Plant available potassium assessment through chemical prediction methods

Understanding factors that affect the potassium (K) status of soil is important to chemical prediction of plant-available K in corn (Zea mays L.). Soils vary widely in their ability to replenish labile K from nonlabile forms. No research has been done to investigate the effects clay fraction mineralogy of South Dakota soils have on the efficacy of chemical prediction methods. The objectives of this study were to (i) evaluate the ability of 1.0 M NH4OAc-extractable soil K test and other common chemical extractants to predict plant-available K, and (ii) relate clay fraction mineralogy to chemical prediction of tissue K concentration. Composite soil surface samples (0-15 cm) and corn tissue samples were collected from eight different corn production field sites across east-central South Dakota. Plants grown at these sites either exhibited or did not exhibit K deficiency symptoms. Soils were extracted and plant-available K was determined by four common procedures. X-ray diffraction and total colloidal CEC analyses were also determined for all soils using standard procedures. When exchangeable Ca2+ and Mg2+ were included in the prediction model. NH4OAc- and CaCl2-extractable K+ explained 80 and 85% of the variation in plant K concentration, respectively. The NH4OAc and CaCl2 methods proved superior to NaBPh4 5- and 15-minute extraction in estimating plant K availability. Since the clay fraction mineralogy of the soils in this study was montmorillonite, there was no advantage to using the NaBPh4 method to estimate plant K concentration.