CHAPTER ONE

   INTRODUCTION

Phosphorus (P) is an important plant nutrient and the reactions of phosphate with soil components have been extensively studied from the point of view of soil fertility, soil chemistry and environmental concerns (Parfit, 1978; Sanyal and De Datta, 1991; Hue et al., 1994; Wang et al., 2007). In tropical and subtropical acidic soils, low P availability becomes one of the limiting factors for plant growth; at the other extreme, accumulation of soil available P has negatively affected water quality (Sharpley, 1995). The misapplication of phosphate fertilizers usually causes eutrophication of water bodies, unbalanced plant nutrition and low P utilization efficiency.  When soil phosphate levels are too low, P deficiency in plant represents a major constraint to agricultural production (Palomo et al., 2006). One problem is that P fertilizer  can largely be fixed by the oxides, hydroxides and oxyhydroxides of Iron (Fe)  and Aluminium (Al) and clay minerals in an acidic soils, which makes it less available or effectively unavailable to plants (Fankem  et al., 2006). This is because the availability of both applied and native P is controlled largely by, the sorption and desorption characteristics of the soil.

Variable charge minerals are also the major components of most soils of the tropics that affect P unavailability to plants. Such is the case with soils of Nigeria which is dominated by sesquioxides and low activity clays (Bala, 1992). The most likely areas appear to be those dominated by Oxisols, Ultisols and Alfisols. The low amount of total and available P in these soils make investigation into problems associated with phosphorus availability imperative. Already, the widespread occurrence of P deficiency in most arable land in Nigeria has led to the intensive use of P fertilizer.  It has been reported that land utilization also influences P sorption capacity (Odunze, 2009).

Due to the low solubility and high sorption capacity of P in soil, the supply of phosphate can be a major constrain to plant growth. There is overwhelming evidence, however, to suggest that some plants can directly modify the rhizosphere to gain access to previously unavailable soil P reserves. This can include the deregulation of P membrane transport systems, the manipulation of root hair length or density, the release of phosphates to replace organically bound soil P and the release of organic acid and H+ to solubilize inorganic P (Tinker and Nye, 2000).

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