The nutrient phosphorus present in phosphate fertilizers is usually expressed in terms of phosphoric anhydride or simply as phosphorus pentaoxide, P2O5 the availability of phosphorus in which it is present. The amount of phosphorus available to the plants depends upon the extent to which the fertilizer supplies HPO4 or H2PO4 - ions. According to the solubility, the phosphatic fertilizers are divided in following groups.
These fertilizers contain phosphorus in available form in neutral soils, which can be readily absorbed by young plants. In acid, soils and free iron, aluminum hydroxy phosphates but in alkaline and calcareous soils, water soluble phosphorous is converted into insoluble calcium phosphate.
The common examples of these fertilizers are:
The citric soluble fertilizers are suitable for acidic soils because at low pH citrate soluble phosphorus is converted to monocalcium phosphate not as water soluble phosphate and therefore, phosphorus is not fixed as iron and aluminum phosphate.
These mineral fertilizers contain phosphorus, which is insoluble in water as well as in citric acid. They are suitable in strongly acid soils or organic soils. These fertilizers are given in green manured fields. The phosphorus is very slowly released by microbes at action and remains in soil for long time.
Following are the other phosphatic fertilizers.
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