Fertilizer grade:

• Fertilizer grade refers to the guaranteed minimum percentage of N, P2O5 and K2O contained in fertilizer material.
• The number representing the grade is separated by hyphens and are always stated in the sequence. Example: Urea (46-0-0), DAP (18-46-0) etc.

A) Nitrogenous fertilizers

• Those fertilizers, which are sold in the market for their nitrogen content. They are classified into four classes on the basis of N present.

a. Nitrate (NO3-) containing nitrogenous fertilizers: Most of the field crops except rice in the early stage of plant growth take up in nitrate form. They are suitable for top and side dressing and on dry soils. In moist soils they leach rapidly e.g. Sodium nitrate (NaNO3) (16%N), Calcium nitrate [Ca (NO3)2 ] (15.5%N).

b. Ammonium (NH4+) containing nitrogenous fertilizers: Ammonia fertilizers are prerferred by the rice crop in the early stage. For other crops these fertilizers need to be nitrified to nitrate. These are more resistant to loss by leaching e.g. Ammonium sulphate (NH4)2 SO4 (20%N), ammonium chloride (NH4Cl) (24-26%N, anhydrous ammonia (NH3) 82% N.

c. Both ammonium and nitrate containing fertilizers: These fertilizers contain both ammonium and nitrate These are acidic in nature. Leaching losses are less. e.g. Ammonium nitrate (NH4NO3) (33-34% N), Calcium ammonium nitrate (CAN) (20% N).

d. Amide fertilizers: These are organic form of N containing fertilizers, which are readily soluble in water and are easily decomposed in the soil where they are quickly changed into ammoniacal and thence to nitrate form e.g. Urea CO (NH2)2 (46%N), Calcium cyanamide (CaCN2) (21%N). These fertilizers are also known as organic fertilizers since they contain carbon atoms.

B) Phosphatic fertilizers

The phosphate content in such fertilizers is expressed in terms of phosphorus pentaoxide (P2O5), which is readily dissolved in water and produces salts of phosphoric acid (H2PO4, HPO4). They are classified according to solubility and availability to crops are:

a) Water soluble or monocalcium phosphate: These fertilizers are available in the form of monocalcium phosphate of ammonium phosphate. Water soluble phosphates can be absorbed quickly by plants. They should be used on neutral to alkaline soils. However they form insoluble iron and aluminum phosphate in acid soils eg. single super phosphate (16% P2O5), Triple super phosphate (46-48% P2O5), Ammonium phosphate (20% P2O5).

b) Citric acid soluble or dicalcium phosphate: Citrate soluble phosphates are soluble in acid soils where they convert into soluble phosphates and there are less chances of fixation e.g Basic slag (14-18% P2O5), Dicalcium phosphate (34-39% P2O5). These are suitable in acidic soils due to presence of calcium.

c) Insoluble or tricalcium phosphate: These are soluble in strongly acidic or organic soils.

These fertilizers are suitable in strongly acidic or organic soils. The availability of phosphorus from these fertilizers can be increased by ploughing in along with green manures. e.g. Rock phosphate (20-40% P2O5), Raw bone meal (20-25% P2O5), Steamed bone meal (22% P2O5).

• These are suitable for plantation crops like tea, coffee, rubber etc.
• Phosphate content in fertilizers is expressed in oxidized form (P2O5) while its content in soil and plant is expressed in elemental form.
• The conversion factors for elemental to oxidized form and vice versa are 2.29 and 0.43 respectively.

C) Potassic Fertilizers

Potassic fertilizers are classified on the following basis:

a) Having K in the chloride form: Chloride form of K fertilizers are used extensively in all crops except where no chlorine is desired in the fertilizer. Potassium chloride is the most common and cheap fertilizer among potassic fertilizer. e.g. Muriate of potash (KCl) (58-60% K2O.

b) Having K in non-chloride form: Non chloride form of K fertilizers are in demand by cultivators growing special crops such as tobacco, potato and tomato to obtain better quality e.g. sulphate of potash (48% K2O).