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.