1 Soil Factors Affecting Nutrient Bioavailability

The soil supports plants, is the physical determinant of root growth and extent, and is the main reservoir for plant-available water and nutrients. Therefore, the soil controls the availability of most essential plant nutrients. It regulates availability by means of biophysiochemical processes, which are functions of soil and plant properties. This chapter introduces the concept of soil nutrient bioavailability, and the soil factors that regulate it. It summarizes the processes that control nutrient release and movement in the soil, and points out the importance of selected plant root characteristics in nutrient acquisition. The soil–plant system’s capacity to supply/absorb nutrients is termed soil nutrient bioavailability, and is the ability of the soil–plant system to supply essential plant nutrients to a target plant, or plant association, during a specific period of time as a result of the processes controlling (1) the release of nutrients from their solid phase in the soil to their solution phase; (2) the movement of nutrients through the soil solution to the plant root–mycorrhizae; and (3) the absorption of nutrients by the plant root–mycorrhizal system (Comerford 1998). Release of nutrients from the solid phase to the soil solution is controlled by the physiochemical processes of desorption and dissolution. It is also a biochemical process by way of mineralization. In both cases, soil and plant root–mycorrhizal characteristics help regulate the expression and dominance of these processes (Table 1.1). The movement of nutrients through the soil solution to the root–mycorrhizal surface is determined by mass flow and diffusion, which in turn are controlled by the interaction of soil and plant characteristics (Table 1.1). Finally, nutrient uptake at the root surface is dependent on the amount of root–mycorrhizal surface and its uptake characteristics. The following discussion presents the three groups of processes outlined above. It concentrates on delimiting those soil characteristics important to defining nutrient bioavailability. Plant root growth, root function and nutri-