Coamendments to Modify Phosphorus Extractability and Nitrogen/Phosphorus Ratio in Feedlot Manure and Composted Manure

Abstract

Abstract A narrow nitrogen to phosphorus ratio (N/P) exists in animal manure, resulting in P high loading and pollution risks when land applications are made to meet N demands of a crop. Nonhazardous mineral by‐products potentially widen the effective N/P ratio in manure. We determined the effects of alum, caliche, and Class C fly ash on extractable P concentrations in stockpiled and composted cattle ( Bos taurus ) manure at rates of 0, 0.10, 0.25, and 0.50 kg kg −1 manure. The mixtures were also added to Amarillo fine sandy loam (Aridic Paleustalf) and Pullman clay loam (Torrertic Paleustoll) at the rate of 22 Mg ha −1 . Caliche, alum, and fly ash reduced water‐extractable P (WP) in stockpiled manure by 21, 60, and 85% and by 50, 83, and 93% in composted manure at the O.1kg kg −1 rate. Alum and fly ash also significantly reduced Bray‐I P (BP) concentrations by 75 and 90% in stockpiled and composted manure, respectively, and >90% at higher rates. Fly ash also decreased Mehlich III‐P (MP) concentrations, ranging from 50 to 98%. Mixing these amendments with feedlot manure widened the effective manure N/P ratio by a factor ranging from 1.5 to 18. Applying fly ash‐treated stockpiled or composted manure reduced all extractable P fractions from amended soils. Alum and caliche consistently reduced the WP fraction only. Coapplications of mineral and animal by‐products stabilized manure P where uncovered storage is unavoidable or widened its effective N/P ratio, allowing land application rates needed to fulfill plant N requirements without causing water quality impairments.