Chemical fertilizers are classified on the basis of type and form of nutrients they contain:
(a) Nitrogenous fertilizers (N),
(b) Phosphate fertilizers;
(c) Potassic fertilizers.
1. Nitrogenous Fertilizers:
(a) Nitrate fertilizers contain nitrogen in nitrate form; preferred by majority of plants, readily soluble in water and quickly available to plants. In moist and waterlogged conditions, these are leached down/converted into gaseous form and become available to plants. Sodium nitrate (16% N), Calcium nitrate (15.50% N); Potassium nitrate (13% N) are used worldwide.
(b) Ammonical fertilizers contain nitrogen in ammonical form; are quite resistant to leaching; hence, used in water-logged conditions also. Though readily soluble in water, these are not as quickly available to plants as nitrate fertilizers. Some time is required to convert NH4 into NO3; hence, suitable for slow-growing, long duration crops. Ammonium sulphate (20% N), Ammomum chloride (24-26% N), Diammonium phosphate (18% N) are widely used in India.
(c) Ammonical-nitrate fertilizers contain nitrogen in both forms—ammonical and nitrate e.g. Ammonium nitrate (33% N), Calcium ammonium nitrate (26% N).
(d) Amide fertilizers are readily soluble in water, and are easily decomposed by microorganisms in the soil in the soil, these are changed into ammonical and then nitrate form to become available to plants. Urea is the cheapest nitrogen fertilizer. In soil, urea is decomposed by the enzyme urease secreted by soil microorganisms. It is resistant to leaching.
2. Phosphate Fertilizers:
Next to nitrogen, phosphorus is the most deficient primary nutrient element in Indian soils:
(a) Water soluble phosphatic fertilizers contain phosphate in highly soluble form (H2PO4) in water, and therefore are readily available to the plants e.g., Single superphosphate (SSP: 16-18% phosphate); Double super phosphate (DSP: 32% phosphate); Triple super phosphate (TSP: 42% phosphate).
(b) Citric acid soluble phosphatic fertilizers— readily soluble in acidic water/weak acids- contain phosphorus in available form (H3PO4).
Basic slag (18% phosphate) byproduct of iron and steel industries. Dicalcium phosphate. Rich in phosphorus.
(c) Insoluble phosphatic fertilizers— completely insoluble in water; slightly soluble in weak acids like citric acid e.g., Rock phosphates (20-40% P2O5); Bone meal.
3. Potassic Fertilizers:
Chief commercial ones are Potassium sulphate (50% K20), and the muriate of potash (60% K2O). NPK are complex fertilizers designed to supply all the three primary nutrients; are marked in different grades indicating the respective percentage of nitrogen (N), phosphorus (P2O5), and potash (K2O) e.g., NPK 15-15-15 grade means that the fertiliser contains 15 percent each of N, P2O5 and K2O. Production of chemical fertilizers involves huge amounts of energy e.g., for producing 1 kg of nitrogenous fertilizer, phosphatic fertilizer and Potassic fertilizer 80 MJ. 12 MJ and 8 MJ energy is consumed respectively.
Thus, chemical fertilizers are:
(i) Expensive,
(ii) Their manufacture depends upon the dwindling resources of energy like petroleum and coal.
Fertilizers applied to crop lands are lost in run-off, polluting soil and water resources.