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Nitrogenous Fertilizers

Nitrogenous Fertilizers

Nitrogen is absorbed by the plant roots in two forms: nitrate form (NO3) and ammonical form (NH4). Most of the crop plants prefer nitrogen in nitrate form; but paddy and few other higher plants prefer nitrogen in ammonical form. Ammonical form of nitrogen is however, easily covertible into nitrate form. Most of the fertilizers contain nitrogen in these two available forms. Urea, however, contains nitrogen in amide form but this form of nitrogen is swiftly converted by soil micro-organisms into ammonical form and then into nitrate form. Based on the forms of nitrogen they contain, nitrogenous fertilizers are classified into following four categories, viz. (1) nitrate fertilizers, (2) ammonical fertilizers, (3) ammonical-nitrate fertilizers, and (4) amide fertilizers.

1.Nitrate fertilizers: As the name, 'nitrate fertilzers' indicates, ,. these fertilizers contain nitrogen in nitrate form which is preferred by a majority of the plants. These fertilizers are readily soluble in water and quickly available to the plants. If plants need nitrogen urgently, soil use of nitrate fertilizers gives quick response. Use of nitrate fertilizers is not recommended for very moist and waterlogged soils because in these situations nitrate nitrogen is either leached down along with excess moisture or converted into gaseous form and lost to the atmosphere due to lack of oxygen. In both conditions, it becomes unavailable to the plants. In sandy soils also, there use is avoided as the sandy soils are incapable of retaining nutrients and even with a little moisture, nitrate is leached down. If necessary, in sandy soils nitrate fertilizers are applied in 2-3 split doses. There are three main nitrate fertilizers use the world over:

Sodium nitrate                                     16% Nitrogen (N)

Calcium nitrate                                    15.5% N

Potassium nitrate                                13% N

The use of nitrate fertilizers in our country  is very limited due to their limited availability, high cost, difficult handling and also their harmful effect of soil. We have to import these de fertilizers and, so, their cost per unit nutrient  (N) is very high. Secondly, when these e fertilizers are continuously used for a few years, they may deteriorate the physical condition of soil. Plants utilize nitrate portion of these fertilizers and sodium and calcium portions are left in the soil. Excessive build up of sodium or calcium in the soils results in the development of alkalinity or salinity respectively, which is not good for the health of soil.

2. Ammonical Fertilizers: Ammonical fertilizers contain nitrogen in ammonical form. These fertilizers can be applied also to those crops which absorb nitrogen in nitrate form. It is because ammonical part of the fertilizers can be easily converted into nitrate  (NO3) form. As the ammonical portion of the " fertilizer (NH4) is positively charged, it easily , gets attached with very fine clay or humus a, particles of the soil which are negatively charged. Ammonical fertilizers are, therefore, quite resistant to leaching. These fertilizers can also be applied in waterlogging conditions, because in such conditions, they do not so easily convert into gases. Ammonical fertilizers are especially suitable for paddy crop for a couple a of reasons: paddy prefers nitrogen in ammonical  form and ammonical form of nitrogen is not easily converted into gases and lost to atmosphere. Although these fertilizers are readily soluble in water, they are not as quickly available to plants as nitrate fertilizers. A little time is required for the conversion of NH4 to NO3, the most preferred form of nitrogen. Ammonical fertilizers are suitable for slow growing, long duration crops. The following ammonical fertilizers are used in our country:

Ammonium suplphate                                     20% N

Ammonium chloride                                       24-26% N

Monoammonium phosphate                           11-20% N, 20-48 P205 (Phosphate)

Diammonium phosphate (DAP                      18%N,   46%P205

Ammonium solution                                        20-2S%N

All the ammonical fertilizers exert negative residual effect on the soil. The characteristics of some important ammonical fertilizers can be discussed below:

a. Ammonium Sulphate: Ammonium sulphate is a whitish, stable crystalline salt, solube in water and stroes well. When applied in the soil, ammonical part gets absorbed on soil clay particles and becomes resistant to leaching. When plants need nitrogen, absorbed ammonium is flushed out into soil solution where micro organisms convert it into nitrate form. Nitrates are then utilised (absorbed) by the plants. The remaining portion of the fertilizer, i.e., sulphate (S04) is left in  the soil and increases acidity in the soil. Application of 100 kg of ammonium sulphate  exerts so much of acidity in the soil that it requires 110 kg of calcium carbonate (limestone) to neutralise it. Within a month of its application, ammonium sulphate is utilised by the  plants.

b. Ammonium chloride: Ammonium chloride is also crystalline ammonical fertilizer available in granular form. It contains 26 per cent nitrogen. This fertilizer is more hygroscopic (absorbing moisture easily from atmosphere) and acidic in residual action than ammonium sulphate. A hundred kg of this fertilizer needs 120 kg neutralizer (CaC03). The mode of action of this fertilizer is also the same as of ammonium sulphate.

c. Diammonium phosphate: Besides nitrogen, diammonium phosphate also I' contains the second most important primary nutrient element, phosphorous (46% P205)' It is  completely solublee in water. It has a very high a content of plant food per bag and has good storage properties. This fertilizer is mainly I considered for supplying phosphate (P205)' The amount of nitrogen virtually supplied through this n fertilizer is then deducted from the total nitrogen requirement of the crop and the rest is applied - through another straight nitrogenous fertilizer.

3. Ammonical-nitrate fertilizers: Ammonium-nitrate fertilizers contain nitrogen in both the forms: ammonical and nitrate. Fertilizers of this category can be used in a wide variety of soils and cropping conditions. Nitrate meets the immediate demand of nitrogen and ammonical form supplies nitrogen to meet the growth and developmental needs of N as the plant ages. Fertilizers of this category are acidic in reaction. The following fertilizers of this category are marketed:

Ammonium nitrate                                          33%N

Calcium ammonium                                       26%N

Ammonium sulphate Nitrate (ASN)                26%N

Some salient characteristics of these fertilizers are as follows:

a. Ammonium nitrate: Although ammonium nitrate is nutritionally very rich, it has several undesirable features. It is highly hygroscopic and shows a marked tendency to absorb moisture from the air and to cake (lump formation). It is highly explosive and, therefore, requires careful handling and storage. This fertilizer is not manufactured in India but imported on a limited scale. The following recommendations can be made to minimize the explosive hazards of this fertilizer:

-do not store any longer than is absolutely necessary;

-keep all bags tightly sealed until ready to use;

-keep all bags indoor in a dry place;

-pile the stacks not higher than 6 bags; and

-change position of bags once a week.

If caking occurs, drop bags on a hard floor.

Some factories have started using conditioners such as clay and one per cent water repellent material like wax, grease or oil to improve the quality of fertilizer. Such granulated material are stored in moisture proof polythene bags.

b. Ammonium sulphate nitrate: Ammonium sulphate nitrate fertilizer has the advantage of having nitrogen in both the forms of nitrate and ammonical and also sulphur. It has, however, one disadvantage in that when it is stored it sets into a hard cake which requires to in be broken up. This fertilizer is resistant to leaching as 3/4th of its total nitrogen is in be ammonical form. The acidity released by 100 kg an of this fertilizer requires 93 kg of calcium (a carbonate CaCo3 to neutralize it.

c. Calcium ammonium nitrate: Calcium ammonium nitrate fertilizer has very good physical condition. It is attractive, dust free; easy flowing and granular in form. This  fertilizer is now-a-days not used in India due to its high cost. This is, in fact, a chemical mixture of ammonium nitrate with calcium carbonate. Calcium improves the effectiveness of  fertilizer due to the following reasons.

-The explosiveness and hygroscopic tendency -of ammonium nitrate is reduced.

-Fertilizer become neutral in reaction  leaves neither acidity nor alkalinity as residual effect on soil and can be used in any type of soil.

-Granule size becomes bigger enabling easy application of ferti.1izer to the field.

-Soil is enriched with calcium, a secondary essential nutrient.

4. Amide fertilizers: Amide group fertilizers are agronomically very significant. They are readily soluble in water and easily decomposed by micro organisms in the soil. In the soil, they are quickly changed into ammonical and then nitrate form and becomes available to the plants. The two main [ amide fertilizers used in our country are: i (a) Urea -46%, (b) Calcium cynamid -21%N.

a. Urea: Urea is readily soluble in water and is, perhaps, the cheapest among all the nitrogenous fertilziers based on per unit cost of nutrient. It is a white, granular, solid and slightly hygroscopic fertilizer. It can also be applied in solution form as spray. Urea exerts acidity equivalent to neutralizing amount of 80 kg calcium carbonate per 100 kg of fertilizer material. Most of the crops respond effectively to the urea application. When applied to the soil, urea - undergoes decomposition in the presence of  enzyme urease',  secreted by soil  microorganisms. Conversion of urea in to nitrate form due to decomposition is completed in about - one week’s time. Urea is somewhat resistant to  leaching.

b. Calcium cynamide: Calcium cynamide is not used in India. This fertilizer creates a high basicity/salinity in the & soil as residual effect which requires sulphur as neutralizer. This fertilizer, if available, can be used in acidic soils.

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Please note that this is the opinion of the author and is Not Certified by ICAR or any of its authorised agents.