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

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, Kfix) 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 Kfix is advisable to avoid fertility loss of soil with low K supplying capacity.