**4. Expected depletion of RP**

The high pH of the soils from the Savannah zones of Nigeria resulted in increased adsorption of P when varying quantities of phosphate were added in these soils. The suggested respon‐ sible factor causing this fixation was the high activity of hydroxy-aluminum at high pH which has strong attraction with phosphate compared to hydroxyl. This attraction was found enough to displace hydroxyl from the hydroxy-aluminum-phosphate attraction. Due to this, increase in phosphate buffer capacity and the amount of phosphate required to attain the desired level of P in the equilibrium solution was noted [51]. In order to study the mechanisms behind the fixation of P in the soils, another study was conducted which showed that P adsorption by the amphoteric soil surface decrease with increasing pH from 4.0–7.0. But in soils high in ex‐ changeable Al, increasing pH results in the formation of highly reactive adsorbing surfaces for P as Al-ions precipitate and insoluble polyhydroxy-Al cation species. So if acidic soils are reacted with lime without intervening air drying, this will result in the adsorption of more P in the soils. Alternatively, it was found that the application of lime to the acidic soils after intervening air-drying results in decreasing the P adsorption in the soil as clear through isotherm studies [52].

As mentioned earlier, precipitation, and adsorption of P with the soil colloids are the main responsible mechanisms/reactions for the removal of P from the soil solution. The former is induced by the presence of Ca2+ ion in the soil solution while latter depends on the chemical properties of the soil colloids. There is high probability of having the precipitates of calcium phosphate in the soils rich in exchangeable cations. It has also been found that calcareous soils were poor in plant available P compared to the limed acid soils [53].

If we look at the manufacturing process of phosphatic fertilizers, we would found that the major factor responsible for their high prices is the use of very high energy in their manufac‐ turing process. Coming to the initial step of the process is the raw material that is used for the manufacture of most of the phosphatic fertilizers, the phosphate rock or RP that is a naturally occurring mineral source of P. One strategy could be the use of this raw material, the cheaper source of P, directly in the field and getting the benefit of phosphatic fertilizers. To cope with this alarming situation, there is a need to find alternative cheaper sources of P, so that we could have a sustainable agriculture to feed out the ever growing population of the world.
