THE ESTIMATION OF LOSSES OF POTASSIUM AND MAGNESIUM FROM CHALKY SOILSIN SOUTHERN ENGLAND - LABORATORY STUDIES

Ten chalk topsoils (0-25 cm) were repacked into columns in the laborat ory. After leaching similar to one year's throughflow in the field, lo ss of K was equivalent to between 9 and 74 kg K/ha. This represented b etween 3 and 30% of the initial exchangeable K with which loss was poo rly correlated, Loss was dependant on the soil solution concentration and was inversely proportional to potassium buffer power The loss of m agnesium in the same columns was between 10 and 22 kg Mg/ha (6 - 21% o f the initial exchangeable Mg). Magnesium loss was poorly correlated w ith exchangeable Mg. When KCl fertilizer was incorporated into the soi ls, the increase in leaching of potassium was 1-35% of the K addition. Application to the top of the column resulted in less leaching than w hen the K as incorporated. Leaching of magnesium was increased by up t o 5 kg Mg/ha. Potassium leaching may be delayed by the underlying A/C horizon but pure chalk, with an extremely low buffer power for K, has little ability to retain K. Extremely calcareous topsoils were the mos t leaky although in practice it is the organic chalk soils on which it is most difficult to attain adequate K levels. On all chalk soils, ma intenance of a high K level with K fertilizer is likely to cause unnec essary long-term leaching losses. Annual, rather than biennial, fertil izer applications are to be preferred.