Effect of sample pretreatment on extractable soil potassium

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Abstract
Effect of soil sample treatment on potassium (K) release characteristics of Northern Great Plains soils was studied. Samples of agricultural soils were extracted with neutral normal ammonium acetate for K after selected pretreatment. Field-moist sub-soil samples with approximately 100 mg/kg extractable K release nearly double this amount after they were air or oven-dried prior to analysis. The percentage increase in extractable K in dried soils decreased as the K level in the field-moist soils increased. At 420 and 500 mg/kg, the extractable K content of air-dried and oven-dried soils, respectively, equaled the extractable K content of field-moist samples. Above these concentrations, drying decreased extractable K. Oven-drying (60 C) affected extractable K more severely than did air-drying the soils. Drying and grinding the 15- to 30-cm depth samples significantly increased exchangeable K compared to field-moist samples. Rehydration of oven-dried soils to 40% water did not affect extractable K. However, results indicated that rehydration of dried soils to 40% water, followed by freezing (-29 C air temperature) for 15 hours, and redrying the samples to a range of 0.5 to 5% water, returned extractable K to the level which existed in the original field-moist samples.