Potassium fertilization on sandy soils in relation to soil test, crop yield and K‐leaching

To study the influence of potassium (K) fertilizer rate on soil test K values, crop yield, and K‐leaching in sandy soils, four long‐term fertilizer 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 available soil K was estimated with the double lactate (DL) method. Small grain 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 decreased from ca. 90 to ca. 30 mg KDL kg−1 within 3 years. This value remained almost constant thereafter. Crop removal (including straw) of 75 kg K ha−1 a−1 was therefore apparently supplied from nonexchangeable K fractions. Compared to the optimum, no K application reduced the 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 KDL kg−1 soil. Maximum potato yield was also obtained at 40 mg KDL kg−1 soil, however, with a single application of 200 kg K ha−1. Similar results were obtained with sugar beet. This indicates that for maximum yield, even for K demanding crops, it is not necessary to maintain KDL values above 40 mg 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 required to maintain the initial KDL of 90 mg kg−1. The K concentration of the soil solution in the top soil measured after harvest was increased exponentially by K fertilizer level and so was K leaching from the 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 Kha−1 a−1 in plots supplied with 60, 120 and 180 kg K ha−1 a−1 respectively. Soil test values around 100 mg KDL kg−1 on sandy soils, as often found in the plow layer of farmers fields, lead to K leaching below the root zone that may exceed the critical K concentration of 12 mg K T−1 for drinking water.