Phosphorus for agriculture

Phosphorus (P) is found in every cell of all living organisms. Human beings, animals and plants depend entirely on phosphorus to live and reproduce. It is a vital nutrient for virtually all biochemical reactions that occur within the plant including energy storage and utilization, photosynthesis, nutrient movement and genetic transfer.

P is the second most important nutrient for plants after nitrogen (N). It is classified as a major nutrient because it is required in relatively large amounts by crops. Beyond its effect on improving the efficiency of other vital nutrients such as nitrogen (N) and potassium (K), it has positive effects on water use efficiency, plant disease resistance, crop maturity, and crop quality.

Phosphate is entirely taken by plants from the soil. Unlike most of the other essential nutrients, phosphorus release from the soil is too slow and the soil reserves in most cultivated lands are generally insufficient to compensate for the removal of nutrients by plants. There are critical levels of P below which crop yields are severely depressed and farm profit is compromised.

Phosphate Rock and P Fertilizers

The only economic source containing native phosphorus is Phosphate rock (PR). It is primarily used as a raw material in the manufacture of phosphoric acid, mineral P fertilizers and other phosphate products. Many geological types of PR are extracted throughout the world with varying chemical and physical properties. They are composed mainly of one or more complex fluorine containing calcium phosphates. They originate either from volcanic action or from sedimentary deposits on the seabed, usually in shallow coastal areas, which subsequently became land. Sedimentary deposits account for the bulk of commercial phosphate production. Most of these PRs are considered to be "unreactive" because they contain little if any water-soluble P in their natural form. When directly applied to the soil, very little of the P in these PRs becomes available to plants. For this reason, the P in the PR must be converted into plant available forms by industrial process in order to obtain P fertilizers with high water solubility (WS) such as Superphosphates with 80-90% WS or ammonium phosphates containing over 90% of water soluble P. 

However, a small number of PRs such as GAFSA have proved to be useful P fertilizers when directly applied to the soil under appropriate rainfall and soil conditions. They are called Reactive PRs or RPRs.

Expression of the phosphorus content

Grade of commercial phosphate rock is usually defined in terms of its phosphorus pentoxide (P2O5) content. This expression is also used for mineral P fertilizers, even though the P2O5 form does not exit in either soils or fertilizers as such. The phosphorus concentration in phosphate rock is also commonly expressed in terms of tricalcium phosphate Ca3(PO4)2 known in the trade as "Bone Phosphate of Lime" or BPL. This term originated when tricalcium was thought to be the main constituent of bone and phosphate rock. It is now known that the phosphorus component of both bone and most phosphate rocks is substituted apatitic compounds.