Crop Removal in Coastal Plain Soils Receiving Long−term, Intensive Applications of Swine Manure

Trace metals like copper (Cu) and zinc (Zn) are added to animal feed as dietary supplements, and trace metals not assimilated by the animals gut are excreted in manure. The accumulation and movement of Cu and Zn in soils has not been as well documented when compared to nitrogen (N) and phosphorus (P). In cases where plant−available Cu and Zn concentrations accumulate to phytotoxic levels, yield may decline for sensitive crops. We investigated the accumulation of Cu and Zn in the topsoil (0 to 15 cm) and the soil profile (0 to 183 cm deep) and estimated uptake by Coastal Bermuda grass (Cynodon dactylon L.) in a Coastal Plain spray field after 4 and 10 years of swine manure effluent application. Effluent was initially applied to a 1 ha portion of this field, which was later enlarged to 5 ha. Top and subsoil samples were collected at nearby control sites that received no swine manure. All soil samples were extracted using Mehlich 3 (M3) and double acid reagent for plant−available and total Cu and Zn concentrations. Comparison of trace metal concentrations in the topsoil after 4 and 10 years of effluent application with control soils showed that total Cu, and M3 and total Zn had accumulated. Mehlich 3 and total Cu and Zn accumulation rates both declined after an increase in spray field area and decrease in effluent application rates. In a 1 ha region of the field, total Cu and Zn concentration both increased between 45 and 90 cm deep after 4 years of effluent applications. Across the 5 ha field, higher subsoil total Cu concentrations relative to controls suggest that Cu leaching occurred; however, leaching of Zn into the total pool was not significant. Estimated times through Coastal Bermuda grass uptake to reduce topsoil M3 Zn concentrations to background levels ranged between 21 to 152 years. Results from this study show that after 10 years of swine manure effluent application, mean topsoil M3 Cu and Zn concentrations were far below concentrations considered phytotoxic to sensitive crops.