Keywords: potassium recommendations, forms in soil, critical levels, soil analysis, plant-preferred osmoticum, potassium in tissue water, N x K interactions.
Potassium (K) is an essential nutrient for plants and animals, including humans for whom it presents no hazards to their health. Its use in agricultural production is well established and using it has no adverse environmental impact. There are large reserves of high quality K-containing deposits suitable for exploitation so there should be no limitation (other than financial) on its use. There are issues about categorising soil in terms of crop response to plant-available soil K and applied K fertiliser. This review aims to link the behaviour of K in soil with the agronomic response of crops to K.
Potassium in soils and plants exists as a positively charged ion (K+). Conceptually, soil K can be considered to exist in four pools related to its availability for uptake by plant roots. Soil solution K is immediately available for uptake, exchangeable K (Kex, readily available), fixed K (slowly/less readily available, mainly residues of past K applications) and lattice/matrix K (an integral part of the clay mineral and very slowly available). Fixed and lattice K are not determined separately and are often referred to as non-exchangeable K (Knex). Analytically there are no issues with determining Kex but measuring Knex and lattice/matrix K is not straightforward. Reversible transfer of K between the soil solution, Kex and Knex, and establishing an equilibrium between the amount of K in these pools could explain issues about the reliability of Kex for categorising plant-available soil K. Improving the prediction of crop response to soil and fertiliser K will require a better knowledge of the amount of both Kex and Knex and the amount and rate of transfer of K between these two pools of soil K.
Potassium has two roles in the functioning of plant cells. Small, but essential amounts of K are required for biochemical functions involved in fundamental metabolic processes. Larger amounts of K are needed for the biophysical function of maintaining the water content of plant cells and hence the turgor (rigidity) of each cell and the upright stance of the plant essential for optimum light interception and production of sugars. The plant responds to applied nitrogen (N) both by increasing cell number and cell size so that the plant contains more water and thus more K is needed to maintain turgor. The agronomic responses to N and K derive from the physiologically interacting effect of N and K on tissue hydration and consequential osmotic adjustments in shoot tissues. To ensure crops use applied N efficiently and effectively to produce optimum yields there must be sufficient K accessible to the roots throughout active growth, for which a better measure of the ability of the soil to supply K during this period is essential.
A.E. Johnston, P.R. Poulton, K.W.T. Goulding and A.J. Macdonald, Department of Sustainable Soils and Grassland Systems, Rothamsted Research, Harpenden, Hertfordshire. AL5 2JQ, UK.
M.J. Glendining, Department of Computational and Systems Biology, Rothamsted Research, Harpenden, Hertfordshire. AL5 2JQ, UK.
52 pages, 18 figures, 10 tables, 88 references