CHAPTER ONE
INTRODUCTION
BACKGROUND TO THE STUDY
Besides genetic and climatic factors, the growth and yield of crops are mainly determined by the amount of nutrients available in the soil (Jungk and Rademacher, 1983). The vast area of tropical soils of the humid tropics are acidic. A number of studies have shown that high proportions of soils which belong to the great soil groups of Oxisols and Ultisols have a marked ability to fix applied inorganic P, and usually have low extractable P (Sanchez and Salinas, 1981). Higher soil acidity is associated with increases in precipitation, leaching, weathering, hydrolysis, organic matter, nitrification, fertiliser application, oxidation of sulphides, uptake of ions, and more (Adams, 1984). Phosphate can readily be rendered unavailable to plant roots because it is the most immobile of the major plant nutrients and whose efficiency can be affected by P fertiliser distribution and distance of application from the plant (Eghball and Sander, 1989). The quantity of P in soil solution is in the range of 0.3 to 3 kg P2O5 ha-1 as growing crops absorb about 1 kg P2O5 ha-1 per day. The labile fraction in the topsoil layer of 20 cm is in the range of 150 to 500 kg P2O5 ha-1, which could replenish soil solution P (Mengel and Kirby, 1996). The phosphate concentrations of the soil solution and its buffering capacity are the most important parameters governing the P supply to plant roots. Thus, the rate of desorption is higher in soils with a higher phosphate buffer capacity (Dear et al., 1992). The consideration of significant varietal and species differences in tolerating low available P2O5 and low pH effect is also important. At similar yield levels, upland rice usually requires less P than maize. The general recommendation for these crops, in acidic soils, ranges from 100 to 150 kg P2O5 ha-1 for maize and 0 to 60 kg ha-1 for upland rice (Sanchez, 1976).