Facts about Garden Fertilizer – You can make your own


Fertilizers provide plants with nutrients; they are commonly listed in catalogues under: Straight fertilizers, Compound fertilizers, Liquid fertilizers.

Straight fertilizers

These are used to supply a specific nutrient. If you wish to make your spring cabbages grow away more quickly in the spring you could top-dress with Nitro-chalk. If your tomatoes are not ripening quickly enough in dull weather sulphate of potash could help. Straight fertilizers are either inorganic `artificials’ or `organics’.

Artificials, which may be manufactured in factories, or are the purified salts from natural underground deposits, are more correctly known as inorganic fertilizers. They generally dissolve easily in water and when applied to moist soils act quickly. But this does not mean that they will be washed out of your soil. Both phosphates and potash are absorbed by soil constituents and so is nitrogen when applied as ammonium fertilizers. Nitrates, however, may be lost from the soil if applied during the winter or too far away from plant roots. Hence the need for top dressings.

Most inorganic fertilizers are fairly concentrated and we know exactly how much of each nutrient is present in any weight of fertilizer and so you can calculate how much fertilizer to apply to the soil to provide a desired quantity of any particular nutrient. But great care must be taken in handling them because being concentrated, overdoses are often harmful.

‘Organics’ are of animal or vegetable origin and their nutrients are locked away inside the complex structure of proteins and other materials. They must break down into soluble forms—nitrates or in some instances ammonia—before they can be used by plants. Since bacteria and other living organisms in the soil break these down, their effectiveness largely depends upon the soil conditions being satisfactory for the organisms; they are most effective when used in moist, well-aerated, well-limed soils. Many are fairly concentrated, but their nutrient content often varies.

Fine dusty particles break down much more quickly than the coarse fragments in organic fertilizers. For example a fine grade of hoof-and-horn meal works very nearly as quickly as some inorganic fertilizers, while coarse particles break down slowly and release their nitrogen over a long period of time.

Synthetic organics such as Urea-Form are chemical combinations of urea and formaldehyde and are designed to give a slow release of nitrogen for several months. Their granules are almost insoluble and do not break down in the soil. The outer surface of each granule is gradually worn away in much the same way as you would suck a sweet. The process begins within a few days in warm moist soils and continues to release nutrients as the plants need them for long pre-determined periods. So, a ‘one-shot’ application may nourish plants throughout the growing season, whereas several applications of quickly available forms may be necessary.

Fruits and vegetables, properly fertilized with quickly available forms of nutrients are as healthful and tasty as those fertilized with slowly available forms; plants obtain their nutrients as simple chemicals through the roots. And these chemicals are exactly the same whether they come from an ‘artificial’ or an ‘organic’ fertilizer.

Compound fertilizers

These contain two, or more usually, all three of the major nutrients, nitrogen, phosphorus and potassium. These are the ones that are used by plants in the largest amounts and are, therefore, most likely to be deficient in soils. When you buy a fertilizer, therefore, you generally buy it for its content of these nutrients in order to give a balanced feed before sowing or planting.

You can make your own compounds by simply mixing together two or more straight fertilizers. The following are often made at home:

A fertilizer for general use:

Sulphate of ammonia 5 parts by weight Superphosphate 5 parts by weight Sulphate of potash 2 parts by weight

Analysis 8% N, 8% P205 8% K20.

John Innes Base Fertilizer for potted plants :

Hoof and Horn meal 2 parts by weight Superphosphate 2 parts by weight Sulphate of potash 1 part by weight

Analysis 5.1% N, 7.2% P205, 9.6% K20


When you buy a fertilizer you want to know how much of each plant food it contains and this information is given in the analysis provided with the fertilizer. The analysis states the content of nitrogen as N, phosphorus as P205 (phosphoric acid) and potassium as K20 (potash). Plants do not absorb their plant foods in these forms, since nitrogen is an inert gas, pure phosphorus and potassium are very active chemically and burn if exposed to the air or water. So these elements can only be absorbed by plants when combined with other elements to form materials suitable for use in fertilizers. So the figures mean that the fertilizer contains the equivalent of the elements.

As an example, nitrate of soda contains 16% of nitrogen; what does the remainder consist of? The chemical name for nitrate of soda is sodium nitrate which is a chemical compound of nitrogen, sodium and oxygen. So there is about 26% of sodium and 58% of oxygen in this compound.

You are advised to read the labels very carefully and look for the analysis very carefully in order to save disappointment and money by avoiding ‘miracle’ or `wonder-working’ fertilizers bearing no guaranteed analysis.

Ready for use compounds

Garden supply shops offer for sale a wide variety of materials for feeding garden plants and lawns. Some of these products are much more expensive than others. They vary in price because of:

Nutrient content

Fertilizers with a high percentage of plant nutrients cost more per pound than those containing a small percentage of nutrients. So always find out from the supplier what the guaranteed content of nitrogen phosphoric acid and potash is. The plant nutrient content of a compound is often indicated by its grade—a series of three numbers separated by dashes. The numbers show the percentage of nitrogen, phosphoric acid and potash, in that order, contained in the product.

Form Pelleted or granular fertilizers, and soluble fertilizer concentrates cost more than powdered fertilizers. But the granular form may be more convenient for you to use.

Powdered fertilizers often contain a lot of very fine dusty material which may blow away or be objectional to use on a windy day. They may become damp more easily and may cake and fail to spread properly through your fertilizer spreader.

Granular fertilizers are not as dusty as powdered fertilizers and they do not cake so easily; they flow freely through fertilizer spreaders. The granules roll off plant foliage, reducing the danger of fertilizer burn.

Ingredients Nitrogen is the most expensive ingredient in a fertilizer compound. Slowly available forms derived from organic sources and Urea-Form are more expensive than the quickly available forms. So, the more nitrogen a compound contains—especially slowly available forms of nitrogen—the more expensive the product is.

Added materials

Products that contain added trace elements, pesticides or herbicides cost more than plain fertilizers. Fertilizer weed killer combinations are generally prepared for use on lawns. These combinations can be quite satisfactory if:

  1. the best time for applying the fertilizers and the best time for applying the weedkiller are the same.
  2. the nutrient content and the weedkiller concentration of the mixture are adjusted so that each is applied at the proper rate.

Package size

Fertilizers sold in small containers cost more per pound than the same product in larger packages.

Liquid fertilizers are simply fertilizers in solution; if you mix 28g (1 oz) of sulphate of ammonia with 41 (1 gall) of water you have a very weak solution containing nitrogen. You can make up feeds to your own prescription, using concentrated chemicals such as potassium nitrate, ammonium phosphate and urea, but this does require a fair amount of technical knowledge and it is usually more convenient to purchase one of the readymade products which are of two main types:

Concentrated liquids that have to be diluted with water according to the maker’s instructions. Mixtures of solid chemicals for dissolving in water. These are the cheapest since you do not have to pay for the cost of transporting the water.

Liquid compound fertilizers are much more expensive than solids, as they have to be manufactured from purer materials. But they are popular for the ease and speed with which nutrients can be applied in balanced form to meet the changing needs of plants at different stages of growth and weather conditions. Also feeding and watering is done in one operation—a distinct advantage when there are large batches of plants to be dealt with.

But they are not necessarily better than solid feeds, merely more convenient, and of course liquid feeding is essential when using certain forms of irrigation equipment.

Some facts to remember when using fertilizers

Never guess at amounts; overdoses can be harmful or even fatal to plants; too little may be ineffective. Either weigh on household scales or buy a special graduated gardener’s measure.

A match box will do if you have no scales available. When full a standard match box will hold:

15g (oz) of superphosphate

22g (oz) of sulphate of ammonia, bone meal, many compounds

28g (1 oz) of sulphate of potash

Always follow the instructions on the label of the container when using proprietary compounds.

Always scatter them as evenly as possible otherwise patchy growth will result. Distributors can be used for lawns and large beds.

You can buy dilutors for liquid feeds which meter the correct amounts.

Always rake fertilizer into the top 5 or 7cm (2 or Sin) of soil or rotary cultivate, but do not bury deeply. Slow acting ones are best mixed thoroughly with the top 15cm (6in) by raking or rotary cultivation.

Never apply liquid fertilizers to dry soil or composts. Always water first.

Keep fertilizers in a dry place and keep the tops of the containers closed up; always keep bags off the floor.

Do not allow fertilizers to touch leaves or flowers—they may scorch. NPK These are the chemical symbols for the three plant foods that are needed in the largest amounts by plants, and are the ones most likely to be deficient in soils. When you buy a fertilizer you generally buy it for its content of these plant foods. Hence the value of a fertilizer depends upon its analysis which should be stated on the bag or other container.

  1. stands for nitrogen which is a gas and cannot be absorbed by plants in this form; it is taken from nitrates which result from the chemical combination of nitrogen with oxygen.
  2. stands for phosphorus, which is a chemical element that catches fire when exposed to air; so plants cannot take it up in this form. Plants get their phosphorus from soluble phosphates which are a combination of this element and oxygen. Unfortunately an archaic expression is still used to denote the content of phosphorus in a fertilizer—this is p205—which is commonly called `phosphoric acid’.
  3. stands for potassium, the Latin name of which is Kalium. Since potassium is a metal that bursts into flame when it comes into contact with water it cannot be used by plants in the raw state. It is taken up from soluble potassium salts in soils.

The word potash, the chemical name of which is potassium oxide-K20, arose from the old custom of concentrating the solution of ashes, which contain potassium, in pots. The plant food content of a compound fertilizer may be indicated by its grade—a series of three numbers separated by dashes. The numbers show the percentage of nitrogen, ‘phosphoric acid’ and potash, in that order, contained in the compound.

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