

**Year**

June July

Fig. 2. Monthly rainfall of the study sites (source: Uganda Meteorological Division, Ministry

of Water, Lands and Environment, Kampala, Uganda).

Scale: 1cm : 250,000

**amount of rainfall (mm)**

constitute green spots with considerable species diversity. The ability of rehabilitation areas to recruit and sustain new life forms is a true measure of their contribution to biodiversity and forest resource conservation (Yost and Eswaran, 1990; Tucker and Murphy, 1997).

In protected areas, it is generally believed that the land resources such as soil, wild flora and fauna, or water will be protected from degradation. Although the restoration ecology and buffering effect of protected areas have been well studied, there are relatively few studies in the region, which would provide a measure of the success or failure of protected areas as one strategy to help prevent decline of soil fertility and adverse effect of water erosion, thereby increasing agricultural productivity. Above all there are no quantitative studies that analyse the effectiveness of protected areas in improving soil chemical and physical properties.

Therefore, the objectives of this study were to asses the impact of protected areas on soil properties, and investigate the relationship between age of protected areas and their effectiveness in improving soil chemical and physical properties.

#### **2. Materials and methods**

#### **2.1 Study area**

Nakasongola District is located on the Bombo - Gulu highway 114 Km north of Kampala. It covers an area of 3,424 km2 of land and boarders with Apac district in the northeast, Mukono district in the east, Masindi in the west and Luwero district in the south (Figure 1) . The area lies on the central plateau between 1000 and 1400 m above sea level. The topography is characterized by extensive uniform undulating plains with broad seasonal swamps.

The soils are mainly weathered basement complex formations of the precambrian age, which consists mainly of metamorphic and igneous rocks, largely composed of gneisses and granites. Remnants of the older mid-tertiary surface are found as relic murram and iron stones in some places. The annual rainfall varies from 875 – 1120 mm with two marked dry seasons. The main vegetation types are woodland and woodland savanna, thicket and soft wood plantations*.* Much of the cultivated land exists as patches with in the woodland.

The district has a total population of 528,126 people, the population density is about 230 persons per km2, and the growth rate is 2.7 %. There are three ethic groups; the Baganda (70%) and Baruli (28) and others (2%). Subsistence agriculture is the major economic activity employing about 89% of population.

Rocks outcropping in the study area belong to the Mesozoic sedimentary series and Tertiary basalt flows. Soils of the specific study sites are developed on lime rich parent material. Using the FAO-UNESCO soil classification system, they were classified as calcaric Cambisols. In the study area, the erosion rate is extremely serious and it is common to observe ground surfaces that have been incised strongly by rill and gully erosion.

The study area has a semi arid continental climate with an average annual rainfall of 615 mm/yr with extreme values of 290 and 900 mm (Figure 2). The rainy season starts in June, peaks in July and August and trails off in September. The study sites have 53-104 rain days per year with mean of 80 days. According to the agro-climatic classification system, which is traditionally used in Uganda, all the study sites are classified as mid altitude.

constitute green spots with considerable species diversity. The ability of rehabilitation areas to recruit and sustain new life forms is a true measure of their contribution to biodiversity and forest resource conservation (Yost and Eswaran, 1990; Tucker and Murphy, 1997). In protected areas, it is generally believed that the land resources such as soil, wild flora and fauna, or water will be protected from degradation. Although the restoration ecology and buffering effect of protected areas have been well studied, there are relatively few studies in the region, which would provide a measure of the success or failure of protected areas as one strategy to help prevent decline of soil fertility and adverse effect of water erosion, thereby increasing agricultural productivity. Above all there are no quantitative studies that analyse the effectiveness of protected areas in improving soil chemical and physical

Therefore, the objectives of this study were to asses the impact of protected areas on soil properties, and investigate the relationship between age of protected areas and their

Nakasongola District is located on the Bombo - Gulu highway 114 Km north of Kampala. It covers an area of 3,424 km2 of land and boarders with Apac district in the northeast, Mukono district in the east, Masindi in the west and Luwero district in the south (Figure 1) . The area lies on the central plateau between 1000 and 1400 m above sea level. The topography is characterized by extensive uniform undulating plains with broad seasonal

The soils are mainly weathered basement complex formations of the precambrian age, which consists mainly of metamorphic and igneous rocks, largely composed of gneisses and granites. Remnants of the older mid-tertiary surface are found as relic murram and iron stones in some places. The annual rainfall varies from 875 – 1120 mm with two marked dry seasons. The main vegetation types are woodland and woodland savanna, thicket and soft wood plantations*.* Much of the cultivated land exists as patches with in

The district has a total population of 528,126 people, the population density is about 230 persons per km2, and the growth rate is 2.7 %. There are three ethic groups; the Baganda (70%) and Baruli (28) and others (2%). Subsistence agriculture is the major economic activity

Rocks outcropping in the study area belong to the Mesozoic sedimentary series and Tertiary basalt flows. Soils of the specific study sites are developed on lime rich parent material. Using the FAO-UNESCO soil classification system, they were classified as calcaric Cambisols. In the study area, the erosion rate is extremely serious and it is common to

The study area has a semi arid continental climate with an average annual rainfall of 615 mm/yr with extreme values of 290 and 900 mm (Figure 2). The rainy season starts in June, peaks in July and August and trails off in September. The study sites have 53-104 rain days per year with mean of 80 days. According to the agro-climatic classification system, which is

observe ground surfaces that have been incised strongly by rill and gully erosion.

traditionally used in Uganda, all the study sites are classified as mid altitude.

effectiveness in improving soil chemical and physical properties.

properties.

**2.1 Study area**

swamps.

the woodland.

employing about 89% of population.

**2. Materials and methods**

Fig. 1. Location of the study district and specific study site at Kakooge sub-county

Fig. 2. Monthly rainfall of the study sites (source: Uganda Meteorological Division, Ministry of Water, Lands and Environment, Kampala, Uganda).

Effect of Slope Position and Land-Use Changes to Bio- Physical Soil

**2.2 Experimental design**

lower part of each study site.

**2.3 Soil sampling and laboratory analysis**

method and calculation using bulk density values.

protected areas.

**2.4 Statistical analysis**

Properties in Nakasongola Pastoral Rangeland Areas, Central Uganda 245

Five and ten year's old protected areas and free grazing lands were selected in the middle altitude agro-ecological zone (mid latitude) for this study. Each of the sites was replicated three times over the study area. Each of the study sites (protected area and free grazing lands) was divided into three slope positions, upper slope (US), middle slope (MS) and foot slope (FS). The US position is the uppermost portion of each study site. It receives little or no overland flow but may contribute runoff to downslope areas. The MS position receives overland flow from the upperslope and contributes runoff to the FS. The FS represents the

Within each position in protected areas and free grazing land, transects were laid out and eight soil sampling points along transect were selected using systematic random sampling. Samples of 0-15 cm soil depth were taken from the eight points of each transect. The eight samples were combined in a large bucket and mixed thoroughly to form a composite soil sample; 36 composite samples were collected from all the sites. Major live plant materials (roots and shoots) in each sample were separated by hand and discarded. The soil samples were air dried, grinded and passed through a 2 mm sieve before the determination of soil nutrients. Standard soil test procedures for observation and analysis in Kyambogo University Soil Science Laboratory were used for all nutrients and physical soil parameters. Organic matter was determined by Walkley Black method and total Nitrogen was determined by Kjeldahl method (Bremmer and Mulvany, 1982). Available Phosphorus was determined by Olsen method (Olsen and Sommers, 1982). Ammonium and Sodium acetate methods were used to determine exchangeable bases and cation exchange capacity (Osiru and Hahn, 1994). Calcium and Magnesium values were then red using Atomic Absorption Spectrophotometer while Potassium and Sodium were determined by flame photometer. The pH and EC of the soil samples were determined by pH and conductivity meter using supernatant suspension of 1:5 soil water ratios. Particle size analyses of the sampled soils were determined in soil suspension by Hydrometer method (Gee and Bauder, 1982). Bulk density was determined by core method through measurement of volume and mass (Blake and Hartge, 1986). Total porosity was determined experimentally by the water displacement

Moreover, the Universal Soil Loss Equation (USLE) was used (Wischmeier, 1976, Table 1) for the purpose of characterizing the severity of erosion in the study sites and generates supplemental data (Yost and Eswaran, 1990). Individual interviews using structured questionnaires were also conducted to understand farmers' perception on the role of

Statistical analyses were performed to test the influence of land use conversion and age of protected areas on soil chemical and physical properties using one-way analysis of variance (ANOVA). Mean comparisons were made using the Tukey Honest Significant Difference (HSD) test with p < 0.05. Two - way ANOVA was also used to see whether there is an interaction effect between the two main independent variables: age and slope gradient. The independent variables used in this study were land use, slope gradient and age of protected

areas. All the analyses were conducted through Statistica 6.0 program.

The typical land use in the study area is range land and protected areas on the steep slopes and crop land in the flats. The vegetation of the study area is largely dominated by *Acacia etbiaca*, and *Euclea shimperi.* Under-storey vegetation of the study sites is dominated by a very diverse assemblage of grass and herbs most of which are palatable for livestock. Crops are mainly cassava, maize, and sweet potatoes. Crop yields are low (500kg/ha on average), with great variability depending on rainfall and the location of the cultivated lands. According to the district's Agricultural office, the average land holding is 0.46-0.76 ha per household.

Within the Nakasongola district, twelve hillslopes ranging from 3.5 to 48.5 ha were selected based on similarities in lithology, soils, climate and land use. Out of the twelve sites selected, six are protected areas and the remaining six are free grazing lands. To examine the influence of age since protection five and ten year's old protected areas each having three replicates were selected for the study. Features of the specific study sites are included in table 1.

10AC, 5 AC: 10 and 5 years old protected areas respectively. C1 and C2 are free grazing lands used as a control for 10 and 5 years protected areas. US, MS, and FS are Upper, Middle and Foot slope. TWSBG, GTWSB, and TWSB; tall weeds and short bushes with grasses, grasses with tall weeds and short bushes and tall weeds with short bushes respectively. BU (2m) = bushes of 2 meter effective height; Bu (0.5) =

bushes of 0.5 meter effective height and SBU= short bushes. 1 = predicted through application of Universal Soil Loss Equation (USLE);.

Table 1. Average characteristics (n = 3) of the specific study sites

#### **2.2 Experimental design**

244 Soil Erosion Issues in Agriculture

The typical land use in the study area is range land and protected areas on the steep slopes and crop land in the flats. The vegetation of the study area is largely dominated by *Acacia etbiaca*, and *Euclea shimperi.* Under-storey vegetation of the study sites is dominated by a very diverse assemblage of grass and herbs most of which are palatable for livestock. Crops are mainly cassava, maize, and sweet potatoes. Crop yields are low (500kg/ha on average), with great variability depending on rainfall and the location of the cultivated lands. According to the district's Agricultural office, the average land holding is 0.46-0.76 ha per

Within the Nakasongola district, twelve hillslopes ranging from 3.5 to 48.5 ha were selected based on similarities in lithology, soils, climate and land use. Out of the twelve sites selected, six are protected areas and the remaining six are free grazing lands. To examine the influence of age since protection five and ten year's old protected areas each having three replicates were selected for the study. Features of the specific study sites are included in

> Slope Length (m)

US -- 12.7 22 147 SBU 4 TWSB 5 57.4 MS 10.2 25 81 SBU 5 TWSB 5 80.2 FS 6.5 11 111 SBU 10 TWSB 9 27.9

US -- 10.7 27 82 SBU 6 TWSB 6 121.5 MS 7.7 29 64 SBU 7 TWSB 11 96.2 FS 7.3 13 80 SBU 4 TWSB 6 25.0

10AC, 5 AC: 10 and 5 years old protected areas respectively. C1 and C2 are free grazing lands used as a control for 10 and 5 years protected areas. US, MS, and FS are Upper, Middle and Foot slope. TWSBG, GTWSB, and TWSB; tall weeds and short bushes with grasses, grasses with tall weeds and short bushes and tall weeds with short bushes respectively. BU (2m) = bushes of 2 meter effective height; Bu (0.5) =

Vegetal canopy Ground

(%) Type

(2m) <sup>43</sup> TWSBG <sup>58</sup> 16.3

(2m) <sup>31</sup> TWSBG <sup>60</sup> 6.8

(2m) <sup>32</sup> TWSBG <sup>52</sup> 2.6

m) <sup>28</sup> GTWSB <sup>23</sup> 98.5

m) <sup>30</sup> GTWSB <sup>38</sup> 68.7

m) <sup>29</sup> GTWSB <sup>38</sup> 27.7

Type Cover

vegetation

Cover (%)

Estimated soil loss t/ha/ye ar (USLE)1

household.

table 1.

Land use

10 AC

5 AC

C1

C2

Slope class

Year of closing

US 1994/9

US 1999/0

Area (ha)

Slope Steepness (%)

<sup>5</sup> 20.3 <sup>24</sup> <sup>125</sup> BU

<sup>0</sup> 6.8 <sup>37</sup> <sup>80</sup> BU(.5

MS 40.8 <sup>19</sup> <sup>274</sup> BU

FS 34.5 <sup>10</sup> <sup>290</sup> BU

MS 6.8 <sup>33</sup> <sup>73</sup> BU(.5

FS 5.4 <sup>18</sup> <sup>86</sup> BU(.5

bushes of 0.5 meter effective height and SBU= short bushes. 1 = predicted through application of Universal Soil Loss Equation (USLE);.

Table 1. Average characteristics (n = 3) of the specific study sites

Five and ten year's old protected areas and free grazing lands were selected in the middle altitude agro-ecological zone (mid latitude) for this study. Each of the sites was replicated three times over the study area. Each of the study sites (protected area and free grazing lands) was divided into three slope positions, upper slope (US), middle slope (MS) and foot slope (FS). The US position is the uppermost portion of each study site. It receives little or no overland flow but may contribute runoff to downslope areas. The MS position receives overland flow from the upperslope and contributes runoff to the FS. The FS represents the lower part of each study site.

#### **2.3 Soil sampling and laboratory analysis**

Within each position in protected areas and free grazing land, transects were laid out and eight soil sampling points along transect were selected using systematic random sampling. Samples of 0-15 cm soil depth were taken from the eight points of each transect. The eight samples were combined in a large bucket and mixed thoroughly to form a composite soil sample; 36 composite samples were collected from all the sites. Major live plant materials (roots and shoots) in each sample were separated by hand and discarded. The soil samples were air dried, grinded and passed through a 2 mm sieve before the determination of soil nutrients. Standard soil test procedures for observation and analysis in Kyambogo University Soil Science Laboratory were used for all nutrients and physical soil parameters. Organic matter was determined by Walkley Black method and total Nitrogen was determined by Kjeldahl method (Bremmer and Mulvany, 1982). Available Phosphorus was determined by Olsen method (Olsen and Sommers, 1982). Ammonium and Sodium acetate methods were used to determine exchangeable bases and cation exchange capacity (Osiru and Hahn, 1994). Calcium and Magnesium values were then red using Atomic Absorption Spectrophotometer while Potassium and Sodium were determined by flame photometer. The pH and EC of the soil samples were determined by pH and conductivity meter using supernatant suspension of 1:5 soil water ratios. Particle size analyses of the sampled soils were determined in soil suspension by Hydrometer method (Gee and Bauder, 1982). Bulk density was determined by core method through measurement of volume and mass (Blake and Hartge, 1986). Total porosity was determined experimentally by the water displacement method and calculation using bulk density values.

Moreover, the Universal Soil Loss Equation (USLE) was used (Wischmeier, 1976, Table 1) for the purpose of characterizing the severity of erosion in the study sites and generates supplemental data (Yost and Eswaran, 1990). Individual interviews using structured questionnaires were also conducted to understand farmers' perception on the role of protected areas.

#### **2.4 Statistical analysis**

Statistical analyses were performed to test the influence of land use conversion and age of protected areas on soil chemical and physical properties using one-way analysis of variance (ANOVA). Mean comparisons were made using the Tukey Honest Significant Difference (HSD) test with p < 0.05. Two - way ANOVA was also used to see whether there is an interaction effect between the two main independent variables: age and slope gradient. The independent variables used in this study were land use, slope gradient and age of protected areas. All the analyses were conducted through Statistica 6.0 program.

Effect of Slope Position and Land-Use Changes to Bio- Physical Soil

areas and free grazing lands (at p < 0.05) for pH and EC values.

However, such trend was not clearly seen in free grazing lands.

parameters for multiple comparisons.

positions in protected areas.

areas were nearly similar.

**3.2 Soil nutrient stocks and land uses**

**value**

Properties in Nakasongola Pastoral Rangeland Areas, Central Uganda 247

and 0.76, Ca between 2.7 and 27 and Mg between 0.2 and 1.8 cmol(+)/kg-soil. Except Ca, the level of exchangeable bases in the sampled soils showed the same trend as the other soil

The result of pH tests revealed that 75 % of the sampled sites had moderately alkaline soils (pH between 7.4 and 8.4) and 25 % of the sampled sites had neutral soils (pH 6.6-7.3). The level of soluble salts in the sample sites was also low, that is less than 1 mmhos/cm. Analysis of variance shows that there were no a significant differences between protected

In protected areas, an increasing trend was observed in soil organic matter, total Nitrogen, available Phosphorus and exchangeable Potassium from US to FS position (Figure 3).

> Soil organic matter, primary nutrients and clay fraction for five and ten years protected areas

> > US MS

In this study, total amount of nutrients stored at 0-15 cm soil depth were determined using bulk density values of each slope position of each site (Table 3). There were considerable differences in total amount of nutrients stored in the upper 15 cm soil between protected areas and free grazing lands. The mean value of SOM stock of ten year protected areas was higher by 40%, TN by 31% and AP by 40% than its control C1. Mean value of SOM, TN, and AP stock of five year protected areas was higher by 44%, 39%, and 40% than its control C2. However, the nutrient stocks in five and ten years protected

Fig. 3. Soil organic matter, primary soil nutrients and clay fraction in relation to slope

### **3. Results**

#### **3.1 Soil chemical properties and land uses**

The soils of free grazing lands and protected areas differed considerably in content of soil organic matter (SOM), total Nitrogen (TN), available Phosphorus (AP) and exchangeable bases and cation exchange capacity (CEC) (Table 2), with significantly higher (p < 0.05) values found in protected areas than in free grazing lands. Two - way analysis of variance also revealed that age of protected areas was highly significant (p < 0.01). Slope effect was not significant. There was a tendency of significant interaction effect (p = 0.07) indicating that the influence of slope changes with age.

Different letters along the column indicate significant differences between mean values of the different land use types at p < 0.05 (Tuckey HSD).

٭ , ٭٭ significant at 0.05 and 0.01 level of significance respectively.

SOM - soil organic matter; TN -total Nitrogen; AP – available Phosphorus; and CEC – cation exchange capacity.

5 AC – five years protected areas; 10 AC – ten years protected areas; C1 – free grazing land used as a control for 10 years protected areas and C2 – free grazing land used as a control for 5 years protected areas. US, MS and FS are upper, middle and foot slopes respectively.

Table 2. Average soil nutrient content (n = 3) of 0-15 cm soil depth for the land use types and slope position

The mean SOM content varied between 1.4 and 3.2 % (Table 2). Multiple comparison of SOM revealed that SOM levels under five and ten years protected areas were significantly higher than in C1 and C2. The mean CEC of the sampled soils varied between 39.6 and 58.0 cmol (+)/kg and had a similar pattern to SOM. Mean TN content varied between 0.20 and 0.57 %. Comparison of means revealed that the protected areas had significantly higher total Nitrogen content than C1 and C2. Mean AP content varied between 2.8 and 6.8 ppm and displayed similar pattern to TN. The mean value of exchangeable K, varied between 0.22

The soils of free grazing lands and protected areas differed considerably in content of soil organic matter (SOM), total Nitrogen (TN), available Phosphorus (AP) and exchangeable bases and cation exchange capacity (CEC) (Table 2), with significantly higher (p < 0.05) values found in protected areas than in free grazing lands. Two - way analysis of variance also revealed that age of protected areas was highly significant (p < 0.01). Slope effect was not significant. There was a tendency of significant interaction effect (p = 0.07) indicating

Soil nutrients

2.4ab 0.27ab 4.5ab 0.32ab 9.2ab 0.4a 52.2ac 2.8ab 0.34ab 5.8ab 0.34ab 5.7a 0.5a 54.5a 3.0a 0.34ab 6.0ab 0.36ab 8.7ab 0.8ab 55.5a 2.8ab 0.35ab 4.6ab 0.59ab 11.0ab 1.8b 56.4a 3.1a 0.37ab 5.1ab 0.63ac 11.0ab 0.8ab 56.6a 3.2a 0.57a 6.8a 0.70ac 5.1a 0.4a 58.0a 1.5b 0.20b 3.0b 0.22b 15.4ab 1.3ab 39.6b 1.5b 0.21b 3.7ab 0.23b 17.6ab 1.4ab 40.9b 1.9ab 0.24b 4.1ab 0.22b 27.0b 1.4ab 41.0b 1.8ab 0.23b 3.7ab 0.76c 2.7a 0.6a 48.3dc 1.8ab 0.26ab 2.8b 0.70c 18.0ab 0.5a 45.5db 1.4b 0.25ab 3.6ab 0.55bc 6.4a 0.2a 45.4db

Ca cmol(+)/kg

Mg cmol(+)/kg

CEC cmol(+)/k g

K cmol(+)/kg

٭٭9.015 1.733 ٭٭3.488 ٭٭7.633 ٭2.473 ٭2.506 ٭٭5.398 value F Different letters along the column indicate significant differences between mean values of the different

SOM - soil organic matter; TN -total Nitrogen; AP – available Phosphorus; and CEC – cation exchange

5 AC – five years protected areas; 10 AC – ten years protected areas; C1 – free grazing land used as a control for 10 years protected areas and C2 – free grazing land used as a control for 5 years protected

Table 2. Average soil nutrient content (n = 3) of 0-15 cm soil depth for the land use types and

The mean SOM content varied between 1.4 and 3.2 % (Table 2). Multiple comparison of SOM revealed that SOM levels under five and ten years protected areas were significantly higher than in C1 and C2. The mean CEC of the sampled soils varied between 39.6 and 58.0 cmol (+)/kg and had a similar pattern to SOM. Mean TN content varied between 0.20 and 0.57 %. Comparison of means revealed that the protected areas had significantly higher total Nitrogen content than C1 and C2. Mean AP content varied between 2.8 and 6.8 ppm and displayed similar pattern to TN. The mean value of exchangeable K, varied between 0.22

**3. Results**

Land use & slope position 10 AC US 10 AC MS 10 AC FS 5 AC US 5 AC MS 5 AC FS C1 US C1 MS C1 FS C2 US C2 MS C2 FS

capacity.

slope position

**3.1 Soil chemical properties and land uses**

that the influence of slope changes with age.

land use types at p < 0.05 (Tuckey HSD).

٭ , ٭٭ significant at 0.05 and 0.01 level of significance respectively.

areas. US, MS and FS are upper, middle and foot slopes respectively.

SOM (%) TN (%) AP

(ppm)

and 0.76, Ca between 2.7 and 27 and Mg between 0.2 and 1.8 cmol(+)/kg-soil. Except Ca, the level of exchangeable bases in the sampled soils showed the same trend as the other soil parameters for multiple comparisons.

The result of pH tests revealed that 75 % of the sampled sites had moderately alkaline soils (pH between 7.4 and 8.4) and 25 % of the sampled sites had neutral soils (pH 6.6-7.3). The level of soluble salts in the sample sites was also low, that is less than 1 mmhos/cm. Analysis of variance shows that there were no a significant differences between protected areas and free grazing lands (at p < 0.05) for pH and EC values.

In protected areas, an increasing trend was observed in soil organic matter, total Nitrogen, available Phosphorus and exchangeable Potassium from US to FS position (Figure 3). However, such trend was not clearly seen in free grazing lands.

Fig. 3. Soil organic matter, primary soil nutrients and clay fraction in relation to slope positions in protected areas.

#### **3.2 Soil nutrient stocks and land uses**

In this study, total amount of nutrients stored at 0-15 cm soil depth were determined using bulk density values of each slope position of each site (Table 3). There were considerable differences in total amount of nutrients stored in the upper 15 cm soil between protected areas and free grazing lands. The mean value of SOM stock of ten year protected areas was higher by 40%, TN by 31% and AP by 40% than its control C1. Mean value of SOM, TN, and AP stock of five year protected areas was higher by 44%, 39%, and 40% than its control C2. However, the nutrient stocks in five and ten years protected areas were nearly similar.

Effect of Slope Position and Land-Use Changes to Bio- Physical Soil

**3.3 Soil nutrient correlations**

٭0.54 TN

Numbers are correlation coefficients (r). N = 36 ٭ significant at 0.05 level of significance

Table 4. Correlation between soil nutrients

**3.4 Land use and soil physical properties**

all sampled sites (Table 5).

**4.1 Effects of protected areas**

**4. Discussion**

than 10% (Table 1).

٭0.45 ٭0.36 AP

٭0.56 ٭0.55 ٭0.55 CEC

٭-0.52 -0.21 -0.28 -0.31 Ca

The mean values of soil bulk density (BD) at a depth of 0-15 cm varied between 1.11 and

Analysis of variance shows that there was no significant difference between land uses (at p< 0.05) on their BD and porosity values. Texture of the 0-15 cm soil depth was sandy loam at

The higher SOM, TN and AP contents in protected areas compared to those from free grazing lands can be explained by the difference in soil erosion and biomass return. Reduced erosion is expected to occur in well developed protected areas because the canopy formed by the mature shrubs and under-story vegetation shields the soil from the erosive energy of the falling raindrops and thereby protects it from splash erosion and surface or sheet erosion. The lowest soil loss (8.5 ton/ha /yr) calculated using USLE was found in ten years old protected areas and is mainly due to the relatively high vegetation canopy cover ( 31-43%) by 2 m effective height bushes, and ground vegetation cover of 52 – 60% (Table 1). In contrast, the higher soil loss (81 ton/ha /yr) predicted for free grazing land is the result of very low vegetation cover (4 - 7%) and ground vegetation cover of less

Our result is in agreement with other studies. The study conducted by Jiang *et al*., (1996), showed that soil loss decreases exponentially with increasing degree of cover by mulch. Siriri and Bekunda (2001) revealed that the median rate of soil erosion in polygons of different land uses decreased in the following pattern: bare soil > open canopy forest > pasture > protected canopy forest. A report by Mullar-Harvey *et al*., (1985), indicates that under a tropical monsoon climate, the establishment of forest on

1.34 g/cm3 (Table 5). The mean values of total porosity varied between 49 and 58 %.

CEC (at p < 0.05).

Properties in Nakasongola Pastoral Rangeland Areas, Central Uganda 249

Some values found in this study were significantly correlated with each other (Table 4). Soil organic matter was correlated positively and significantly with total Nitrogen (TN), AP, and

SOM TN AP CEC Ca K

See table 2 for abbreviations

Table 3. Average (n = 3) soil nutrient stocks of 0-15cm soil depth for the land uses and slope positions

#### **3.3 Soil nutrient correlations**

248 Soil Erosion Issues in Agriculture

10 AC US 45810 4938 8.2 223 3286 84

10 AC MS 50598 6097 10.7 256 2126 110

5 AC US 53548 6658 8.7 446 3743 377

5 AC MS 52289 7204 9.0 411 4176 167

C1 US 29211 4714 4.4 177 6062 305

C1 MS 27607 3880 6.3 172 6715 316

C2 US 35544 4588 6.0 598 1084 149

C2 MS 30446 4621 5.0 498 6578 108

C1 FS 36671 4024 7.7 167 10474

C2 FS 27125 4829 7.0 416 2302

Table 3. Average (n = 3) soil nutrient stocks of 0-15cm soil depth for the land uses and slope

**50176 5703 10.2 239 2863**

**55958 7705 10 456 3301**

**31163 4206 6.1 172 7750**

**31038 4679 6 504 3321 105**

10 AC FS 54119 6073 11.7 237 3175

5 AC FS 62037 9255 12.0 512 1985

Soil nutrient stocks (kg/ha)

SOM TN AP <sup>K</sup> Ca Mg

157

**117**

90

**211**

322

**314**

57

Land use and slope position

Mean for the land use type

Mean for the land use type

Mean for the land use type

Mean for the land use type

See table 2 for abbreviations

positions

Some values found in this study were significantly correlated with each other (Table 4). Soil organic matter was correlated positively and significantly with total Nitrogen (TN), AP, and CEC (at p < 0.05).

Numbers are correlation coefficients (r). N = 36

٭ significant at 0.05 level of significance

Table 4. Correlation between soil nutrients

#### **3.4 Land use and soil physical properties**

The mean values of soil bulk density (BD) at a depth of 0-15 cm varied between 1.11 and 1.34 g/cm3 (Table 5). The mean values of total porosity varied between 49 and 58 %.

Analysis of variance shows that there was no significant difference between land uses (at p< 0.05) on their BD and porosity values. Texture of the 0-15 cm soil depth was sandy loam at all sampled sites (Table 5).

#### **4. Discussion**

#### **4.1 Effects of protected areas**

The higher SOM, TN and AP contents in protected areas compared to those from free grazing lands can be explained by the difference in soil erosion and biomass return. Reduced erosion is expected to occur in well developed protected areas because the canopy formed by the mature shrubs and under-story vegetation shields the soil from the erosive energy of the falling raindrops and thereby protects it from splash erosion and surface or sheet erosion. The lowest soil loss (8.5 ton/ha /yr) calculated using USLE was found in ten years old protected areas and is mainly due to the relatively high vegetation canopy cover ( 31-43%) by 2 m effective height bushes, and ground vegetation cover of 52 – 60% (Table 1). In contrast, the higher soil loss (81 ton/ha /yr) predicted for free grazing land is the result of very low vegetation cover (4 - 7%) and ground vegetation cover of less than 10% (Table 1).

Our result is in agreement with other studies. The study conducted by Jiang *et al*., (1996), showed that soil loss decreases exponentially with increasing degree of cover by mulch. Siriri and Bekunda (2001) revealed that the median rate of soil erosion in polygons of different land uses decreased in the following pattern: bare soil > open canopy forest > pasture > protected canopy forest. A report by Mullar-Harvey *et al*., (1985), indicates that under a tropical monsoon climate, the establishment of forest on

Effect of Slope Position and Land-Use Changes to Bio- Physical Soil

impact on decomposition and nutrient cycling at ecosystem levels.

protected areas and free grazing land soils.

management history.

trampling effect.

material.

crop production (Siriri *et al.*, 2000).

Properties in Nakasongola Pastoral Rangeland Areas, Central Uganda 251

and accumulation of absorbent humus on the soil surface, thereby significantly reducing the volume, velocity, and erosive and leaching capacity of surface runoff (Jiang *et al*., 1996). However, land use for free grazing land is related to soil management practices that have commonly been very destructive to the soil and have caused serious erosion. Therefore, differences in soil erosion control contribute to the significant difference in nutrients in

In addition, less biomass return causes the reduction of SOM, TN and AP in free grazing lands (Mullar - Harvey *et al*., 1985; Shariff *et al.*, 1994). The most evident impact of grazing on the rangeland ecosystem is removal of a major part of above ground biomass by livestock. Therefore the input of aboveground litter to the soil decreases. Any reduction in litter inputs may have important consequences for soil nutrient conservation and cycling (Shariff *et al*., 1994). Grazing may also have an indirect effect on soil characteristics through change in plant species composition. This is mainly because plant species has a significant

The similar pattern displayed among SOM, TN and AP for multiple comparison of means can be explained by the influence of soil organic matter in nutrient retention and supply (Brubaker *et al*., 1993). Among soil properties, total organic carbon is a sensitive soil quality indicator suggesting that within a narrow range of soil, it may serve as a suitable indicator of soil quality (Mullar-Harvey *et al*., 1985). Moreover, the soil organic matter fraction may offer further insight into soil fertility changes and the sustainability of

The insignificant differences in soil bulk density and porosity among protected areas and free grazing lands can be explained by the coarse texture nature of the study sites and low amount of preexisting soil moisture. Preexisting soil texture and moisture conditions are important variables to consider when investigating the relationship between soil compaction and grazing intensity. The effect of grazing intensity in increasing bulk density is pronounced in wet and fine textured soils. The slight difference found in this study can be explained by their difference in SOM content and compaction of the sites by livestock

The role of soil organic matter in storing and supplying nutrients explains the significant correlation between SOM and soil nutrients such as TN and AP and CEC. Despite the fact that one of the effects of organic matter is to retain cations and protect them from leaching and from removal by runoff, organic matter is inversely correlated with Calcium. This is mainly due to the effect of soil erosion and parent material. In free grazing areas, where soil erosion is more severe than in protected areas, most of the top soil is removed and the soils that were sampled partly included subsoils, highly influenced by lime rich parent

The positive impact of protected areas on the amount and availability of soil nutrients has many influences on the livelihood of the local people near the study sites. The amount of grass produced in a given area increases and the local people get supplemental animal feed and thatching material. The regeneration of indigenous trees is also improved by the increased availability of water and nutrients. Besides, farmers benefit from the protective effect of protected areas. This is mainly because protected areas reduce runon and soil erosion in farm lands located below the protected areas. This in turn leads to an increase in

eroded slopes reduced annual soil erosion from about 15000 to 3000 m3/ km2 over a period of 10 years.

SL = Sandy loam; ns = non significant (at p < 0.05, Tukey HSD).

Table 5. Average (n = 3) soil physical properties of 0-15cm soil depth of the land uses and slope positions

The higher SOM in protected areas also improves the soil physical properties such as soil structure and total soil porosity. This in turn increases the amount of water infiltrated into the soil and decreases the amount of runoff that can be generated from a given amount of rainfall. Water infiltration in the soil is enhanced by both preferential flow along trees roots

eroded slopes reduced annual soil erosion from about 15000 to 3000 m3/ km2 over a

Porosity (%)

5 AC Upper slope 1.11 58.02 57.9 28.6 13.5 SL

**Average value 1.22 53.96**

10 AC Upper slope 1.19 54.75 61.23 24.5 14.22 SL

**Average value 1.23 53.60**

C1 Upper slope 1.34 49.4 62.3 18.68 19.02 SL

**Average value 1.29 51.14**

C2 Upper slope 1.34 49.29 59.64 27.9 12.44 SL

**Average value 1.27 52.02** F value 0.933ns 0.936ns

Table 5. Average (n = 3) soil physical properties of 0-15cm soil depth of the land uses and

The higher SOM in protected areas also improves the soil physical properties such as soil structure and total soil porosity. This in turn increases the amount of water infiltrated into the soil and decreases the amount of runoff that can be generated from a given amount of rainfall. Water infiltration in the soil is enhanced by both preferential flow along trees roots

SL = Sandy loam; ns = non significant (at p < 0.05, Tukey HSD).

slope positions

Middle slope 1.26 52.33 53.1 29.6 17.3 SL

Foot slope 1.28 51.51 52.1 29.6 18.3 SL

Middle slope 1.28 51.62 62.3 22.4 15.3 SL

Foot slope 1.20 54.4 59.9 24.8 15.3 SL

Middle slope 1.26 52.33 62.83 21.34 15.82 SL

Foot slope 1.28 51.68 62.3 21.34 16.35 SL

Middle slope 1.20 54.5 61.24 25.24 13.51 SL

Foot slope 1.26 52.25 58.57 27.38 14.04 SL

Particle size distribution

Sand Silt Clay

(%) Textural

class

period of 10 years.

types slope position Bulk density

(gm/cm3)

Land use

and accumulation of absorbent humus on the soil surface, thereby significantly reducing the volume, velocity, and erosive and leaching capacity of surface runoff (Jiang *et al*., 1996). However, land use for free grazing land is related to soil management practices that have commonly been very destructive to the soil and have caused serious erosion. Therefore, differences in soil erosion control contribute to the significant difference in nutrients in protected areas and free grazing land soils.

In addition, less biomass return causes the reduction of SOM, TN and AP in free grazing lands (Mullar - Harvey *et al*., 1985; Shariff *et al.*, 1994). The most evident impact of grazing on the rangeland ecosystem is removal of a major part of above ground biomass by livestock. Therefore the input of aboveground litter to the soil decreases. Any reduction in litter inputs may have important consequences for soil nutrient conservation and cycling (Shariff *et al*., 1994). Grazing may also have an indirect effect on soil characteristics through change in plant species composition. This is mainly because plant species has a significant impact on decomposition and nutrient cycling at ecosystem levels.

The similar pattern displayed among SOM, TN and AP for multiple comparison of means can be explained by the influence of soil organic matter in nutrient retention and supply (Brubaker *et al*., 1993). Among soil properties, total organic carbon is a sensitive soil quality indicator suggesting that within a narrow range of soil, it may serve as a suitable indicator of soil quality (Mullar-Harvey *et al*., 1985). Moreover, the soil organic matter fraction may offer further insight into soil fertility changes and the sustainability of management history.

The insignificant differences in soil bulk density and porosity among protected areas and free grazing lands can be explained by the coarse texture nature of the study sites and low amount of preexisting soil moisture. Preexisting soil texture and moisture conditions are important variables to consider when investigating the relationship between soil compaction and grazing intensity. The effect of grazing intensity in increasing bulk density is pronounced in wet and fine textured soils. The slight difference found in this study can be explained by their difference in SOM content and compaction of the sites by livestock trampling effect.

The role of soil organic matter in storing and supplying nutrients explains the significant correlation between SOM and soil nutrients such as TN and AP and CEC. Despite the fact that one of the effects of organic matter is to retain cations and protect them from leaching and from removal by runoff, organic matter is inversely correlated with Calcium. This is mainly due to the effect of soil erosion and parent material. In free grazing areas, where soil erosion is more severe than in protected areas, most of the top soil is removed and the soils that were sampled partly included subsoils, highly influenced by lime rich parent material.

The positive impact of protected areas on the amount and availability of soil nutrients has many influences on the livelihood of the local people near the study sites. The amount of grass produced in a given area increases and the local people get supplemental animal feed and thatching material. The regeneration of indigenous trees is also improved by the increased availability of water and nutrients. Besides, farmers benefit from the protective effect of protected areas. This is mainly because protected areas reduce runon and soil erosion in farm lands located below the protected areas. This in turn leads to an increase in crop production (Siriri *et al.*, 2000).

Effect of Slope Position and Land-Use Changes to Bio- Physical Soil

383-411.

55.

20002

449.

9: 403-430.

*African Crop Science Journal* 2: 153-160.

*Ecology and Management.* 99: 133 – 152.

*conservation.* 31: 5-9.

Agroforestry. AFRENA, Kabale, Uganda.

Properties in Nakasongola Pastoral Rangeland Areas, Central Uganda 253

Brubaker, S.C., Jones, A.J., Lewis, D.T., Frank, K., 1993. Soil properties associated with landscape positions. *Soil Science Society of America Journal*, 57: 235-239. Buyinza, M., Kiiza, S., and Nabalegwa, M., 2004. Multiple cropping patterns and

in Nakasonogala district, Uganda. *Journal of the Uganda Society*. 50: 32-45.

Gee, G.W. and Bauder, J. W. 1982. Particle size analysis. In: A. Klute (ed.). Methods of soil

Jiang, Z., Wang, Z. & Liu, Z., 1996. Quantitative study on spatial variation of soil erosion in a

Lal R. 1974. Soil erosion and shifting agriculture. Desertification Control Bulletin, 19, 38-

MWLE (Ministry of Agriculture, Animal Industries and Fisheries), 2000. National

NAADS task Force and Joint Donor Groups. MAAIF, Entebbe pp. 32 - 39 Mullar-Harvey, I., Juo, A.S.R., Wilde, A. 1985. Soil C, N and P after forest clearance: mineralization rates and spatial variability. *Journal of Soil Science*, 36: 585-591. NEMA. 2001. The State of the Environment Report Uganda. National Environment

Olsen, S. R and L. E Sommers. 1982. Phosphorus. In: A. L. Page, et al. (ed.). Methods of soil

Osiru, D.S.O. and Hahn, S.N. K. 1994. Effects of mulching on growth, development and

Siriri,D and Bekunda, M.A., 2001. Soil fertility Management in Uganda: The potential of

Siriri,D., Raussen, T. and Poncelet, P., 2000 . The development potential of Agroforestry

Shariff, A.R., Biondini, M.E., C.E. Grygiel. 1994. Grazing intensity effects on litter

Tucker, N. I. and T.M. Murphy. 1997. The effects of ecological rehabilitations on vegetation

Wischmeier,W.H. 1976. Use and misuse of the Universal Soil Loss Equation. *Jn. Soil Water*

Xiaoliang. 1977. Effect of artificial vegetation on soil conservation of littoral hilly slopes. Diambai country, Kwantung, China. In: *Desertification Control Bulletin*, 19: 38-55.

technologies in Southwestern Uganda. Agroforestry Trends 2000.

FAO, 1986. Highland reclamation study, Final report, Vol. 1 and 2. FAO, Rome. FAO, 1995. Global and national soils and terrain digital database (SOTER). Fournier, R. 1967. Research on soil erosion in Africa. Aff. Soils, 12: 53-96.

*Conservation*, 2: 1-9, (in Chinese, with English abstract).

domestication of woodland fruit tree species to improve household food security

analysis: part 1. Physical and mineralogical methods. ASA Monograph number 9:

small watershed in the loess hilly region. *Journal of Soil Erosion and Soil and Water*

Agricultural Advisory Services Programme (NAADS); Master Document of the

Management Authority, Ministry of Water, Lands and Environment, Kampala.

analysis: part 2. Chemical and microbiological properties. ASA Monograph number

yield of white yam. African Crop Science Society, Makerere University, Uganda.

decomposition and soil nitrogen mineralization. J. Range Management. 47: 444 –

recruitment: some observations from the wet Tropics of North Queensland. *Forest*

#### **4.2 Effect of slope position**

Because the different slope positions have similarities in vegetal canopy and ground vegetation cover (table 1), the variation in soil nutrients at different slope positions in protected areas can only be explained through the difference in soil erosion rates. Results from this study showed that upper slope positions in protected areas had higher rate of soil erosion. This is mainly due to the steepness of the US positions compared to MS and FS positions. Nutrients in the upper slope positions can be dissolved and washed by runoff and might be deposited in the middle and foot slope positions.

However, slope position effect in free grazing lands was inconsistent. This can be explained by differences in vegetation removal, related to accessibility by livestock and human beings; depending on the position of villages nearby. This difference creates a difference in C input which influences SOM content, and thereby related soil nutrients such as total Nitrogen and available Phosphorus. The amount of predicted soil loss was not also consistent like in protected areas where it showed a decreasing trend from US to FS position. Hence, there is no clear trend among the slope positions in free grazing lands.

#### **5. Conclusions**

This study assessed the effects of land-use conversion and slope position on soil chemical and physical properties. Significant differences (at p < 0.05) between land uses were found for SOM, TN and AP and exchangeable bases and CEC. Five and ten years' protected areas had higher levels for SOM, TN, AP and CEC compared to free grazing lands. The difference in soil nutrients content between the land use types is mainly due to differences in soil erosion and biomass return.

From the technical point of view, under the present land use management and climate conditions of the study area, free grazing areas in hilly lands should be changed to protected areas before soil organic matter and other nutrient contents are depleted more. Besides, the erosion processes may be very active resulting in further degradation. However, the socioeconomic dimensions of protecting free grazing lands and changing them into protected areas (for example, impact on livestock and crop production in short run) should also be considered before making a decision.

#### **6. Acknowledgement**

Funds for this study were provided by Ministry of Water, Lands and Environment, Plan for Modernization of Agriculture (PMA). We thank the Soil laboratory technicians of both Kyambogo and Makerere University, Nakasongola district authorities and farmers for their constructive input and assistance.

#### **7. References**

Blake, G.R and K.H Hartge, 1986. Bulk density. In: A. Klute (ed.). Methods of soil analysis: part 1. Physical and mineralogical methods. ASA Monograph number 9:363-375.

Bremmer, J.M. and C. S. Mulvaney. 1982. Nitrogen total. In: A. L. Page, *et a*l. (ed.). Methods of soil analysis: part 2. Chemical and microbiological properties. ASA Monograph 9: 595-624.

Because the different slope positions have similarities in vegetal canopy and ground vegetation cover (table 1), the variation in soil nutrients at different slope positions in protected areas can only be explained through the difference in soil erosion rates. Results from this study showed that upper slope positions in protected areas had higher rate of soil erosion. This is mainly due to the steepness of the US positions compared to MS and FS positions. Nutrients in the upper slope positions can be dissolved and washed by runoff and

However, slope position effect in free grazing lands was inconsistent. This can be explained by differences in vegetation removal, related to accessibility by livestock and human beings; depending on the position of villages nearby. This difference creates a difference in C input which influences SOM content, and thereby related soil nutrients such as total Nitrogen and available Phosphorus. The amount of predicted soil loss was not also consistent like in protected areas where it showed a decreasing trend from US to FS position. Hence, there is

This study assessed the effects of land-use conversion and slope position on soil chemical and physical properties. Significant differences (at p < 0.05) between land uses were found for SOM, TN and AP and exchangeable bases and CEC. Five and ten years' protected areas had higher levels for SOM, TN, AP and CEC compared to free grazing lands. The difference in soil nutrients content between the land use types is mainly due to differences in soil

From the technical point of view, under the present land use management and climate conditions of the study area, free grazing areas in hilly lands should be changed to protected areas before soil organic matter and other nutrient contents are depleted more. Besides, the erosion processes may be very active resulting in further degradation. However, the socioeconomic dimensions of protecting free grazing lands and changing them into protected areas (for example, impact on livestock and crop production in short run) should also be

Funds for this study were provided by Ministry of Water, Lands and Environment, Plan for Modernization of Agriculture (PMA). We thank the Soil laboratory technicians of both Kyambogo and Makerere University, Nakasongola district authorities and farmers for their

Blake, G.R and K.H Hartge, 1986. Bulk density. In: A. Klute (ed.). Methods of soil analysis: part 1. Physical and mineralogical methods. ASA Monograph number 9:363-375. Bremmer, J.M. and C. S. Mulvaney. 1982. Nitrogen total. In: A. L. Page, *et a*l. (ed.). Methods

of soil analysis: part 2. Chemical and microbiological properties. ASA Monograph

**4.2 Effect of slope position**

**5. Conclusions**

erosion and biomass return.

**6. Acknowledgement**

**7. References**

considered before making a decision.

constructive input and assistance.

9: 595-624.

might be deposited in the middle and foot slope positions.

no clear trend among the slope positions in free grazing lands.

**Erosion Control**

Yost, D. and Eswaran, H. 1990. Major Land Resources Areas of Uganda. Soil Management Services, USAID, Washington D.C., USA 218p. **Part 4**  Services, USAID, Washington D.C., USA 218p. **Part 4**

**Erosion Control**

254 Soil Erosion Issues in Agriculture

Yost, D. and Eswaran, H. 1990. Major Land Resources Areas of Uganda. Soil Management

**11**

**Pastoral Hill Slope Erosion in New Zealand**

**1. Introduction**

significantly across the country.

**1.1 The geology of New Zealand and its contribution to erosion**

every organisational level, of repeated erosion and flooding events.

New Zealand is a predominantly hilly and mountainous country. An area of 18 million hectares (69% of the country) has slopes greater than 12°, and is commonly called 'hill country'. This is further divided into 'hill-land' (12–28°) and 'steepland' (if slope exceeds 28°) (DSIR 1980). The range of slope and elevation, coupled with a wide latitudinal range, a mid-oceanic setting encompassing subtropical to cool temperate climates, and complex geologic and tectonic regimes means that New Zealand's 'hill country' is physically diverse. As a consequence of this diversity, the productive potential of New Zealand's' hill country, and its response to climatic events, land use pressure and environmental change varies

Erosion is a significant environmental issue facing agricultural and forestry land uses in large parts of the hill country of New Zealand. It causes both on-site (loss of soil productive capacity and water holding capacity) and off-site (declining water quality, river aggradation, increased vulnerability of infrastructure to severe climatic events) effects. As well as the environmental costs there are major financial risks and social consequences, at

Of the 6.3 million hectares in the North Island, the majority is developed on soft rock and crushed soft rock terrain in the south-east and west of the island (2 825 000 ha, 45%). Volcanic ash and loess-mantled terrain comprises another 23% (1 456 000 ha), largely on the periphery of the Central Volcanic Zone. Hard rock hill country, exclusive of the igneous hard rock hill country, is largely concentrated on the margins of the axial ranges (919 000 ha, 14.5%), whereas the hill country developed on deeply weathered sedimentary and igneous rocks (863 000 ha, 13.6%) is located predominantly in Northland and on the Coromandel Peninsula. In the North Island approximately 200 000 ha has a mapped erosion severity of severe, very severe or extreme. The North Island hill country is dominated by mass movement erosion. Shallow landslide (soil slip) and sheet erosion are the most widely

**and the Role of Poplar and**

**Willow Trees in Its Reduction**

Ian McIvor1, Grant Douglas2, John Dymond3,

Garth Eyles4 and Michael Marden3

*1Plant & Food Research Ltd,*

*4Erosion Management Consultant*

*2AgResearch Ltd, 3Landcare Research Ltd,*

*New Zealand*

## **Pastoral Hill Slope Erosion in New Zealand and the Role of Poplar and Willow Trees in Its Reduction**

Ian McIvor1, Grant Douglas2, John Dymond3, Garth Eyles4 and Michael Marden3 *1Plant & Food Research Ltd, 2AgResearch Ltd, 3Landcare Research Ltd, 4Erosion Management Consultant New Zealand*

#### **1. Introduction**

#### **1.1 The geology of New Zealand and its contribution to erosion**

New Zealand is a predominantly hilly and mountainous country. An area of 18 million hectares (69% of the country) has slopes greater than 12°, and is commonly called 'hill country'. This is further divided into 'hill-land' (12–28°) and 'steepland' (if slope exceeds 28°) (DSIR 1980). The range of slope and elevation, coupled with a wide latitudinal range, a mid-oceanic setting encompassing subtropical to cool temperate climates, and complex geologic and tectonic regimes means that New Zealand's 'hill country' is physically diverse. As a consequence of this diversity, the productive potential of New Zealand's' hill country, and its response to climatic events, land use pressure and environmental change varies significantly across the country.

Erosion is a significant environmental issue facing agricultural and forestry land uses in large parts of the hill country of New Zealand. It causes both on-site (loss of soil productive capacity and water holding capacity) and off-site (declining water quality, river aggradation, increased vulnerability of infrastructure to severe climatic events) effects. As well as the environmental costs there are major financial risks and social consequences, at every organisational level, of repeated erosion and flooding events.

Of the 6.3 million hectares in the North Island, the majority is developed on soft rock and crushed soft rock terrain in the south-east and west of the island (2 825 000 ha, 45%). Volcanic ash and loess-mantled terrain comprises another 23% (1 456 000 ha), largely on the periphery of the Central Volcanic Zone. Hard rock hill country, exclusive of the igneous hard rock hill country, is largely concentrated on the margins of the axial ranges (919 000 ha, 14.5%), whereas the hill country developed on deeply weathered sedimentary and igneous rocks (863 000 ha, 13.6%) is located predominantly in Northland and on the Coromandel Peninsula. In the North Island approximately 200 000 ha has a mapped erosion severity of severe, very severe or extreme. The North Island hill country is dominated by mass movement erosion. Shallow landslide (soil slip) and sheet erosion are the most widely

Pastoral Hill Slope Erosion in New Zealand

**2. Hill country erosion control practices**

and the Role of Poplar and Willow Trees in Its Reduction 259

In contrast, gully (e.g. De Rose et al. 1998, Betts et al. 2003; Marden et al. 2005; Parkner et al. 2006, 2007, Fuller & Marden 2011, Marden et al. 2011) and earthflow erosion (e.g. McConchie 1986; Zhang et al. 1991, 1993; Trotter 1993, Marden et al. 2008) are far less well studied. However, in at least some East Coast areas gullies cover much less area than landslides but make a far larger contribution to sediment yield as low-magnitude/high

New Zealand is unique in the way it uses its steep and often unstable hill country for pastoral farming, grazing predominantly sheep and beef cattle but also deer. Stock remain

**2.1 History of using poplar and willow for erosion management in pastoral hill country**  Following deforestation (1.1), pastoral hill country erosion became so severe, the Water and Soil Conservation Act was passed by the New Zealand Government in 1941. This Act set in place an administrative structure of Catchment Boards tasked with controlling soil erosion and flooding, directed by central government policies. Central government funded applied research and grants to Catchment Boards till 1988. In 1988 the Catchment Boards became Regional or Unitary Councils responsible for managing a wide range of environmental activities including soil erosion and flood control. A new act, the Resource Management Act

Erosion control measures for pastoral hill country were required to be established in the presence of the grazing animal, with permanent retirement from grazing recommended only in the most extreme situations. Only tree-based control measures were affordable on the scale required. The main beneficial effects of trees on the mass stability of the slopes are well described by Gray and Sotir (1996, pp 59-61). The two most suitable tree species proved to be poplar (*Populus* spp) and willow (*Salix* spp). Both species are readily established from large poles in the presence of stock, with a minimum of protection, are able to be produced cheaply in nurseries, and are easily transported and planted on steep unstable hill country. They grow quickly, can tolerate wet soil conditions for long periods, and do not shade pasture growth to any degree unless planted at close spacing. They are protected from stock browse by plastic sleeves placed over the poles at planting time which are removed after 4 or 5 years when the bark is able to resist stock rubbing and stripping. Their added values to the pastoral site through shade, shelter, quality fodder (especially during drought periods)

frequency rainfall events activate gully erosion (Page et al. 2000, Marden et al. 2008).

on the hills all year round, except in the high country during winter.

(RMA) of 1991, is now the guiding legislation for erosion control.

and carbon sequestration, increase their utilisation by landowners.

**2.2 Using poplar and willow for erosion control on pastoral hill country**

against landslides. Control of other erosion types will be covered briefly.

This section describes recommended practices employed by landowners or land managers to prevent or reduce the more prevalent forms of erosion on pastoral hill land. Usually the erosion control is operating where earlier erosion events have occurred. Almost all pastoral hill slopes in New Zealand show signs of past erosion events. Landslide erosion control is given most attention for the reasons given in 1.1. Landslides affect much greater areas of land, are more widespread geographically, and have significant economic impact (see 3.1). The focus of the root research activity described later in this chapter is on slope protection

distributed followed by earthflow and gully erosion. Sediment yield is highest in the northern East Coast Region, and high throughout much of the rest of the East Coast, inland Taranaki and parts of Northland. Mean sediment yields are highest in the crushed soft rock hill country and hilly steeplands, and steep soft rock hill country (Basher et al., 2008).

In contrast to the North Island, 56% of the South Island's 3.7 million hectares of hill country is developed on hard rock terrain. Only 569 000 ha is developed on soft rocks. Steep weathered hill country is associated with both schist and greywacke. In the South Island less than 103 000 ha has an erosion severity ranking of severe, very severe or extreme. Unlike the North Island, the South Island is dominated by surface erosion types. Sheet and soil slip erosion are the most common forms of erosion in the South Island hillcountry. Tunnel gullying is far more common in the South Island while gully and earthflow erosion are far less common. Sediment yields tend to be far lower in the South Island and are highest on the West Coast because of the higher rainfall. Mean sediment yields are highest in the steep soft rock hill country (Basher et al. 2008).

#### **1.2 Erosion processes on Hill country**

New Zealand is widely recognised as having high rates of both natural and man-induced erosion. The extent and type of natural erosion is determined by the complex interplay between geology (rock type, weathering, structure, and regional plate tectonics), climate (particularly rainfall amounts and intensities, frequency of large storms) and vegetation, producing regional variation in the susceptibility of the land to erosion. Severe earthflow and gully erosion are closely related to the extensive areas of mudstone and crushed rock terrain in the North Island, whereas in the South Island gully erosion is largely associated with hill country developed on easily eroded soft mudstone, sandstone, weakly consolidated conglomerate, and regional fault and crush zones. Annual rainfall is the dominant factor influencing rates of erosion, as measured by suspended sediment yields, and numerous studies point to the importance of large storms as triggers for widespread landsliding (Griffiths 1981, 1982; Hicks et al. 1996).

Imposed on these natural drivers of erosion is the effect of historical deforestation and land use which has increased erosion to a greater or lesser degree depending on the lands' inherent susceptibility to erosion. Numerous studies demonstrate the linkage between increased sedimentation rates, European settlement and deforestation (e.g. Page et al. 1994a, b). Deforestation can lead to at least a 10-fold increase in erosion both in the long term and during large storms, while short-term sediment yield increases of up to 100 times after forest harvesting have been recorded (e.g. Fahey et al. 1993). Much of the increase occurs when the tree roots lose their strength, about 2–5 years after forest removal. However, the impact of deforestation persists in many deforested areas with thick regolith or soft rocks which continue to erode faster than similar forested areas for at least a century (Page et al. 2000).

Landslides are the best studied erosion process, are very widespread, and a major contributor to hillslope erosion and sediment yield. Crozier (2005) concludes that most common landslide events are triggered by storm rainfall with critical intensities governed by the prevailing antecedent moisture conditions, rainfall duration and amount. Since the onset of European deforestation, increased sediment production over much of New Zealand has largely been determined by landslides (Glade 2003). On unstable slopes, thousands of landslides can be triggered by high-magnitude/low-frequency climatic events during storms with estimated return periods in excess of 50 years (e.g. Glade 1997, 1998, 2003).

distributed followed by earthflow and gully erosion. Sediment yield is highest in the northern East Coast Region, and high throughout much of the rest of the East Coast, inland Taranaki and parts of Northland. Mean sediment yields are highest in the crushed soft rock

In contrast to the North Island, 56% of the South Island's 3.7 million hectares of hill country is developed on hard rock terrain. Only 569 000 ha is developed on soft rocks. Steep weathered hill country is associated with both schist and greywacke. In the South Island less than 103 000 ha has an erosion severity ranking of severe, very severe or extreme. Unlike the North Island, the South Island is dominated by surface erosion types. Sheet and soil slip erosion are the most common forms of erosion in the South Island hillcountry. Tunnel gullying is far more common in the South Island while gully and earthflow erosion are far less common. Sediment yields tend to be far lower in the South Island and are highest on the West Coast because of the higher rainfall. Mean sediment yields are highest in the steep soft

New Zealand is widely recognised as having high rates of both natural and man-induced erosion. The extent and type of natural erosion is determined by the complex interplay between geology (rock type, weathering, structure, and regional plate tectonics), climate (particularly rainfall amounts and intensities, frequency of large storms) and vegetation, producing regional variation in the susceptibility of the land to erosion. Severe earthflow and gully erosion are closely related to the extensive areas of mudstone and crushed rock terrain in the North Island, whereas in the South Island gully erosion is largely associated with hill country developed on easily eroded soft mudstone, sandstone, weakly consolidated conglomerate, and regional fault and crush zones. Annual rainfall is the dominant factor influencing rates of erosion, as measured by suspended sediment yields, and numerous studies point to the importance of large storms as triggers for widespread

Imposed on these natural drivers of erosion is the effect of historical deforestation and land use which has increased erosion to a greater or lesser degree depending on the lands' inherent susceptibility to erosion. Numerous studies demonstrate the linkage between increased sedimentation rates, European settlement and deforestation (e.g. Page et al. 1994a, b). Deforestation can lead to at least a 10-fold increase in erosion both in the long term and during large storms, while short-term sediment yield increases of up to 100 times after forest harvesting have been recorded (e.g. Fahey et al. 1993). Much of the increase occurs when the tree roots lose their strength, about 2–5 years after forest removal. However, the impact of deforestation persists in many deforested areas with thick regolith or soft rocks which continue to erode faster than similar forested areas for at least a century (Page et al. 2000). Landslides are the best studied erosion process, are very widespread, and a major contributor to hillslope erosion and sediment yield. Crozier (2005) concludes that most common landslide events are triggered by storm rainfall with critical intensities governed by the prevailing antecedent moisture conditions, rainfall duration and amount. Since the onset of European deforestation, increased sediment production over much of New Zealand has largely been determined by landslides (Glade 2003). On unstable slopes, thousands of landslides can be triggered by high-magnitude/low-frequency climatic events during storms with estimated return periods in excess of 50 years (e.g. Glade 1997, 1998, 2003).

hill country and hilly steeplands, and steep soft rock hill country (Basher et al., 2008).

rock hill country (Basher et al. 2008).

**1.2 Erosion processes on Hill country**

landsliding (Griffiths 1981, 1982; Hicks et al. 1996).

In contrast, gully (e.g. De Rose et al. 1998, Betts et al. 2003; Marden et al. 2005; Parkner et al. 2006, 2007, Fuller & Marden 2011, Marden et al. 2011) and earthflow erosion (e.g. McConchie 1986; Zhang et al. 1991, 1993; Trotter 1993, Marden et al. 2008) are far less well studied. However, in at least some East Coast areas gullies cover much less area than landslides but make a far larger contribution to sediment yield as low-magnitude/high frequency rainfall events activate gully erosion (Page et al. 2000, Marden et al. 2008).

#### **2. Hill country erosion control practices**

New Zealand is unique in the way it uses its steep and often unstable hill country for pastoral farming, grazing predominantly sheep and beef cattle but also deer. Stock remain on the hills all year round, except in the high country during winter.

#### **2.1 History of using poplar and willow for erosion management in pastoral hill country**

Following deforestation (1.1), pastoral hill country erosion became so severe, the Water and Soil Conservation Act was passed by the New Zealand Government in 1941. This Act set in place an administrative structure of Catchment Boards tasked with controlling soil erosion and flooding, directed by central government policies. Central government funded applied research and grants to Catchment Boards till 1988. In 1988 the Catchment Boards became Regional or Unitary Councils responsible for managing a wide range of environmental activities including soil erosion and flood control. A new act, the Resource Management Act (RMA) of 1991, is now the guiding legislation for erosion control.

Erosion control measures for pastoral hill country were required to be established in the presence of the grazing animal, with permanent retirement from grazing recommended only in the most extreme situations. Only tree-based control measures were affordable on the scale required. The main beneficial effects of trees on the mass stability of the slopes are well described by Gray and Sotir (1996, pp 59-61). The two most suitable tree species proved to be poplar (*Populus* spp) and willow (*Salix* spp). Both species are readily established from large poles in the presence of stock, with a minimum of protection, are able to be produced cheaply in nurseries, and are easily transported and planted on steep unstable hill country. They grow quickly, can tolerate wet soil conditions for long periods, and do not shade pasture growth to any degree unless planted at close spacing. They are protected from stock browse by plastic sleeves placed over the poles at planting time which are removed after 4 or 5 years when the bark is able to resist stock rubbing and stripping. Their added values to the pastoral site through shade, shelter, quality fodder (especially during drought periods) and carbon sequestration, increase their utilisation by landowners.

#### **2.2 Using poplar and willow for erosion control on pastoral hill country**

This section describes recommended practices employed by landowners or land managers to prevent or reduce the more prevalent forms of erosion on pastoral hill land. Usually the erosion control is operating where earlier erosion events have occurred. Almost all pastoral hill slopes in New Zealand show signs of past erosion events. Landslide erosion control is given most attention for the reasons given in 1.1. Landslides affect much greater areas of land, are more widespread geographically, and have significant economic impact (see 3.1). The focus of the root research activity described later in this chapter is on slope protection against landslides. Control of other erosion types will be covered briefly.

Pastoral Hill Slope Erosion in New Zealand

**2.2.4 Gully and tunnel gully erosion**

**2.2.3 Slump erosion control**

movement will reduce.

**2.2.5 Stream bank erosion**

size and reduce risk of bank destabilisation.

consequences of silt incursion into waterways.

**3.1 Effect of hill slope erosion on pasture production**

and the Role of Poplar and Willow Trees in Its Reduction 261

These events are usually sufficiently large that tree planting alone will not control them. They require surface water to be drained, the surface smoothed, retired from grazing and close planted in trees. Once in trees they need to be managed as continuing movement will overturn or topple trees requiring clearing to keep drainage open, and replanting. Over time

Discontinuous gullies are very common in pastoral hill country. Large erosion events fill the valley bottoms and smaller events begin to scour them. Control of further erosion is achieved by pair or single planting willows up the valley bottom at 15-20 m spacings and, at points with serious erosion, developing a planted block with 2-3 rows of willows planted at 1.5m spacings across the valley bottom and in an area fenced out from grazing. Erosion from small gullies has been successfully arrested by construction of debris dams at intervals up the gully coupled with pair planting of willows. The willow root mats growing across the gully bed and over the surface of the dam cover and isolate the eroding surface. Drainage lines supplying the gully are pair planted with tree willows at 15-20 m spacings and measures taken to ensure the grassed surface is not broken, allowing a gully head to form. Terrace edges are protected by either grassed waterways or constructed flumes. To stabilise very large gullies often requires the whole catchment to be retired from grazing and planted in closed canopy trees. Where climate allows trees to grow, tunnel gully erosion is controlled by planting a poplar or willow pole in the collapsed tunnels. Further tunnel erosion is prevented by planting poles in the characteristic sunken drainage lines supplying

the tunnel. The tree roots hold up the sediment gradually filling up the holes.

**3. Effectiveness of trees in reducing erosion on pastoral slopes**

In hill country, storm events create flash floods resulting in bank scour and deepening the channel to form a gully. Stream bank protection is achieved by planting willows or poplars along the banks. The roots stabilise the banks and protect the stream bed from degrading. Large limbs are removed from mature stream bank trees to rejuvenate them, reduce their

Knowledge of the effectiveness of trees in stabilising soil and reducing erosion potential on slopes is important when considering time scales, costs, landowner attitudes to topsoil displacement and loss and the achievement of catchment scale objectives for managing

Landslide erosion results in immediate and often dramatic reductions in pasture production on steep hill country (Lambert et al. 1984). Reductions in land productivity from soil erosion occur directly through the loss of topsoil, and indirectly through reduced pasture yields on eroded ground (Blaschke et al. 1992). Quantification of economic and biophysical losses associated with landslide erosion is crucial to justify the need for soil conservation and erosion control activities on steep hill country. An understanding of the rate of recovery of

#### **2.2.1 Landslide erosion control**

Landslides of the nature seen in figure 1 (L) follow a major rainstorm event and can happen at any stage of the year. Herbaceous plant cover alone on these pastoral hill slopes offers insufficient resistance to landsliding. To prevent the occurrence of landslide, or to prevent further landsliding, willow or poplar poles are planted up to 15 m spacings across the slope in a regular pattern or, more usually, where further landsliding is considered most likely to occur (Figure 1 R). Landslide prone soils are often shallow and exposed to climatic extremes.

Fig. 1. Severe slipping on soft sandstone (L); young space planted poplars reduce the risk of further erosion on an unstable slope (R).

The recommended approach is to plant a mix of clones and actively manage them (e.g. protection, replace dead trees, no cattle exposure for 2-3 years) for the first five years of growth to ensure a healthy tree develops. Willows are usually preferred in the wetter areas and lower on the slope where they are better adapted and the soil moisture is higher, while poplars are favoured further up the slope. Spacings are reduced where there is water ponding (Hathaway 1986). Pole planting is done in winter, either by ramming the sharpened pole directly into the moist soil or planting into an augured hole. Newly planted poles are revisited in summer to ram contracted soil around the pole. Survival rates of 90% are often achieved for poplars and willows planted from poles. Any dead poles are replaced the following winter. Tree death from animal damage is reduced by covering the poles with a plastic sleeve and excluding cattle from planted areas up to three years. Where the potential for slip erosion is very severe to extreme a closed canopy tree cover is recommended. However, this requires permanent retirement from pasture, a change many farmers are not willing to undertake.

#### **2.2.2 Earthflow erosion control**

Willows are preferable to poplars for slowing earthflows as their roots form thick mats when in a wet environment. Earthflow control measures rely on removing surface water to minimise infiltration by surface smoothing and constructing diversion banks, tying together the surface with willow roots, pair planting willows where gullies are likely to form and planting at the toe of the movement to hold up the toe (essential when the toe is being undercut by a waterway). Spacing is closer for wetter or more active earthflows.

#### **2.2.3 Slump erosion control**

260 Soil Erosion Issues in Agriculture

Landslides of the nature seen in figure 1 (L) follow a major rainstorm event and can happen at any stage of the year. Herbaceous plant cover alone on these pastoral hill slopes offers insufficient resistance to landsliding. To prevent the occurrence of landslide, or to prevent further landsliding, willow or poplar poles are planted up to 15 m spacings across the slope in a regular pattern or, more usually, where further landsliding is considered most likely to occur (Figure 1 R). Landslide prone soils are often shallow and exposed to

Fig. 1. Severe slipping on soft sandstone (L); young space planted poplars reduce the risk of

The recommended approach is to plant a mix of clones and actively manage them (e.g. protection, replace dead trees, no cattle exposure for 2-3 years) for the first five years of growth to ensure a healthy tree develops. Willows are usually preferred in the wetter areas and lower on the slope where they are better adapted and the soil moisture is higher, while poplars are favoured further up the slope. Spacings are reduced where there is water ponding (Hathaway 1986). Pole planting is done in winter, either by ramming the sharpened pole directly into the moist soil or planting into an augured hole. Newly planted poles are revisited in summer to ram contracted soil around the pole. Survival rates of 90% are often achieved for poplars and willows planted from poles. Any dead poles are replaced the following winter. Tree death from animal damage is reduced by covering the poles with a plastic sleeve and excluding cattle from planted areas up to three years. Where the potential for slip erosion is very severe to extreme a closed canopy tree cover is recommended. However, this requires permanent retirement from pasture, a change many

Willows are preferable to poplars for slowing earthflows as their roots form thick mats when in a wet environment. Earthflow control measures rely on removing surface water to minimise infiltration by surface smoothing and constructing diversion banks, tying together the surface with willow roots, pair planting willows where gullies are likely to form and planting at the toe of the movement to hold up the toe (essential when the toe is being

undercut by a waterway). Spacing is closer for wetter or more active earthflows.

**2.2.1 Landslide erosion control**

further erosion on an unstable slope (R).

farmers are not willing to undertake.

**2.2.2 Earthflow erosion control**

climatic extremes.

These events are usually sufficiently large that tree planting alone will not control them. They require surface water to be drained, the surface smoothed, retired from grazing and close planted in trees. Once in trees they need to be managed as continuing movement will overturn or topple trees requiring clearing to keep drainage open, and replanting. Over time movement will reduce.

#### **2.2.4 Gully and tunnel gully erosion**

Discontinuous gullies are very common in pastoral hill country. Large erosion events fill the valley bottoms and smaller events begin to scour them. Control of further erosion is achieved by pair or single planting willows up the valley bottom at 15-20 m spacings and, at points with serious erosion, developing a planted block with 2-3 rows of willows planted at 1.5m spacings across the valley bottom and in an area fenced out from grazing. Erosion from small gullies has been successfully arrested by construction of debris dams at intervals up the gully coupled with pair planting of willows. The willow root mats growing across the gully bed and over the surface of the dam cover and isolate the eroding surface. Drainage lines supplying the gully are pair planted with tree willows at 15-20 m spacings and measures taken to ensure the grassed surface is not broken, allowing a gully head to form. Terrace edges are protected by either grassed waterways or constructed flumes. To stabilise very large gullies often requires the whole catchment to be retired from grazing and planted in closed canopy trees. Where climate allows trees to grow, tunnel gully erosion is controlled by planting a poplar or willow pole in the collapsed tunnels. Further tunnel erosion is prevented by planting poles in the characteristic sunken drainage lines supplying the tunnel. The tree roots hold up the sediment gradually filling up the holes.

#### **2.2.5 Stream bank erosion**