Impacts of Moisture Content On Soil Test Potassium.

Soil testing results are critical to determine accurate soil fertilizer application rates. Soil samples collected under different moisture contents can cause variations in soil test K levels. Additionally, soil testing labs oven dry soil samples at low temperature, which might not be a best assessment of soil available K because previous studies have found that soils may fix or release K upon drying.  This study was conducted to evaluate the impacts of moisture content on soil test K levels and the relationship between soil test K level changes and soil K critical values. Five field sites were established throughout Indiana from 1998 to 2002. Sites were cropped in soybean-corn rotation. Four different rates of K fertilizer (0, 67, 134, 202 kg K₂O ha^-1) were applied annually. Several soil samples from each site were picked to provide a range in Mehlich-3 soil test K values from 30 to 200 mg kg^-1. The dry soil samples were rewetted to field capacity and incubated at 25 ºC for 21 days before splitting into three subsamples for different drying methods (moist, air-dry, and oven-dry at 60 º C). Each sample was extracted by both Mehlich-3 and Sodium Tetraphenylborate (STPB) to assess soil available K. The results showed that soils with low test K levels released K, while soils with high test K levels fixed K upon drying. Mehlich-3 soil test K levels did not differ between the air-dried and oven-dried treatment. However, oven-drying increased the low STPB soil test K levels and decreased the high STPB soil test K levels in comparison with air-drying. The relationship between soil test K level changes and the percentage of total soil fixed K is being evaluated.