Nonexchangeable Potassium Removal from Soils by Successive Acid Extractions as Related to Removal by Greenhouse Crops

NUMEROUS attempts have been made to measure nonexchangeable potassium availability in soils by chemical methods. Reitemeier et al. (7) made measurements of the rate of potassium release from several soils by chemical and cropping methods. In these comparisons, the methods of boiling the soil in N HNO3 for 10 minutes (5), of 30-day electrodialysis, and of using a modified Neubauer technique (8) gave good agreement with conventional cropping methods. Attoe and Truog (2) distinguished between readily, moderately, and difficultly available forms of potassium in soils. The extraction of a soil with ammonium acetate removed the readily available fraction; whereas, the leaching of a soil for 1 hour with 0.5 N HC1 also removed the moderately available fraction. Alternate wetting and drying (9) of soils has been shown to fix soluble potassium when present in large amounts and, conversely,to cause release of the fixed forms at low levels of soluble potassium (3). A part of the present investigation was based on the above findings and partly on the premise that repeated extractions with intervening dryings of a soil might cause fixed forms of potassium to become soluble in rates or amounts indicative of the potassium-supplying capacity of the soil in 'question. The purpose of the present investigation was to gain information regarding the status of nonexchangeable or fixed forms of potassium in several agriculturally important soils. Nonexchangeable potassium availability in these soils was measured by a cropping method using alfalfa as the test plant. These availabilities were compared with the amount of potassium solubilized in soils when subjected to repeated NH4Ac or HC1 extractions with intervening dryings.