The following points highlight the six important types of fertilizers. The types are: 1. Nitrogenous Fertilizers 2. Organic Nitrogenous Fertilizers 3. Phosphate Fertilizers 4. Potassic Fertilizers 5. Compound Fertilizers 6. Complete Fertilizers (NPK).
Type # 1. Nitrogenous Fertilizers:
The nitrogenous fertilizers are divided into four groups — nitrate, ammonia and ammonium salts, chemical compounds containing nitrogen in the amide form, and plant and animal byproducts.
(i) Sodium Nitrate:
It occurs in natural deposits in northern Chile and is refined before use. The refined product contains about 16% nitrogen in the nitrate form, which renders it directly available to plants. For this reason it is applied as a source of nitrogen, specially to young plants and garden vegetables, which need readily available nitrogen for quick growth.
Sodium nitrate is easily soluble in water and is quickly leached out from the soil. It is particularly useful for acidic soils. Its continued and abundant use in soils causes de-flocculation and develop a bad physical condition in the regions of low rainfall.
(ii) Ammonium Sulphate:
It is the most widely used fertilizer in the country. It is a white crystalline salt, containing 20 to 21 % ammoniacal nitrogen. It is very suitable for wetland crops, for example, paddy and jute. Ammonium sulphate is easy to handle and is stored well under dry conditions.
It is also suitable for wheat, cotton, sugarcane, potatoes and many other crops grown on a wide variety of soils. Its continuous use increases soil acidity and lowers the yield. Its application to acid soils improves the yield of tea plantation considerably. It is advisable to use this fertilizer along with bulky organic manures to avoid its ill effects.
Ammonium sulphate can be applied before sowing, at sowing time, or as a top dressing to the growing crop. It should not be applied during germination, as in concentrated form it affects the germination very adversely.
(iii) Ammonium Nitrate:
Ammonium nitrate is a white crystalline salt, containing 33 to 35% nitrogen, 50% as nitrate nitrogen and another 50 percent as the ammonium form. In the ammonium form it is not leached out easily from the soil. It is quick-acting and highly hygroscopic and cannot be stored. Under certain conditions, it is explosive and, therefore, should be handled cautiously.
‘Nitro Chalk’ is the trade name of the product formed by mixing ammonium nitrate with about 40% of limestone or dolomite. It contains 20.5% nitrogen, 50% in the form of ammonia and half as nitrate. Its continuous use makes the soil acidic. The presence of lime makes it useful for acid soils.
(iv) Ammonium Sulphate Nitrate:
It is a mixture of ammonium sulphate and ammonium nitrate. It is available in a white crystalline form or as dirty white granules. It contains 26% nitrogen, three-fourths of it in the ammoniacal form and the rest as nitrate nitrogen.
It highly soluble in water and very quick-acting and non-explosive. It is useful for all crops. It slightly acidifies the soil. It is applied before sowing, during sowing or as a top dressing, but it is unsuitable for application along with the seeds.
(v) Ammonium Chloride:
It is quite crystalline compound possessing a good physical condition. It contains 26% ammoniacal nitrogen. It is extensively used on paddy in Japan. It is used largely in- industries in India. It is similar to ammonium sulphate in action. It is not recommended for certain types of crops like tomatoes, tobacco, etc., as they may be injured by chlorine.
(vi) Urea:
Urea is a white crystalline organic compound. It is highly concentrated nitrogenous fertilizer containing 45 to 46% of organic nitrogen. It is highly hygroscopic and cannot be stored well in humid regions. To overcome this difficulty it is also produced in granular pellet forms coated with a non-hygroscopic inert material.
It is highly soluble in water and rapidly leached out from the soil. It is very quick-acting and rapidly changed into ammonia when applied. It is applied during sowing or as top-dressing but never during germination. It is suitable for most crops and can be applied to all types of soils.
(vii) Calcium Ammonium Nitrate:
It is a fine, light brown or gray granular fertilizer. It is prepared from ammonium nitrate and ground limestone. It is almost neutral and can be applied even to acid soils. Its nitrogen content varies from 25 to 28 percent. Of the total nitrogen 50 percent remains in the ammoniacal form and the remaining 50 percent in nitrate form.
Type # 2. Organic Nitrogenous Fertilizers:
These fertilizers include plant and animal by-products, such as oil cakes, fish manure and dried blood from slaughter-houses. Before use by the crops these materials are converted by bacterial fermentation into utilizable ammonium-nitrogen and nitrate-nitrogen. These fertilizers are, therefore, slow acting, but supply available nitrogen for a longer period to the crops.
Oil-cakes are usually supplied as organic fertilizer throughout the country. They contain not only nitrogen but also some phosphoric acid and potash. A large quantity of organic matter is also present in the oil-cake. In addition to the three fertilizing constituents like, N, P2O5 and K2O, the oil-cakes contain 2 to 15 percent of oil.
Dried blood or blood meal contains 10 to 12 percent highly available nitrogen and 1 to 2 percent phosphoric acid. It is effective on all types of crops and all types of soils.
Fish manure is available either as dried fish or as fish meal or powder. After extraction of oil from the fish the residue can be used as a manure, Fish manure contains 5 to 8 percent organic nitrogen and 4 to 6 percent of phosphoric acid. It is quick acting and suitable for all crops and soils. It is usually used as powder.
Type # 3. Phosphate Fertilizers:
Phosphate fertilizers are classified as natural phosphates, treated phosphates, by-product phosphates and chemical phosphates.
(i) Rock Phosphate:
It occurs as natural deposits of rock in different countries. Very little rock phosphate is used directly as a fertilizer. Much more of it is used to manufacture superphosphate, the phosphoric acid of which is water soluble and becomes available to the crops.
(ii) Super phosphate:
It is the most widely used phosphoric fertilizer in India. It is now manufactured from ground phosphate rocks treating with sulphuric acid. The brownish- gray product after treatment contains mono-calcium phosphate and calcium sulphate (Gypsum) in practically equal quantities.
There are three grades of super phosphate:
Single superphosphate containing 16 to 20 percent phosphoric acid; di-calcium phosphate, 35 to 38 percent; and triple superphosphate, 44 to 49 percent.
Single superphosphate is the most commonly available grade in Indian market. The fertilizer is suitable for all crops and can be applied to all soils. It should be used along with organic manures in acid soils. It should be applied before or at sowing or transplanting.
(iii) Basic Slag:
Basic slag is a by-product of steel factories. It contains 6 to 20 percent of phosphoric acid (P2O5). The European slag contains 15 to 18 percent P2O5 and used as a popular phosphatic fertilizer in central Europe. But slag from Indian steel mills is poor in P2O5 and is not used as a fertilizer. The European slag is suitable for acid soils as it is alkaline in reaction. For effective use, it must be pulverized before application.
(iv) Bone-Meal:
The ground bone is called bone-meal. It is now widely used as phosphate fertilizer.
It is available in two forms:
(a) raw bone-meal
(b) steamed bone-meal.
The steaming up bones under pressure removes fats, nitrogen and glue making substances. It contains 25 to 30 percent phosphoric acid. Steamed bones are more brittle and can be readily ground.
As it is slow acting, bone-meal should not be used as a top dressing. It must be incorporated into the soil in order to become available. It is applied either at sowing time or a few days before sowing and should be broadcast. It is particularly suitable for acid soils.
It is used for all crops. In some places of the country charred and powdered bones are used as manure. Charring destroys about 50 percent of nitrogen, but the whole of P2O5 remains in a quickly available form.
Type # 4. Potassic Fertilizers:
In India most of the soils contain sufficient amount of potash. So, potassic fertilizers are applied only to those soils which are deficient in potash.
Potassic fertilizers are used as:
(a) muriate of potash (potassium chloride)
(b) sulphate of potash (potassium sulphate).
(i) Muriate of Potash:
It is a gray crystalline material containing 50 to 63 percent of potash (K2O), the whole of which is available to the crops. It remains absorbed on the colloidal surfaces and is not leached out from the soil. It is applied at sowing time or before sowing.
(ii) Sulphate of Potash:
It is more costly as it is prepared by treating potassium chloride with magnesium sulphate. It contains 48 to 52 percent K2O. It dissolves readily in water and becomes available to the crops almost immediately after application. It can be applied at any time up to sowing. In certain crops like tobacco, chillies, potato and fruit-tree it is considered better than muriate of potash.
Type # 5. Compound Fertilizers:
These fertilizers contain two or three plant nutrients simultaneously. When both nitrogen and phosphorus are deficient in soil, a compound fertilizer, e.g., amorphous, can be used. It contains 16 percent nitrogen and 20 percent P2O5. Two different fertilizers can be mixed in correct proportion to produce the compound fertilizer.
Type # 6. Complete Fertilizer (NPK):
Compound fertilizers are not always well adapted to different kinds of soils. For that reason mixed fertilizers containing two or more materials in suitable proportions are used according to the needs of different soils. Mixtures usually fulfil the nutrient deficiencies in a more balanced manner and require less labour to apply than different fertilizers used separately.
These mixtures containing all the three principal nutrients (N, P and K) are called complete fertilizers as most soils usually remain deficient in these three elements. A special mixture for different crops are also produced by the manufacturers.
In some cases insecticides, fungicides and weed-killers, such as DDT, BHC and mercury or copper salts and 2, 4-D are mixed into the complete fertilizers. The component fertilizers must be compatible to ensure mutual reaction. Uneven mixing must be avoided. Bone- meal, muriate of potash and sulphate of potash can be mixed with all fertilizers.