Changes in Soil Phosphorus from Manure Application

Abstract

Manure and mineral fertilizer P sources vary in their contributions to soil P pools. We conducted incubation experiments to (i) assess temporal changes in soil P concentration for 84 d following application of KH2PO4 or manure (beef, dairy, poultry, or swine), and (ii) to evaluate interactive effects of P rate, P source, and background P level on soil P pools 90 d after P application. Changes in soil P over time were evaluated following the amendment of a coarse-loamy, mixed, frigid Typic Haplorthod soil (pH 5.8) at a rate of 100 mg total P kg−1 Water soluble P (WSP), 0.01 M CaCl2–extractable P (CaCl2–P), and modified Morgan P (MMP) declined to <3 mg P kg−1 soil within 21 d of application, following an exponential decay function; P extracted by anion-exchange membrane (AEMP) and Mehlich-III (M3-P) also declined rapidly and are attributed to the high level of exchangeable Al and Fe in this soil. Ninety days after application, all soil P pools exhibited linear increases in concentration for application rates up to 800 mg P kg−1 soil, regardless of P source. Phosphorus applied as KH2PO4 was more efficient at increasing CaCl2–P and M-3P. Manure P sources generally had a greater effect on MMP, and poultry manure was more efficient than all other sources. Efficiency of P sources at increasing soil P concentration (b, slope of linear regression) varied from <1% for rapidly available P pools (CaCl2–P) to nearly 50% for more recalcitrant P (M3-P). Efficiency also increased as background M3-P increased from 150 to 750 mg kg−1 soil. The amount of CaCl2–extractable P increased rapidly when soil P saturation ([Mehlich III-P]/[Mehlich-III Al + Fe]) exceeded 0.25 mol mol−1 Manure and KH2PO4 contribute to different soil P pools, and these differences are magnified at high application rates and high background soil P levels.