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Insoluble phosphatic fertilizers

Insoluble phosphatic fertilizers

As the name suggests, insoluble phosphatic fertilizers are completely insoluble in water. They are very slightly soluble in weak acids like citric acid but are readily soluble in strong acids. Their use is, therefore, always recommended for strongly acidic soils. These fertilizers also contain phosphorus in the available form PO4 but this form is very slowly available in comparison to pervious two forms, i.e. H2PO4 and HPO4. These fertilizers are applied in large quantities and also ploughed under with green manuring crops or other organic materials. This group of phosphatic fertilizers is available in two main forms:

a. Rock phosphate -                                     20-40% P205

b. Bonemeal -a. Raw bonemeal                     2- 25% P205

c. Steamed bonemeal                                   22% P205

a. Rock phosphates: In India, rock phosphate is mainly utilised for the manufacturing of various other phosphatic fertilizers. It is also used directly to the field, as a phosphatic fertilizer in strongly acidic soils. After mining, it is processed and finally  ground and sold in the polythene bags for field use. Rock phosphate is suitable for permanent, long duration crops like fruit and plantation  crops (orange, apple, tea, coffee, etc.). Rock phosphate responds very well in soils rich in organic matter, because acids released by decomposing organic matter help in solubilising the rock phosphate. Legumes which have a high Ca and P requirement, give greater response to rock phosphate than non-legumes.

Areas having strongly acid soils are the most suitable ones for application of rock phosphate. Such areas are found in the whole of Kerala. Shimogha, Coorg and Mysore districts of Karnataka, Chotanagpur region  of Bihar, parts of  Assam, Meghalaya, West Bengal and rice cropped laterite soils of Orissa.

b. Bonemeal: Bonemeal is the oldest phosphatic fertilizer used by Indian farmers. Presently. it is marketed: in two forms.

i. Raw bonemeal -The bones collected from city slaughter houses and from the countryside, are dried and crushed without any treatment. Besides 20-25% P205' raw bonemeal also contains 2-4% nitrogen.

ii. Steamed bonemeal -The bones collected are given a steam treatment in a sterilization chamber. They are then dried in warm, rotating ovens. Bones become brittle and easy for grinding.

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Please note that this is the opinion of the author and is Not Certified by ICAR or any of its authorised agents.