Bioavailability of Soil P for Plant Nutrition

The poor mobility of soil inorganic phosphorus is due to the large reactivity of phosphate ions with numerous chemical, mineralogical and biological soil constituents. Adsorption-desorption and precipitation-dissolution equilibria control the concentration of P in the soil solution and P chemical mobility. P bioavailability depends on soil pH, concentrations of anions that compete with P ions for ligand exchange and metals that can coprecipitate with P ions. The mineralization and hydrolysis of organic phosphorus also maintain the phosphorus solution concentration. Thus, only a marginal proportion of soil phosphorus is present as P ions in the soil solution. Due to lack of appropriate methods for studying its speciation and biogeochemical behavior, the mobility of inorganic P in most soils is still poorly understood and hardly predictable. This is even worse when considering the problem of the bioavailability of P to plants. The characterisation and quantification of soil phosphorus and the factors that control the availability of P to plants are of utmost importance to define the bioavailability of soil P. Thus, the measurement of intensity and capacity factors together can describe P supply with considerable precision. Phosphorus flux to the root system is mainly controled by diffusion. Soil water content and diffusion coefficient, including the soil buffer power are key factors in the diffusion of P to plant roots. Root systems have the ability to increase the bioavailable pool of P due to their influence on soil chemistry either directly by the activity of plant roots or indirectly by enhancing the activity of rhizospheric microflora. Phosphorus uptake by roots, effect of soil pH, anion/cation balance, gaseous exchanges and release of root exudates are major rhizosphere processes controling the bioavailability of soil P. Soil P bioavailability can also vary with plant species or genotypes, plant nutritional status and ambient soil conditions. Various extractions procedures are now widely used to assess P bioavailability based on the knowledge of phosphorus dynamics in soils and P mobilization by plants. Furthermore, physiological processes occurring in the rhizosphere also provide a better understanding of P availability. 31P-NMR technique clearly indicates the distribution of inorganic phosphorus in living cell. Kinetics have elucidated the functional characteristics of plasma membrane and tonoplast inorganic phosphate transporters. Molecular studies have confirmed the presence of multiple genes encoding phosphate transporters. Modeling the P bioavailability is a logical approach to understand the complexity of the P nutrition. Though, the authenticity of the models will largely depends on the accuracy and quality of input data that are very subjective to varying soil conditions. Inclusion of bioavailability of P as a parameter in crop modeling will help in solving the large black box of processes and mechanisms of P uptake from soil that has perplexing and hitherto not well understood. The aim of this chapter is to give an overview of P uptake processes and mechanisms involved in plants and the chemical processes that are directly induced by plant roots which can affect the concentration of P in soil solution and, ultimately, the bioavailability of soil phosphorus to plants.