Assessment of plant available potassium in east-central South Dakota soil as related to quantity-intensity, reduction -oxidation, and chemical predictio

Potassium (K) deficiencies in corn (Zea mays L.) have been increasing throughout eastern South Dakota. No research has been done in South Dakota to evaluate the affects which soil chemical properties have on K deficiency in corn. The specific objectives of this research were to (1) identify production soils in east-central South Dakota which may demonstrate K deficiency potential for corn; (2) evaluate K quantity-intensity relationships and determine the potential of several chemical extractants to predict available K levels for corn; and (3) determine the effects which reduction-oxidation and phyllosilicate clay mineral type have on soil-K availability.;Soil and plant tissue samples were collected from eight different corn production field sites across east-central South Dakota in 1997. At each field site, samples were collected from areas where corn plants exhibited classic K deficiency symptoms (marginal chlorosis of lower leaves), and from areas where corn plants appeared healthy (K sufficient). Complete chemical analyses were conducted on plant tissue and soil (0--15 cm) samples. In addition, quantity-intensity relationships and typical Q/I parameters were determined. Soil samples (0--15 cm) from 4 of the 8 study sites were collected in 1998. The soils were used in a greenhouse study to evaluate reduction-oxidation effects on K+ fixation. Total colloidal cation exchange capacity and X-ray diffraction were also determined.;The soils studied in east-central South Dakota appear to be at, or near, a critical labile K level. Quantity-intensity relationships show these soils to be very low in plant-extractable soil K. The DeltaK0 (exchangeable K+) values ranged from 0.176 to -0.470 cmolc kg-1, and the ARke values ranged from 0.0013 to 0.013 moles L-1. The soils' potential to buffer plant-depleted K was also low. Without proper K fertilization these soils will be unable to sustain K levels necessary for proper plant growth and production.;Montmorillonite is the dominant phyllosilicate species in the clay-sized fraction of east-central South Dakota soil. These soils would not be expected to "fix" or release much added K. A single redox event does not increase K+ fixation, and biochemical reduction is not the cause of K deficiency in east-central South Dakota soil. Further research over a wider range of eastern South Dakota soils is needed to verify the findings of this study. Release kinetics of K+ from K-bearing minerals require further study.