Effect of Potassium Budget on Evolution of Soil Potassium in Different Crop Sequences and Site Conditions

The influence of six different crop sequences on the dynamics of topsoil available and fixed potassium (K_avail and K_fix) was studied within a 4-year period at 6 sites of different soil and climatic conditions. The highest K off takes were recorded for 3-year alfalfa / clover cropping followed by winter wheat (-151 kg K/ha/year) and for the crop sequence including one year of sugar beet / potato (-0124 kg K/ha/year). K_avail and K_fix were significantly decreased especially at crop sequence with alfalfa / clover cropping, compared to other crop sequences. At three sites of lower K_fix content (340-830 mg K/kg), differences in K_avail and K_fix between crop sequences were more frequent, whereas almost no significant differences between treatments were observed at sites of higher K_fix content (1,000-1,380 mg K/kg). Changes of both K forms and K budget correlated significantly at sites of lower K_fix content. At these sites, K_avail decreased at the rate of 0.045 mg K per 1 kg/ha of K offtake; the rate of K_fix decrease was 0.059 mg K per 1 kg/ha of K offtake. At sites of higher K_fix, no significant relation between K forms and K budget was observed. The results show an importance of K_fix as a K source in intensive agricultural systems with low or no K inputs.