Amounts, forms, and solubility of phosphorus in soils receiving manure

Continually land-applying manure at rates exceeding crop removal can change soil 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^-1 yr ^-1) for 10 to 25 yr. For all untreated and manured soils, the pH averaged 5.9 and 6.6; exchangeable Ca, 0.9 and 6.2 g kg^-1; organic C, 15.7 and 32.6 g kg^-1; and total P, 407 and 2480 mg kg^-1, respectively. As Mehlich-3 P increased (64-2822 mg kg^-1), the proportion that was water extractable (14-3%) declined as exchangeable soil Ca increased (R² = 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.