Keywords: Potassium Cycle, Fixed Potassium, Foliar Application, Modelling.

Potassium (K) is an essential nutrient for plants and animals, including humans; its role in agricultural production is firmly established. Also, K constitutes no hazards to human health and has no deleterious effects on water quality. It is highly unlikely that the world’s supply of high quality K sources will be exhausted within the next few centuries. The needs and opportunities for further work on K in agriculture relate to economic issues, particularly improved recommendations for K fertilizer use. To achieve this a better understanding and integration of the relevant soil and plant factors is required.

The chemistry of K in soils is relatively simple, compared to that of nitrogen and phosphorus. Exchangeable K is a good indicator of soil K status and the likelihood of obtaining a response to K fertilizer in many soils. However, for soils containing partially weathered micaceous minerals, fixed K (which is slowly available) is a potentially important source of K for crops. Work in the United Kingdom in the 1950s and 1960s emphasised the likely significance of fixed K (often referred to as non-exchangeable K) as a source of K for crop growth. As exchangeable K levels have been built up and the likely contribution of fixed K has diminished, this interest has waned but it would be worth reconsidering the relationship between non-exchangeable K and initially exchangeable K in soils where mineralogical composition is known. This is because it may be possible to moderate exchangeable K values, based on an estimate of the supply of fixed K, in developing more precise fertilizer recommendations for K.

Expressing plant K concentrations for cereals on a tissue water basis provides essentially constant values for K concentration which are little affected by fertilizer N and P and water supply. This method offers promise for diagnosing K deficiency for these crops but commercialisation of the possibility has been slow. Foliar application of K has produced very positive results in some experiments but further work is required for this method of application in specific situations, such as high magnesium soils. Crop offtake of K has increased substantially in recent years as yields have increased and more straw has been removed. Fertilizer recommendations for K are being modified accordingly.

Modelling offers the best prospects for improving our understanding of the dynamic interactions between the soil supply of K and crop demand for K. There have been advances in modelling the uptake of K by crops but there is a shortage of data to validate such models. To find widespread acceptance at the field level, such mechanistic models require a better integration between the soil and crop components, simplification, and validation.

Prof. J Keith Syers, University of Newcastle, Newcastle upon Tyne, UK.

32 pages, 10 figures, 2 tables, 73 refs.