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In the Czech Republic, negative potassium (K) budget in agricultural soils is caused by non-fertilization by K and by a decline of manure application. We investigated soil available, fixed (acid-extractable, K fix ) and structural K pools in the field trial with graduated K application rate, established in 1972 at 8 sites of different climate and soils. The content of K-bearing minerals was evaluated on semi-quantitative scale by XRD diffraction. K-feldspars were a dominant source of structural K. Total soil K consisted of 1.7–7.1% of fixed K, which was in a positive relation to mixed-layer phyllosilicates. Differences in available K in treatments with K budget lower than –30 kg K/ha/year were small compared to those of fixed K. In control treatments, calculated average depletion of available K was –18 kg K/ha/year and the average depletion of fixed K was –12 kg K/ha/year; however at sites of higher altitude fixed K depletion prevailed. Fixed K accounted for 6–31% of the K budget. In negative K budget, monitoring of K fix is advisable to avoid fertility loss of soil with low K supplying capacity.

Long-term effect of low potassium fertilization on its soil fractions