http://orgprints.org/391/ Exchangeable potassium in soil as indicator of potassium status in an organic crop rotation on loamy sand. Askegaard, Margrethe Eriksen, Jørgen Farming Systems 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 whereas 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. 2002 Journal paper NonPeerReviewed application/pdf http://orgprints.org/391/1/AE2002.pdf Askegaard, Margrethe and Eriksen, Jørgen (2002) Exchangeable potassium in soil as indicator of potassium status in an organic crop rotation on loamy sand. Soil Use and Management, 18, pp. 84-90. http://orgprints.org/391/http://orgprints.org/391/1/