Question

Superphosphate of lime is a good fertilizer because it is:
(A) Calcium containing substance
(B) Soluble in water
(C) Containing gypsum
(D) None of these

Answer 1

Hint: Fertilizers are chemical compounds which are added to the soil in order to increase its fertility, water retention capacity along with providing the adequate nutrients to the crops. Superphosphate of lime is an inorganic fertilizer.

Complete step by step solution:
When rock phosphate is treated with sulfuric acid or phosphoric acid, superphosphate or superphosphate of lime, $ Ca({ H }_{ 2 }P{ O }_{ 4 }{ ) }_{ 2 }.2{ H }_{ 2 }O+Ca{ SO }_{ 4 }.2{ H }_{ 2 }O $ is produced which is a mixture of primary calcium phosphate and gypsum.
Given below is the reaction for the preparation of superphosphate of lime:
$ { Ca }_{ 3 }(P{ O }_{ 4 }{ ) }_{ 2 }+2{ H }_{ 2 }{ SO }_{ 4 }+5{ H }_{ 2 }O\longrightarrow Ca({ H }_{ 2 }P{ O }_{ 4 }{ ) }_{ 2 }.2{ H }_{ 2 }O+2Ca{ SO }_{ 4 }.2{ H }_{ 2 }O $
Some of the uses and properties of superphosphate of lime are listed below:
* It is one of the most important fertilizers. It is a form of phosphorus which is used by plants. Generally, the available phosphate in a superphosphate is about 20% whereas double superphosphate (also known as treble superphosphate) have about 40%–50% available phosphate.
* It is a fertilizer that is used during sowing. It is not generally used alone but rather it is complemented by other fertilizers rich in nitrogen and potassium.
* Because of the calcium sulphate content, it is highly suitable for saline soils and for those crops that require more calcium.
* Since it is soluble in water it enriches the soil in calcium and phosphorus.
But it also has its limitations such as:
* The nearby germinating pasture seeds can be damaged by superphosphate and other very soluble fertilisers.
* Fluorine is present in superphosphate which can cause fluorosis in animals if they ingest it directly from recently fertilised pasture.
* Cadmium is present in many rock phosphates. It is toxic to humans. It must be ensured that cadmium does not exceed 280 milligrams per kilogram of phosphorus in order to avoid any excess cadmium entering the food chain.
* Phosphate clings to the clay and that is why it is strongly held in the soil. It gets released slowly and plants acquire it for their growth and nourishment. A very small amount gets leached into the nearby streams or rivers. The little phosphate that does goes into the streams thereby harming the quality of the water as well as the aquatic animals is attached to fine sediments. So if this loss of the sediments can be prevented, then the amount of phosphate entering into the streams will also be significantly less.
Though superphosphate of lime is calcium containing substance but this is not the reason why it is a good fertilizer. Superphosphate of lime is a good fertilizer because it is (b) soluble in water since with water soluble fertilizers it is simple to control the amount of the nutrients required by the plants. They are also easy to add to the soil.

Note: It has been found that the superphosphate of lime increases the exchangeable calcium in an infertile acid soil. Although it showed no involvement in the nitrogen fixation process, it did increase the nitrogen uptake by the plants.