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.