Sixty-one experiments with 15 levels of K fertilizer in the presence of excess N and P fertilizer were carried out on adjacent sites of the same field. Yield was always related to level of K fertilizer by a ‘diminishing returns’ type curve, and a derived equation, which defined relative responsiveness in terms of a single parameter, fitted the data for each crop very satisfactorily. Although the responsiveness of many of the crops was similar there were marked differences and the optimum levels of K (defined as the level at which a further 10 kg/ha increased yield by 1%) varied from 0 to 360 kg/ha, depending on the crop. Responsiveness was largely independent of the plant family to which the crop belonged, but was related to the mean plant weight atharvest; the larger the weight the less reponsive the crop. No general relation existed between responsiveness and duration of growth.
The % K in the dry matter of leaves (including stems) at harvest of crops receiving the optimum levels of K fertilizer was mainly determined by the family. It was generally between 0·9 and 1·1 for the Amaryllidaceae, between 1·1 and 1·2 for the Leguminosae and between 1·9 and 2·5% for the Cruciferae. The difference between the % K in the dry matter with the optimum level of K fertilizer and that with no fertilizer was proportional to responsiveness. Percentage K at harvest was a good indicator of the extent to which crop growth was restricted by lack of potassium.
At harvest crops receiving the optimum levels of K fertilizer contained between 29 and 220 kg/ha of K, but uptake increased asymptotically to a maximum as K applications were raised to higher levels. Maximum uptake for nearly all crops was almost double the uptake with the optimum fertilizer application.
Percentage recovery of 100 kg/ha of added K fertilizer varied between 8 and 70%, roughly in proportion to the total crop dry weight, which varied between 1 and 15 t/ha.
Effects of level of K fertilizer on crop quality were also measured and over the practical range of applications the effects were generally small.
The differences between the K requirements of crops are discussed and it is argued that the responsiveness of one crop relative to that of another would be expected to be similar on a range of soils.