Fertilizer may contain one or more of essential nutrients required for plant growth Fertilizers which contain only one nutrient are know as single, simple or straight fertilizers (eg. urea). Fertilizers which contain two or more nutrients are classified as mixed or compound fertilizers (eg. diammonium phosphate).

Many materials like fertilizers manures and composts can be used to supply nutrients to plants. Mostly fertilizers are; in inorganic form, while manures and composts a organic form, Several important between organic manures and inorganic fertilizers are given in Table 1.

i) Ammoniacal fertilizers (NH₄ - N)

Examples N content

a) Ammonium sulphate (21.2%)

b) Ammonium chloride (26%)

c) Anhydrous ammonia (82%)

ii) Nitrate fertilizers (NO₃ - N)

a) Sodium Nitrate (16%)

b) Potassium Nitrate (13%)

c) Calcium Nitrate (15.5%)

iii) Fertilizers containing ammonium and nitrate ions

a) Ammonium Nitrate (35%)

b) Calcium Ammonium Nitrate (25%)

c) Ammonium sulphate Nitrate (26%)

iv) Amide fertilizers (NH₂)₂ CO)

a) Urea (46.2%)

b) Urea phosphate (17%)

c) Urea sulphate (30-40%)

v) Nitrogen solution (Liquid fertilizers)

a) Anhydrous ammonia (82%)

b) Aqueous ammonia (varies)

vi) Slow release N fertilizers

a) Urea-formaldehyde (38%)

b) Oxamide (31.8%)

c) Metal ammonium phosphate (8%)

vii) Nitrification inhibitors

a) N-serve

b) Nitrapyrin

Liquid (solution) fertilizers are not commonly used in India. Only in foreign countries like USA, UK and European countries they are used. Among many different forms N fertilizers, some-important N fertilizers used in India are discussed below :

Urea is the most important N fertilizer used in India. A high N content and its ease of handling as prills have made urea the most popular form of N fertilizer. More thasc³ 70% of our Indian farmers use urea, since it is the cheapest source of N. Urea contains the highest N content (46%) man any soil fertilizer commonly used.

Urea is manufactured by the reaction of ammonia and carbondioxide in a pressure reaction to form a melt containing urea, ammonium carbonate and water along with some ammonia. The primary source of raw materials for the manufacturing of urea are natural gas, air and water.

Urea is a white, fine crystalline water soluble compound and is hydroscopic (water absorbing substance). When applied to the soil urea undergoes rapid changes. The fate of urea in soil is illustrated in Figure1.

Nitrate is very soluble water and highly mobile. Therefore it is easily lost through leaching. The NO-₃ may also be reduced to gaseous N (nitrous oxide and nitrogen) and lost through denitrification. Due to these losses of NO₃ ammoniacal form of N fertilizer is preferred over nitrate form.

We have seen from the preceding discussion that N added through urea or other chemical fertilizers is subject to may losses. As a result crop utilization of _ N is very low (<40%). To minimize the loss of N and increase the effectiveness slow release N fertilizers have been developed. In these fertilizers, N is released slowly which provides N supply, continuously for longer period and is better matched with plant uptake rates.

i) UF - Urea formaldehyde (38%)

ii) CDU - Crotonylidene diurea (28% N)

iii) Oxamide (32%)

iv) Metal ammonium phosphates (7-10% N)

v) N- serve, USG-Urea Super Granule

To minimise the loss of N from urea, the prills or granules of urea are coated with the indigenously available cheap materials like neem cake, tar, gypsum and sulfur. Coating eliminates the risk of injury to plants from salt concentration. Coating also reduces the solubility of urea in water and its toxicity caused by a heavy rate of application.

i) Gypsum coate urea

ii) Neem blended urea

iii) Rock phosphate coated urea

iv) Lac coated urea

v) Sulphur coated urea

vi) Tar coated urea

Among the coated fertilizers, gypsum coated urea and sulphur coated area are advantageous because they also supply calcium and sulphur nutrients. The preparation of some of the coated fertilizers is discussed in the practical manual of courses.

By inhibiting the process of nitrification, the loss of N can be minimised. Certain substances are toxic to the nitrifying bacteria and inhibit nitrification. A nitrification inhibit (inhibitor) or should have the following important properties:

i) It should be nontoxic to plants and other organisms

ii) It should specifically inhibit the microorganism responsible for nitrification

iii) It should not interfere with other soil processes

i) N-serve (2 Chloro 6 trichloromethy) 1 pyridine

ii) Nitrapyrin

iii) AM (2-amino-4 Chloro-6 Methy l pyrimidine)

iv) DD (Dicyanadiamide)

v) TU (Thiourea)

Indian soils are, generally, low in phosphours (P) status and therefore it is necessary to apply P fertilizer for sustaining soil fertility. Phosphorus fertilizers are classified into three groups based on their solubility. They are as follows :

1. Water soluble phosphates

eg : Superphosphate, diammonium phosphate

2. Citrate soluble phosphates

eg : Dicalcium phosphate. Rock phosphates

3. Phosphates that arc only soluble in strong mineral adds such as sulphuric, nitric or hydrochloric add.

eg : Tricalcium phosphates

Some important phosphorus fertilizers commonly used in India are described below :

Superphosphate (single superphosphate) is widely used on agricultural and horticultural crops. It is manufactured from finely ground rockphosphate and concentrated sulphuric acid, Monocalcium phosphate (Ca (H₂ PO₄)₂) is the most important component of superphosphate.

Ordinary superphosphate is a grey-white granular fertilizer contains about 16-18% of phosphorus (P₂O₅) most of which is water soluble. In addition it also contains 21% calcium and 12% sulphur.

When superphosphate is added to soil, the soluble phosphates react with the soil constituents and become less soluble compounds depending upon the soil. In acid soils (pH less man 7), aluminum and iron phosphates are formed.

Under certain conditions the phosphorous released from superphosphate may be adsorbed on the surfaces of soil minerals. In many soils, a large amount of phosphate reacts with the soil and become less soluble and less available to crops. The conversion of available form into unavailable form is called phosphate fixation.

The response of crops to the superphosphate varies in different soils depending upon the nature of reaction of the superphosphate dissolved phosphate which exists in solution phase can be adsorbed by plant roots. In soils having high levels of organic matter, the dissolved P may be immobilized by microorganism.

Diammonium phosphate is also a widely used P fertilizer. Commercial grade DAP contains 46 - 48% P₂ O₅, and 16-18% N. It is suitable for foliar spray. For pulses crop, foliar applications of DAP is recommended.

Ammonium polyphosphates are slow release P fertilizers. To reduce the P fixation and thus increase the efficiency, ammonium polyphosphate can be used for direct application or for bulk blending. The P content of this fertilizer ranges from 52-80% (P₂O₅).

The raw materials for all phosphate fertilizers are the deposits of rock phosphates which are found in several areas of the world. In India rock phosphates (RP) mined in different parts of the country are being used as sources of phosphorus.

The total P content of RP varies from 11.5 to 18% (27 to 41% P₂ O₅) awl mostly they are insoluble. Rock phosphate is mostly suitable for acid soils. In neutral and alkaline soils rock phosphate can be used along with organic manures or bio-fertilizers. Various rock phosphates available in India and their phosphorus contents list in Table 2.

Potassium fertilizers are often referred to as potash fertilizers. Potassium is mined from abundant mineral deposits. Practically all of the potassium in combination with chloride, sulphate, nitrate. Commonly used potassium fertilizers in India are

i) Potassium Chloride and ii) Potassium sulphate.

In India potassium chloride is commercially marketed as 'muriate of potash'. Fertilizer grade muriate of potash contains about 60-63% K2 O and varies in color from pink or red to white. Muriate of potash is manufactured from the minerals cornallite and sybrite through fractional distillation and crystallization.

When added to soil, muriate of potash dissolves in the soil water readily and is taken up plants. Some of the added potassium may be adsorbed on the soil colloidal complex or lost through leaching.

It is also known as 'sulphate of potash'. It is a fine yellowish salt which contains 50 - 53% K₂O and 17% sulphur. It has one important advantage over muriate of Potash in that it can also supply adequate sulphur to the crop. Sulphate of potash is very effective on potatoes and tobacco, which are sensitive to large applications of chlorides.

Sulphur (S) fertilizers contains S either in the form of sulphate or as elemental S. Sulphate is often present as a secondary constituent of fertilizers used mainly for nitrogen, phosphorus or potassium contents. Some important S containing fertilizers are listed in Table 3.

Table 3 : Important Sulphur containing fertilizers

All sulphates are relatively soluble, and when applied to the soil the S is in a form immediately available for plant uptake. However, sulphate can also be leached readily from the soil and adsorbed on soil clay.

Calcium is not normally formulated as such, but it is present in many other fertilizer materials. Some of the fertilizers containing calcium are given in Table 4.

When fertilizer is added, calcium is held as exchangeable ions by soil particles. Generally calcium is not subject to fixation. The calcium replaces sodium on the exchange complex, and the sulphate is leached through drainage water. This replacement helps to flocculate the soil and makes it more permeable to water.

The most widely used magnesium fertilizers are listed in Table 5.

The reactions of magnesium containing fertilizers in soils are somewhat similar to calcium fertilizers. Magnesium may become fixed in the soil clay mineral and also be easily leached from soil.

Micronutrient deficiencies are widely spread in Indian soils (refer module 3). Micronutrient deficiencies in crops can be treated either by application of micronutrient containing fertilizers to the soil or by the application of micronutrient foliar sprays. Widely used fertilizers containing important micronutrient are given below :

Borax (11%B)

Borates (14 - 21%B)

(different grades)

copper sulphate (25% Cu)

copper chelates (9-13%Cu)

Ferrous sulphate (19%Fe)

Ferrous amonium sulphate (14%Fe)

Pyrite (46.5% Fe)

Manganese sulphate (26 - 28% Mn)

Chelates (5 - 12% Mn)

Zinc sulphate (35% Zn)

Zinc chelates (9 - l4% Zn)

Ammonium molybdate (54% Mo)

Sodium molybdate (39% Mo)

Different micronutrient fertilizers have different reaction mechanisms which are very complicated. Micronutrients in fertilizer form simple cations or anions, as complex inorganic ions or as soluble organic complexes in the soil solution. For some micronutrients particularly copper, organic complexes may constitute a major pro-portion of their total solution concentration. The availability of micronutrients in soil is also influenced by interaction of various nutrients present. For example, high levels of any one of the micronutrient cations viz., iron, copper. Zinc., and manganese may induce deficiencies of the other. Similarly in some situations deficiency of zinc or iron can be induced by high soil phosphate levels.

Chelates are complex organic compounds that can be used to correct micronutrient deficiencies. These chelates are quite effective, and therefore only small amounts are required to correct deficiency of the various micronutrients. Chelates can be applied directly to the soil or as foliar sprays.

Examples : EDTA, DTPA, EDDHA

A mixed of compound fertilizer is a fertilizer containing any combination or mixture of fertilizer materials. Mixed or compound fertilizers contain two or more of the major plant nutrients. Most fertilizers can be mixed together without any adverse effects while some fertilizers react with one another. Balanced manuring with adequate provision for secondary and micronutrients can be achieved through mixed fertilizers.

It is required by legislation that all fertilizer bags should bear the guaranteed analysis of the contents of N,P and K. For example,-the number 17:17:17 means that the fertilizer contains 17% N, 17% P₂ O₅ and 17% K₂O. A guaranteed analysis of 16:17:0:20 means that the fertilizer contains 16% N, 17% P₂ O₅, no K₂O, and 20% Sulphur. Most compound fertilizers are granulated, priHed or pelleted. This makes them easier to handle, store and spread.