A Rapid Assay to Estimate Soil Microbial Biomass Potassium in Agricultural Soils

Various assays for microbial biomass C (C-mic) and other elements in the soil are available but there are none for microbial biomass K (K-mic). Our objective was to develop a rapid chloroform-fumigation extraction (CFE) assay to estimate K-mic. The extractants ammonium acetate (CH3COONH4) and sodium hydrogen carbonate (NaHCO3) were tested at three molarities on a Hoytville clay. A 1 mol L-1 CH3COONH4 yielded the highest levels of extractable K. To calibrate the CFE efficiency for K-mic, bacteria and fungi isolated from soils were cultured, suspended, and mixed with three soils of different texture: a fine sand, a silt loam, or a clay. The known concentrations of K-mic, in the inoculum served as a basis to calculate the microbial biomass K conversion factors (k(EK)) for the three soils tested. Soils from corn (Zea mays L.) fields in Ohio under no-till, minimum tillage, and plow tillage had K-mic values ranging from 68 to 179 mg K kg(-1). The K-mic value was highest in the silt loam under no-till and in the clay soil under minimum tillage. Soil exchangeable K(K-exch) and nonexchangeable K (K-nexch) did not correlate with K-mic, but with total C. On average, K-exch was 2.7 and K-nexch 8.3 times higher than K-mic. The data presented in this study are based on an initial study, and more soils and ecosystems need to be evaluated before the method can be generally adopted for soil samples. Our study shows, however, that K-mic is a significant K pool in soils that has been overlooked in the past.