Nutrient availability in soils

Only a proportion of the total nutrient amount in soil can be taken up and utilized by plants. This proportion varies with nutrient and is influenced by a range of soil, plant, and environmental factors. In this chapter, methods for estimating nutrient availability and their limitations are discussed. In the soil, nutrients move to the soil surface by mass flow and diffusion, and the relative importance of these two processes varies with nutrient. Nutrient movement by diffusion is slow; therefore depletion zones develop around roots. The factors affecting the extent of this depletion zone, namely, root hair length and release of nutrient-mobilizing exudates, are described. The roles of root density, soil structure, and soil water content on nutrient availability are discussed. The chapter concludes with a critical assessment of the usefulness of soil tests and modeling approaches to improve the understanding of nutrient availability in soils.