Exchangeable potassium and potassium balances in organic crop rotations on a coarse sand

Crops on sandy soils (<5% clay) are exposed to K deficiency due to the small release and high leaching losses of K. Reliable tools are needed to improve the K management in cropping systems with limited K input, such as organic farming where import of nutrients are restricted according to the EC regulations. We investigated K balances and exchangeable K (K-exch) changes in an organic crop rotation experiment. Potassium leaching decreased from 42 kg ha(-1) in 1998/99 to 21 kg ha(-1) in 2000/01 as an average of a crop rotation (spring barley, grass-clover, winter wheat and pea/barley) with manure application and without catch crops. In the same period, spring K-exch decreased from 5.0 to 3.0 mg K 100 g soil(-1) (0-20 cm). The retention of the straw K left in the field after harvest increased with decreasing levels of K-exch. The cereal crops did not respond to K application but in the pea/barley mixture the pea yield increased by 46%. The concordance between measured K balances and changes in K-exch was weak. Exchangeable K is suitable as a tool for K management on a rotational basis, and a K-exch above 3 mg 100 g soil(-1) in the autumn should be avoided to minimize K leaching.