Availability and Form of Copper in Three Soils following Eight Annual Applications of Copper‐Enriched Swine Manure

Abstract

AbstractThe use of high levels of Cu in swine (Sus scrofa domesticus) production to stimulate growth and improve feed efficiency is presently being questioned due to the potential environmental hazard from repeated usage of the resulting Cu‐rich manure in agricultural land. Three field studies were established in 1978 and continued through 1985 to determine the effects of long‐term applications of Cu, as either CuSO4 or Cu‐enriched swine manure, at high rates on corn (Zea mays L.) production. The studies were conducted on Bertie fine sandy loam (Aquic Hapludults), Guernsey silt loam (Aquic Hapludalfs), and Starr‐Dyke clay loam (Fluventic Dystrochrepts‐Typic Rhodudults) in the Coastal Plain, Ridge and Valley, and Piedmont regions of Virginia, respectively. These soils have CEC ranging from 5.0 to 12.3 cmolc kg−1. According to USEPA guidelines, which are based on the CEC values and pH levels ≥6.5, the three soils have a maximum safe Cu loading rate of 280 kg Cu ha−1. After 8 yr, 890.4 t ha−1 of wet Cu‐enriched manure had been applied to these soils. The manure contained up to 1550 mg Cu kg−1 and was produced by pigs fed diets with an average of 251 mg Cu kg−1. Application of 336 kg Cu ha−1 as CuSO4, or up to 264 kg Cu ha−1 as Cu‐enriched manure, from 1978 through 1985 did not decrease corn grain yields on the three soils. The eight annual Cu applications increased Cu concentrations in ear leaf tissue by ≤2.1 mg kg−1, but did not affect the Cu concentrations in the grain. The Cu levels in all plant tissue remained within normal ranges. Diethylenetriaminepentaacetic acid (DTPA)‐extractable Cu was linearly related to the amount of Cu applied to the three soils (r = 0.93−0.98, α = 0.01) and was unrelated to Cu levels in grain or leaf tissue. Small increases, up to 1.6 mg Cu kg−1, in extractable Cu in the B horizons of the Bertie and Starr‐Dyke soils indicated little downward movement of the applied Cu. Larger increases in extractable Cu in the Guernsey subsoil were attributed to downward movement caused by plowing. Copper fractionation data indicated that a substantial portion of the applied Cu has reverted to forms not available to plants.