Amounts, Forms, and Solubility of Phosphorus in Soils Receiving Manure

TLDR

Continually land‑applying manure at rates exceeding crop removal can alter soil P chemistry and raise soil P to environmentally concerning levels. To assess the effect of long‑term manure application on soil P forms and solubilities. We measured water‑extractable P, Mehlich‑3 P, Hedley‑P fractions, and crystalline Ca‑P minerals in surface soil from 20 sites across New York, Oklahoma, and Pennsylvania, sampling soils that received dairy, poultry, or swine manure at 40–200 kg ha⁻¹ yr⁻¹ for 10–25 yr. Manure‑treated soils exhibited higher total P and a shift from Al/Fe‑bound to Ca‑P minerals, producing higher Mehlich‑3 P but lower water‑extractable P, a pattern linked to increased exchangeable Ca and suggesting that Mehlich‑3 P may overestimate P losses from heavily manured soils.

Abstract

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.