ИСТИНА

The aims of the work were (I) to determine optimal application rates of KCl based on crop response to K application, crop quality, K balance, and economic parameters; (II) to assess the suitability of the existing K test methods to predict K supply. KCl application rates were 0 to 280 kg/ha for sugar beet, corn, rape, and soybean. In soil samples collected at the start of experiment and during three next years, exchangeable (in 1 M NH4OAc), mobile (in 0.5 N HAc or in 1% (NH4)2CO3), and easily exchangeable K forms (in 0.01 M CaCl2) were determined. It was found that K supply depends on the exchangeable K pool. During the first season of KCl application the positive K balance resulted in increase in the content of exchangeable and mobile K. At the negative K balance, a decrease in the reserve was manifested, especially for exchangeable K, less likely for easily exchangeable K (r was 0.77 and 0.64, respectively). The comparison of different extracts showed that the correlation between the content of exchangeable and CaCl2-extractable K+ was most stable; relationships between mobile and exchangeable as well as plant-available K forms were of more local character. Optimal application rates of KCl based on the set of criteria were proposed with consideration for the partial compensation of K removal by crop.