ROCK PHOSPHATE

Agricultural Uses

Rock phosphate is a natural mineral found as a geological deposit on a large scale in the form of sedimentary rocks containing various amounts of calcium phosphates. Once ground, it is used as a phosphatic fertilizer, called phosphorite or mineral phosphate, or as a primary source of phosphorus.
Rock phosphate deposits are found in many countries, such as the USA, Russia, Tunisia, Malaysia, Sri Lanka, Israel, Jordan, etc. India alone is estimated to have about 140 million tons of rock phosphate deposits, most of which are however, of low grade and with substantial impurities unsuitable for the production of phosphate fertilizers.
A large part of mined rock phosphate is used to manufacture single superphosphate, triple superphosphate, tri-calcium phosphate and calcium metaphosphate. Finely ground rock phosphate can be applied directly to the soil if it is strongly acidic, as it has a high humus content. It is, however, not recommended for neutral or alkaline soils where acid-treated rock phosphate (like superphosphate) is used. The material should optimally be in the mesh size of 60 to 100. Powdered rock phosphate is free-flowing and is easily amenable to handling and storage. Crops like rubber, tea, coffee, apples and fruit plantations of oranges are suitable for direct application of rock phosphate.
The most predominant mineral of phosphate is francolite which is calcium carbonate-fluorapatite of formula [Ca₅(PO₄,CO₃)₃(F,OH)]. Four kinds of phosphate rocks are recognized: hard rock phosphate, sofc rock phosphate, land pebble phosphate and river pebble phosphate, with the phosphorus content varying from 2 to 21 %
Most of the phosphorus in the phosphate rock is in an apatite form. There is, therefore, no water-soluble phosphorus. The citrate solubility can vary from 5 to 17 % of the total phosphorus, depending on the chemical nature of the ruck and the size to which it is ground. The efficiency of the ground rock phosphate can be increased by (a) mixing rock phosphate with soluble phosphorus and fertilizers, (b) mixing rock phosphate with elemental sulphur or sulphur-producing compounds, and (c) using phosphate solubilizing micro-organisms
Over 90% of the rock phosphate production, starting with high-grade rock phosphate without impurities, is used in acidulating factories to produce superphosphate and phosphoric acid. Less than 8 % of rock phosphate is used directly as soil fertilizer and about 2 % as animal and poultry feed.