Phosphorus solubility in response to acidification of dairy manure amended soils

Manure additions beyond crop P needs result in accumulated soil Ca phosphate (Ca-P). Although stable near neutral pH levels, there is concern about the solubility of accumulated soil Ca-P when soil pH conditions become acidic, potentially releasing water-soluble P (WSP). The purpose of this study was to examine changes in WSP among non-acidic, Ca-P-accumulated soils in response to six levels of acidification and acidification timing. Two soils that historically received excessive dairy manure were incubated for 8 wk with pH adjusted by single-point or gradual acid additions. After 8 wk, acid additions ceased and WSP, Mehlich-3 P (M3-P), and pH were determined before continuing the incubation for two additional weeks. Soil pH had little effect on M3-P, while acidification timing interacted with the effect of pH on WSP concentrations. After 8 wk, single-point acidification resulted in WSP decreases with decreasing pH, but gradual acidification showed the opposite effect in the pH range 4.5 to 6.5. This effect disappeared after two additional weeks of incubation, resulting in overall decreased WSP concentrations. Results suggested that among the high-P (Ca-P) soils used in this study, which contained significant Fe and Al, acidification did not increase WSP beyond the original concentrations since dissolved Ca-P was resorbed onto soil Fe and Al. This subsequent resorption of P was not immediate, however, and was dependent on kinetics. Overall decreases in soil WSP resulting from acidification is beneficial to water quality through prevention of nonpoint dissolved P losses from soils to surface waters.