Keywords: Potassium, plant nutrition, arable crops, grass, potassium use efficiency, soil analysis, critical values for soil potassium.

The developing root system of arable crops early in the growing season explores only a very limited volume of soil to take up nutrients. It is essential that this soil contains sufficient readily plant-available potassium (K) during the early stages of growth if the crop is to achieve its economic optimum yield. This is illustrated by the very different pattern of daily uptake of K by spring barley grown on two similar soils but with different amounts of available K and the effects of this on final grain yield. The total amount of K taken up by crops usually exceeds that of nitrogen (N) and with an ample supply of K, crops use N fertilizers more efficiently. The roots of grass crops are efficient in searching soil for nutrients but grass that yields well can remove 800-900 kg K/ha per year. Consequently soils growing grass must also have an adequate supply of readily plant-available K if this demand is to be met. In farming systems where grass and arable crops are grown alternately (ley-arable systems), if the large amounts of K taken off in grass and other ley crops are not replaced, the amount of plant-available K in soil can decline to such an extent that the yields of following arable crops are decreased, and examples are given here. It is possible to determine a critical level of readily plant-available K in soil for crops, farming systems and soil types; examples are given as are ways of maintaining the critical level once it has been achieved. This relies on estimating K balances and periodically analysing soil.

A conceptual framework to describe soil K is presented in which soil K is considered to exist in four pools related to the availability of the K to plants. These pools are: soil solution K, exchangeable K, fixed K and lattice K and the K in these pools is immediately available, readily available, less readily (slowly) available and very slowly available, respectively, for plant uptake. The evidence presented shows that K can transfer reversibly between the first three pools. When K fertilizer is applied to a soil, some K remains in the soil solution and some rapidly transfers to the exchangeable and fixed K pools. As plant roots take up K from the soil solution it is replaced by K in the exchangeable and fixed K pools. It appears that in a 'steady state' situation, there is an equilibrium between exchangeable and fixed K. At equilibrium the amount of K in each pool depends on the number of exchange sites available for holding K and this is related to the amount and type of clay minerals.
A new approach to estimating K use efficiency is presented. Using soil analysis to manage the use of K in crop production is discussed.

A E Johnston, A and E Division, Rothamsted Research, Harpenden, Hertfordshire AL5 2JQ, UK.

39 pages, 11 figures, 13 tables, 64 references.