The importance of appropriate plant nutrient management in agricultural soil cannot be overstated. If agricultural and pastoral activity is to be maximised in a sustainable fashion, it is imperative that plants are supplied by proper nutrition. The available quantity of many of these essential plant nutrients is finite and, once depleted or degraded, cannot be easily restored. In addition to the presence of nutrients in the soil, fertility is also influenced by the structure of the soil and the activity of a vast and diverse range of soil microbes. This management process requires an understanding of the function and interaction of all soil components which can be highly complex and, at times, highly variable or even volatile.