Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/25/2003
Publication Date: 11/1/2004
Citation: Sharpley, A.N., McDowell, R., Kleinman, P.J.A. 2004. Amounts, forms, and solubility of phosphorus in soils receiving manure. Soil Science Society of America Journal. 68:2048-2057. Interpretive Summary: In the last 20 years, the number and size of livestock operations have increased substantially. Large amounts of manure are produced in areas where land available to receive manure applications is limited. Although it is known that continual application of manures can alter such soil properties as pH, organic matter and phosphorus content, less is known about how the forms and solubilities of soil phosphorus are affected. A clearer understanding of the forms and solubility chemistry of phosphorus in manured soils is needed to refine soil test methods and develop soil test phosphorus thresholds for crop response, environmental risk assessment and manure management recommendations in general. For 20 soils from New York, Oklahoma and Pennsylvania, we found that applying manure for more than 15 years can result in an accumulation of large amounts of phosphorus in forms that are readily available for both plant uptake and release to runoff water. Manure application also results in a shift in phosphorus chemistry and reaction products from aluminum- and iron-dominated complexes to Ca minerals. In other words, manured soils tend to behave more like calcareous soils than their untreated analogues. This has important implications to environmental recommendations for agricultural phosphorus management if routine acid-based soil test extractants are used. Our data suggests that an environmental threshold of 500 mg/kg exists for soil test phosphorus (as determined by the Mehlich-3 method), above which this extractant no longer adequately estimates the potential phosphorus enrichment of overland flow by soil phosphorus release.
Technical Abstract: Continually land applying manure at rates exceeding crop removal can change soil phosphorus (P) chemistry and increase soil P to levels that are of environmental concern. To assess the effect of long-term manure application on soil P forms and solubilities, we determined water extractable P, Mehlich-3 P, Hedley P fractions, and crystalline Ca-P minerals in surface soil (0-5 cm) from 20 locations in New York (n = 6), Oklahoma (n = 8), and Pennsylvania (n = 6), which received dairy, poultry or swine manure (40 - 200 kg/ha/yr) for 10 to 25 yr. For all untreated and manured soils, pH averaged 5.9 and 6.6, exchangeable Ca 1.7 and 6.2 g/kg, organic C 15.7 and 32.6 g/kg, and total P 407 and 2481 mg/kg, respectively. As Mehlich-3 P increased (4 - 2835 mg/kg), the proportion that was water extractable declined (14 - 3%) as a function of increasing exchangeable soil Ca (R^2 of 0.81). Results suggest that addition of manure to soils shifts P from Al-and Fe- to Ca-P reaction products, accounting for the relatively greater Mehlich-3 but lower water extractability of soil P. This shift has implications to environmental soil P testing. For instance, the fact that Mehlich-3 P has been shown to overestimate potential losses of P in overland flow from heavily manured soils may be explained by dissolution of Ca-P minerals not soluble in water.