The Effect of Drying and Drying Temperature on Soil Test Values.

Soil samples are dried at various temperatures when samples are prepared for analysis, which may result in different results of the same sample. This project was conducted to determine the effect of drying and drying temperature on the results of common soil test analytes. Twenty-seven different soil samples from major agricultural regions of the U.S. were obtained and prepared for this study.  The samples were hand ground to pass 2 mm sieve and divided into 6 portions. One of the 6 portions was kept at field moist condition, and the other 5 portions were dried at 25, 45, 65, 85 and 105°C overnight. Soil pH, and concentrations of ammonium-N, nitrate-N, plant available K, P, Ca, Mg, Cu, Fe, Zn, Mn, B, organic C, and total N were determined using standard methods. In general, sample drying increased the concentration of most analytes compared with the field moist samples. The contents of Fe, B, Zn, Mn, NH₄-N, organic carbon and total nitrogen were increased by drying temperature. The concentrations of and Ca, NO₃-N, and Cu, however, were not changed by drying temperature. In addition, values of P, K, and pH, however, were not consistently affected by temperature, but Mg was decreased by drying temperature. It is important for laboratories to use a standardized sample drying temperature to accurately make fertilizer and lime recommendations.