POTASSIUM FERTILIZATION ON SANDY SOILS IN RELATION TO SOIL TEST, CROPYIELD AND K-LEACHING

To study the influence of potassium (K) fertilizer rate on soil test K values, crop yield, and K-leaching in sandy coils, four long-term fer tilizer experiments (0-60-120-180 kg K ha(-1) a(-1)) were initiated in 1988 in northern Germany on farmers fields. Clay content of the plow layer was about 4%, and organic matter between 2% and 5%. Plant availa ble soil K was estimated with the double lactate (DL) method. Small gr ain cereals (rye and barley) did not respond to K fertilization in the 7-year period even though the soil test value of the K-0 plots decrea sed from ca. 90 to ca. 30 mg K-DL kg(-1) within 3 years. This value re mained almost constant thereafter. Crop removal (including straw) of 7 5 kg K ha(-1) a(-1) was therefore apparently supplied from nonexchange able K fractions. Compared to the optimum, no K application reduced th e yield of potato by up to 21%, and that of white sugar yield up to 10 %. Maximum potato yield was obtained by annually applying 60 kg K ha(- 1) which resulted in a test value of 60 mg K-DL kg(-1) soil. Maximum p otato yield was also obtained at 40 mg K-DL kg(-1) soil, however, with a single application of 200 kg K ha(-1). Similar results were obtaine d with sugar beet. This indicates that for maximum yield, even for K d emanding crops, it is not necessary to maintain K-DL values above 40 m g K kg(-1) soil throughout the entire crop rotation. Soil test values increased roughly proportional to the K fertilizer level. About 120 kg fertilizer K ha(-1) a(-1), markedly more than crop K removal, was req uired to maintain the initial K-DL of 90 mg kg(-1). The K concentratio n of the soil solution in the top soil measured after harvest was incr eased exponentially by K fertilizer level and so was K leaching from t he plow layer into the rooted subsoil. The leached quantity increased from 22 kg K ha(-1) a(-1) in the plot without K application to 42, 79 and 133 kg K ha(-1) a(-1) in plots supplied with 60, 120 and 180 kg K ha(-1) a(-1) respectively. Soil test values around 100 mg K-DL kg(-1) on sandy soils, as often found in the plow layer of farmers fields, le ad to K leaching below the root zone that may exceed the critical K co ncentration of 12 mg K L-1 for drinking water.