Storage-induced changes in phosphorus solubility of air-dried soils

Soil is commonly stored in an air-dried state for extended periods before chemical analysis. The effect of storage on P solubility was assessed by determining NaHCO3-extractable P concentrations in a range of pasture soils from England and Wales (total C = 28.9-80.4 g kg(-1), clay = 219-681 g kg(-1), pH = 4.4-6.8) immediately following air drying and after 3 yr of storage at ambient laboratory temperature. Following storage, NaHCO3-extractable inorganic P concentrations decreased by between 2 and 60% (mean decrease = 21%), while NaHCO3-extractable organic P concentrations increased by between 48 and 156% (mean increase = 95%). The greatest changes occurred in soils of pH < 5.3. The changes appear to result from the disruption of organic matter coatings on mineral surfaces, continuous solid-phase diffusion of phosphate into soil particles, and decomposition of microbial cells. The results have important implications for the determination of NaHCO3-extractable P in stored soils and highlight the importance of working with fresh samples to derive information with relevance to field conditions.