These are nutrient supplements for plants manufactured in fertilizer factories from chemicals. They are nutrient specific i.e., they may provide only nitrogen, only phosphorus or only potassium to the soil. They are often used when a particular nutrient is required in the soil for a particular crop. Chemical fertilizers contain a higher amount of nutrients as against manures and so are used in very small quantities.

Based on the availability of nutrients in them chemical fertilizers are divided into four groups:

  • Nitrogenous fertilisers
  • Phosphatic fertilisers
  • Potassic fertilisers
  • Complex fertilisers

Nitrogenous Fertilisers

Fertilisers that contain the macronutrient nitrogen come under this group.

Example: Ammonium sulphate, ammonium nitrate, sodium nitrate, urea.

Phosphatic Fertilisers

The fertility of soil can be enhanced by using chemical fertilizers, which provide the essential plant nutrients, potassium, nitrogen and phosphorus.

The most important phosphatic fertilizer is the superphosphate of lime, Ca(H2PO4)2.

This is produced directly from phosphate rock by treatment with concentrated sulphuric acid. In this way the insoluble phosphate rock is rendered soluble in water to improve the release of Phosphorus to the soil for the uptake by the plants.

phosphate rock by treatment with concentrated sulphuric acid

Phosphatic rock generally contains fluoride, which reacts with H2SO4 to give hydrogen fluoride, which in turn generates other side products. The gaseous side-products are removed by washing with water in a scrubber. Almost 90% of the phosphate rock mined goes into the production of phosphatic fertilizers; the remaining 10% is used for the production of elemental phosphorus.

Treatment of phosphate rock with phosphoric acid yields triple super phosphate, Ca(H2PO4)2.H2O which is free from calcium sulphate and hence contains a greater percentage of Phosphorus:

Treatment of phosphate rock with phosphoric acid

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