Exchangeable potassium in soil as indicator of potassium status in an organic crop rotation on loamy sand

In organic agriculture, where K may be a limited resource, reliable tools are important in the assessment of K availability in the soil in order to avoid K deficiency. We investigated the effect of four organic farming systems on the exchangeable K in the plough layer of a six-course crop rotation from 1994 to 1997. The accumulated K balances over the four years varied between -49 and +120 kg K ha(-1) and the corresponding exchangeable K (0-20 cm) in autumn 1997 was 7.1 and 9.6 mg K 100 g soil(-1), respectively, as an average of the crop rotation. The exchangeable K fraction responded to the K application in manure and to the crop in the rotation. In an additional experiment, no yield response to K was found, despite a low level of exchangeable K. The exchangeable K was a useful indicator of changes in the K status in the farming system with the largest positive K balance, but this K fraction was insufficient as an indicator in the other three farming systems. The considerable variation of exchangeable K through the crop rotation makes the soil test method most suitable at the crop rotation level where fluctuations caused by crop and management are smoothed out.