Introduction

Green plants utilise the sun's energy to synthesise organic compounds from carbon dioxide and water. The pioneering work, concurrently carried out by Liebig in Germany and Lawes & Gilbert in the UK more than a century ago, conclusively showed that plants must take up inorganic nutrients from the soil to produce these organic components. Since that discovery it has been established that many elements are necessary for optimum functioning of the biochemical machinery of the plant. Most of these are necessary in such small amounts, however, that the supply from the seed, or from natural sources suffices. In agriculture the situation is often different for the macro-elements nitrogen, phosphorus and potassium that are needed in such large quantities, especially where crop management practices aim at very high yields, that the supply from natural sources falls far short of the demand. Fertiliser experiments show that, up to a certain level, addition of these elements from a fertiliser bag leads to higher yields. Unfortunately, interpretation of these fertiliser experiments seldom exceeds the derivation of the optimum nutrient application rate for the conditions of the experiment, either in physical or in economic terms. The lack of explanatory conclusions hinders the use of such results for predictive purposes, for example, in the formulation of fertiliser recommendations for the farmer.