The effects and fate of copper from pig slurry when applied to soil
Copper is widely used as a growth promoter at levels of up to 250 mg Cu/kg in rations for fattening pigs, resulting in the production of excreta containing high concentrations of this element. Since livestock excreta have traditionally been returned to the land as a fertiliser, concern has been expressed regarding the possible effects on the soil-plant-animal system of disposal of large amounts of copper-rich pig manures or slurries on agricultural land.(a) The effect on copper content of herbage and accumulation of copper in soil following application of copper-rich pig slurry to pasture was investigated in field studies. Slurry produced by pigs receiving supplementary copper at a level of 200 mg Cu/kg DM in the ration contained up to 760 mg Cu/kg DM and was incorporated into soils prepared for seeding with rye grass and white clover. The slurry copper, applied at a rate of 9 kg/ha increased the copper content of the mixed herbage by a maximum of 1.5 mg/kg D,M, on a light sandy soil; herbage Cu content was unchanged on an imperfectly drained clay-loam soil.Application of slurry copper at a similar rate to the established sward increased herbage copper contents by a maximum of 6.5 and 5.6 mg Cu/kg DM on the sandy and the clay-loam soils respectively. Copper in clover was increased to a greater concentration than that in rye grass, the respective maximum concentrations in each species being 24.8 and 13.4, mg Cu/kg DM.Repeated applications of copper-rich slurry to the soil surface in an established rye grass sward increased herbage copper content from 4 to 16 and from 8 to 23 mg Cu/kg DM on the sandy and on the clay-loam soils respectively. Dilution of the slurry with water resulted in greater increases in herbage copper than did undiluted slurry.The addition of up to 30 kg slurry Cu/ha over a 2 year period markedly increased the level of EDTA-extractable copper in the soils; it was estimated that 89 to 90% of slurry copper applied was extractable from the topsoil using this reagent.(b) Less than 5 mg Cu/l were found in the aqueous phase from slurry, while a fine particulate plus colloidal fraction in the solids contained in excess of 2000 mg Cu/kg DM. Evidence was obtained suggesting that copper in the dried slurry solids was associated with the organic matter and was not present as the insoluble sulphide. Copper in the aqueous phase from slurry was found to increase the concentration of copper in rye grass.(c) Carbon mineralisation in soil was unaffected by addition to the soil of a fraction of slurry solids containing 2278 mg Cu/kg DM which increased the total copper content of the soil by 108 mg/kg.(d) The true availability (4.3%) "to sheep of copper in dried slurry solids was found to be similar to that of copper fed as CuS04. The availability (5.8%) to sheep of copper in herbage grown on slurry treated land was significantly greater than that (4°8%) in herbage from untreated land.(e) The potential hazards to crops and livestock of disposal of high copper pig slurries on agricultural land are discussed.