Phosphorus Solubility in Response to Acidification of Dairy Manure Amended Soils

Authors: PENN, CHAD - OKLAHOMA STATE UNIV; Bryant, Ray

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/3/2007
Publication Date: 1/11/2008
Citation: Penn, C.J., Bryant, R.B. 2008. Phosphorus Solubility in Response to Acidification of Dairy Manure Amended Soils. Soil Science Society of America Journal. 72:238-243.

Interpretive Summary: Additions of dairy manure at rates that exceed plant need results in the accumulation of soil phosphorus (P) in the form of calcium phosphate (Ca-P), which is relatively insoluble at neutral pH. Some have expressed concern that if manure additions cease and these soils become acid under a humid leaching environment, then P will become soluble, thereby posing a greater threat to the environment. We subjected two dairy manure amended soils to various rates of acidification over time and observed a short term (two weeks) increase in P solubility. After four weeks, P solubility was less than the untreated soils, indicating that P had reacted with iron or aluminum to form a less soluble chemical compound. These results suggest that soil management operations that result in acidification, such as nitrogen fertilization, may pose a short term increase in the risk of P loss. In the long term, the risk of P loss resulting from acidification of soils that contain at least moderate amounts of iron and aluminum minerals will decrease over time.

Technical Abstract: Phosphorus (P) additions from animal manure beyond plant needs results in accumulated soil calcium 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 at two rates (time). Two soils that historically received excessive dairy manure were incubated for eight weeks with pH adjusted by single point or gradual acid additions. After eight weeks, 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 rate interacted with the effect of pH on WSP concentrations. After eight weeks, single point acidification resulted in WSP decreases with lowering pH, but gradual acidification showed the opposite effect in the pH range of 4.5 to 6.5. This effect disappeared after two additional weeks of incubation, resulting in overall decreased WSP concentrations. These results suggest that for high P soils containing significant Ca-P, Fe and Al, acidification will not increase WSP beyond original concentrations, since dissolved Ca-P is re-adsorbed onto soil Fe and Al. However, this subsequent re-adsorption of P is not immediate and will depend on the kinetics. Overall decreases in soil WSP resulting from acidification is beneficial to water quality through prevention of non-point dissolved P losses from soils to surface waters.