Keywords: nitrogen, potassium, sodium, interactions, tissue water, arable crops, grass, exchangeable potassium.

The potassium (K) status of the soil has a considerable influence on crop uptake and response to nitrogen (N). Yield response to applied fertilizer N is decreased by low concentrations of exchangeable K in the soil. The basis of this interaction is explored using historical and current data from Rothamsted Research's experiments at Rothamsted, Saxmundham, Woburn and Broom's Barn on spring barley, winter wheat, potatoes, sugar beet, mangels and grass. It is argued that the agronomic responses derive from the physiologically interacting effects of N and K on tissue hydration and consequential osmotic adjustments in shoot tissues.

G F J Milford and A E Johnston, Rothamsted Research, Harpenden, Hertfordshire AL5 2JQ, UK.

23 pages, 13 tables, 6 figures, 6 references.