Ammonium sulphate, urea, ammonium chloride, ammonium nitrate and anhydrous ammonia are the important N fertilisers used in crop production.
Fertiliser # 1. Ammonium Sulphate:
Ammonium sulphate is manufactured by synthetic process in which nitrogen and hydrogen form ammonia gas and as a byproduct of coal and steel industry.
Synthetic Process:
The process consists of making ammonia by the synthesis of nitrogen and hydrogen, reacting ammonia with carbon dioxide gas to produce ammonium carbonate and reacting ammonium carbonate with gypsum to produce ammonium sulphate. It is also known as gypsum process.
Byproduct:
When coal containing nitrogen is distilled in retorts or ovens, a portion of nitrogen is volatilised as ammonia and ammonium salts are retained in wash water. Slaked lime is added to this ammoniacal liquor which is then distilled and ammonia so produced is absorbed in sulphuric acid. Ammonia and sulphuric acid combine to give ammonium sulphate.
Ammonia is fixed on soil colloids replacing calcium. Hence, leaching losses are relatively less. The main advantages of ammonium sulphate are low hydgroscopicity, chemical stability and agronomic suitability. It is a good source of N and S.
Strongly acid forming reaction in soil can be advantageous in high pH soils. Main disadvantage of ammonium sulphate is its relatively low N content adding to storage and transport costs. Its unit cost is, therefore, higher than that of urea.
Reactions in Soil:
When ammonium sulphate is added to the soil, ammonia displaces some other cations, usually calcium.
Calcium sulphate is usually lost in drainage water leading to soil acidity. As such ammonium sulphate is an acid forming fertiliser. Application of 100 kg ammonium sulphate utilises 110 kg calcium carbonate from the soil. Ammonium on exchange sites may be taken by plants or nitrified and lost by leaching.
When ammonium sulphate is applied to calcareous soils, considerable loss occur through volatilisation.
Unstable ammonium carbonate decomposes easily to give NH3, C02 and water. Deep placement can considerably minimise the volatilisation loses of ammonia.
Fertiliser # 2. Urea:
Urea is manufactured by reacting ammonia and carbon dioxide under very high pressure in the presence of a suitable catalyst. Since both reactions are produced at an ammonia plant, urea is a well-integrated commodity for manufacture in association with a synthetic ammonia plant.
The reaction proceeds in two steps:
(i) Formation of ammonium carbonate and
(ii) Dehydration of ammonium carbonate.
Urea, a diamide of carbonic acid, is a white crystalline water soluble compound. It is hygroscopic in nature. It is considered slow release nitrogen fertiliser since it must undergo both hydrolysis and nitrification before it becomes abundantly available to most crops. It is less acidic compared to ammonium sulphate, since application of 100 kg urea leaves acidity equivalent to 80 kg of calcium carbonate to neutralise.
Loss of nitrogen by leaching is negligible. Plants can absorb urea through leaves. As such it can be applied as foliar spray to standing crops. It can also be used in small amounts as nitrogenous feed for farm animals.
Reactions in Soil:
Upon application to the soil, urea is acted on by enzyme urease, which hydrolyses it to unstable ammonium carbonate. This unstable compound by microbial oxidation is converted into ammonia and nitrate, in which forms is absorbed by plants.
In the presence of adequate water or other hydrogen ions, ammonia resulting from decomposition of urea will be converted to and retained in soil as ammonium. However, ammonium is slowly transformed by microorganisms into nitrite and nitrates through nitrification.
When urea is applied to submerged soil (waterlogged soil), the nitrate formed from ammonia is leached in lower reduced zone where the nitrate is converted into nitrous oxide and nitrogen gas, which are lost. As such urea should be placed in the reduced zone.
Loss of N from applied urea to waterlogged soil is given in Fig. 6.3:
Biuret Injury:
When urea is heated to temperatures above 140°-170°C. biuret is formed from two molecules of urea by the elimination of NH3.
Application of urea with high biufet content affect germination of wheat and maize, when applied in bands close to seed. However, broadcast application had no influence on germination. When urea is applied as foliar spray, biuret content should not be more than 1.0 per cent, preferably lower than 0.5 per cent. Citrus and pineapple am sensitive to biuret. If the biuret content is more than 0.25 per cent, foliar spray leads to phytotoxicity.
Fertiliser # 3. Ammonium Chloride:
Most of ammonium chloride is produced by the dual salt process, in which ammonium chloride and sodium carbonate are formed simultaneously. It is a modification of the Solvey ammonia soda process used for producing sodium carbonate.
In this process, a 30 per cent sodium chloride solution is ammoniated and then treated with carbon dioxide. The resulting ammonium bicarbonate reacts with sodium chloride to form sodium bicarbonate and ammonium chloride, which is separated out by filtration or centrifuging.
Another production method is the direct neutralisation of ammonia with hydrochloric acid.
Ammonium chloride is best known for its use in rice cultivation. Coconut and oil-palms are very responsive to chlorides. Ammonium chloride is as acidic forming as ammonium sulphate per unit of nitrogen. Hundred kg of it will produce an acidity equivalent to 128 kg of calcium carbonate.
Reaction in Soils:
Ammonium chloride contains chloride, which has decalcifying property. In the presence of calcium carbonate in soil, it gives rise to soluble calcium chloride which leaches down beyond the root zone. Thus, its application leads to loss of calcium from top soil.
Compared to urea and ammonium sulphate, it has lower nitrogen content. Its chlorine content will limit its use to sensitive crops.
Fertiliser # 4. Ammonium Nitrate:
It is manufactured by direct reaction of ammonia with nitric acid.
It is a white crystalline compound with equal proportions of ammonium and nitrate nitrogen. It is fairly resistant to leaching. It is quite hygroscopic. In intimate contact with oxidisable carbonaceous substances, it forms an explosive mixture. It is more prone to leaching and de-nitrification than ammoniacal products.
Reaction in Soils:
Loss of nitrogen in gaseous form occurs due to biological processes or chemical reactions. Nitrate is lost as nitrogen dioxide (NO2), nitrous oxide (N2O) and nitrogen (N2). In calcareous soils, losses from ammonium nitrate are much less than from ammonium sulphate or urea.
Fertiliser # 5. Anhydrous Ammonia:
It contains the highest nitrogen percentage (82%) compared to any of the commonly used nitrogenous fertilisers. High pressure tanks or refrigeration arrangements are needed for storage and high pressure tanks and melting devices for application. To avoid application losses, it should be applied preferably below 10 cm soil depth. It can also be applied through irrigation water.
Reaction in Soils:
For the greatest stability of applied ammonia in the soil, it has to be converted into ammonium (NH4+) ion which, being positively charged, is retained by negatively charged soil colloids. Ammonium ion behaves like K and can be fixed in vermiculite clays where the tetrahedral sheets are the source of the negative charge.
A moist soil can retain more ammonia than dry soil. Fine textured soils can retain more ammonia than coarse textured soils. More ammonia is absorbed by clay minerals in the acid range and by soil organic matter in the alkali range.
Ammonium Fixation by Clay Minerals:
Clays with expanding lattice have the ability to fix NH4+ in nonexchangeable form. Ammonium takes the place of interlayer cations in the expanded lattice of clay minerals and gets fixed. The mechanism of fixation is similar to that of K+ fixation.
The expanding clay minerals like montmorillonite, illite and vermiculite are largely responsible for ammonium fixation. Cations such as Ca2+, Mg2+, Na+ and H+ which expand the lattice can replace the fixed NH4+. In general, the fixation occurs to a great extent in the subsoil than in the surface soil.