Einfluß der Kaliumdüngung auf die Verfügbarkeit des Kaliums in K-fixierenden Böden im Verlaufe der Vegetationszeit

Effect of potassium fertilization on K-availability in potassium fixing soils over the vegetative period The effect of potassium fertilization on the K-availability over the course of the vegetative period was tested on nine potassium fixing soils over two years. The availability of potassium fertilized at the beginning of vegetative period falls (on a soil with 50% clay) to half of the original amount within one month. The availability decreases especially during the time without vegetation. Only after fertilization with 1200 kg K₂O/ha are substantial amounts of potassium available for the plants until the end of the vegetative period (determined by electroultrafiltration). An annual application of 300 kg K₂O/ha only slightly increased the CAL-K and exchangeable K in comparison with the unfertilized plots, although the plants showed marked yield responses. A significant improvement in potassium supply was not obtained with less than 600 kg K₂O/ha. The potassium wet fixation values are relatively constant on the plots without or with low K-fertilization: after high potassium application they show considerable fluctuation. The fixation potential is considerably reduced with an annual application of 1800 kg K₂O/ha, but is not completely removed. Potassium can be removed by repeated extraction with a dilute CaCl₂-solution but the rate at which it is released into the soil solution is too slow compared to the needs of rapidly growing crop plants. The rise in the level of available potassium by fertilization is reverse to the clay content. There was considerable variation between the different available potassium forms over the vegetative period on the highly fertilized plots; consequently a soil test made annually can only give a rough idea of the K-level in potassium fixing soils.