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 ( R 2 = 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.