List of fertilisers containing phosphorus:- 1. Single Superphosphate 2. Triple Superphosphate 3. Ammonium Phosphates 4. Nitro-Phosphates 5. Bone Meal 6. Basic Slag 7. Rock Phosphate.
Fertiliser # 1. Single Superphosphate:
Single superphosphate (ordinary superphosphate) is manufactured by mixing equal parts of rock phosphate and sulphuric acid (batch mix process).
Phosphate originally present as apatite is converted into mono-calcium phosphate. Gypsum, one of the products of reaction is intimately mixed with mono-calcium phosphate. In the later stages of solidification and drying, water is taken up to form superphosphate which contains about three parts by weight of gypsum and two parts by weight of mono-calcium phosphate.
Chemical composition of superphosphate is:
Superphosphate is greyish and slightly moist with acid odour. In addition to 16-18 per cent P2O5, it contains 12 per cent sulphur and 21 per cent calcium.
Reaction in Soils:
In slightly acid, neutral and calcareous (alkaline) soils, di-calcium and tri-calcium phosphates are formed.
Di-calcium phosphate is slowly dissolved in soil water containing carbonic acid and becomes available to plants. In markedly acid soils, mono-calcium phosphate combines with iron and aluminium to form insoluble phosphates.
Fertiliser # 2. Triple Superphosphate:
It is manufactured by adding phosphoric acid to rock phosphate.
It contains 48 per cent water soluble phosphate (mono-calcium phosphate) and about 18 per cent lime. Its high phosphorus content is attractive due to reduction in transportation, storage and handling charges.
Fertiliser # 3. Ammonium Phosphates:
These are produced by reacting ammonia with phosphoric acid or a mixture of phosphoric and sulphuric acids. Mono-ammonium phosphate (MAP), di-ammonium phosphate (DAP) and ammonium phosphate sulphate are commonly used. These are completely water soluble. Ultimately they have an acid effect on soils, even though the initial reaction of DAP is alkaline. There are usually available in granular form.
Fertiliser # 4. Nitro-Phosphates:
These are manufactured by reacting nitric and sulphuric acids with rock phosphate (apatite).
Liquid ammonia is then added to the slurry to form nitrophosphate.
Nitro-phosphates are granular fertilisers with a stabiliser to prevent reversion of citrate soluble phosphate to insoluble tri-calcium phosphate. Usually, about one-third of phosphate is water soluble and the rest citrate soluble. They contain ammoniacal and nitrate nitrogen. In general, they give best results on acid soils for long duration crops.
Fertiliser # 5. Bone Meal:
There are two kinds of bone meals – raw and steamed. Raw bone meal contains about 25 per cent P2O5 and 4 per cent nitrogen. Steamed bone meal contains about 25 to 30 per cent total P2O5 and 1-2 per cent nitrogen. It contains about 25 per cent citrate soluble P2O5. Steamed bone meal can be readily ground to powder. Bone meal is mainly used on acid soils.
Fertiliser # 6. Basic Slag:
It is obtained as a byproduct from steel industry. It is a greyish black powder with high specific gravity. Most commonly accepted formula is (CaO)5P2O5SiO2. It contains 8 to 12 per cent P2O5, of which 3-4 per cent is citrate soluble. It is an ideal source of phosphorus for crops on acid soils.
Fertiliser # 7. Rock Phosphate:
Ground rock phosphate, which is neither water soluble nor citrate soluble should be thoroughly mixed with soil for its availability to plants. It is effective on acid soils with a pH less than 6.0. It should not be applied directly to a short duration crop. Factors limiting the use of rock phosphate are uncertain agronomic value, handling and application problems and relatively low P2Os content.
The initial phosphorus availability of low reactivity rock phosphate can be significantly improved by treatment with 10 to 20 per cent of the usual quantity of H2SO4 necessary for the manufacture of triple superphosphate. Such partly acidulated rock phosphate is more effective source of phosphorus.