Phosphorus, like nitrogen, potassium, calcium, magnesium and sulfur i classed as a macronutrient. It occurs in most plants in concentrations between 0.1 and 0.4%. Plants can absorb phosphorus as either the primary H2PO4 ions o smaller amounts of the secondary HPO4-2 orthophosphate ions. Since the former i most abundant over the range in soil pH prevailing for most crops, it is usually the principal absorbed.
Other forms of phosphorus, including the pyrophosphates and metaphosphates, which are components of certain commercial fertilizers are suitable for crops. Since in aqueous solutions both of these forms hydrolyze to orthophosphate, their absorption probably occurs mainly after conversion to orthophosphate. Phosphorus is associated with early maturing of crops; especially the shortage is accompanied by marked reduction in plant growth. It is considered essential to seed and fruit formation.
The important phosphorus fertilizers are:
I) Super phosphate or calcium superphosphate (16-20% P2O5)
II) Tripe super phosphate (42-46% P2O5)
III) Phosphatic slage (14-22% P2O5 and 40% lime)
SUPER PHOSPHATE OR CALCIUM SUPER PHOSPHATE Ca(H2PO4)2:
Super phosphate fertilizer is a mixture of mono-calcium phosphate and gypsum. It contains about 20% P205.Super phosphate is obtained by treating phosphate rock (3Ca3(PO4)2.CaF2) with 70% H2SO4. The process by which super phosphate is manufactured is called Din’s process. In this process, phosphate rock is first crushed and then finally ground. The ground material is then made to reaet with sulphuric acid in a cylindrical mixture, which is provided with a stirrer. The material is stirred for sometime. The semi-liquid mass is then dumped mechanically into one of the dens ‘D,’ and ‘D2″ through the values ‘V1” or “V2′.To ensure complete reaction, the “batch”‘ is left in the den for about 6 to 24 hours.
Gaseous compounds of fluorine and silicon are allowed to escape through the pipe provided at the top of the din. The product left in the den is porous and dry in appearance. This porous and dried material is then subjested to “curing”. The term “curing’ means further reaction of the free acid with the unreacted portion of the phosphate rock. The material is then transferred to a storage tank for a period of 4 to 6 week to Complete the reaction. This material is then ground in ball mills and packed in bags.