Copper adsorption/desorption characteristics on copper amended soils
Laboratory tests were conducted to determine Cu availability of three Virginia soils amended with Cu as either Cu-rich pig manure or CuSO₄ with the same Cu content as that in manure. The study also assessed the capability of Mehlich-3 to estimate deficient and toxic Cu levels in soil. Application of Cu as manure or CuSO₄ did not affect grain yield or grain Cu concentration in corn grown at each site. Young corn plant height was reduced by CuSO₄ application on the Bertie sandy loam, an indication of greater toxicity from inorganic than organic Cu.
Very little exchangeable or solution Cu was present in any of the three soils. The vast majority of Cu was distributed between the organic and soil oxide fractions. The ratio of soil oxide to organic matter determines the relative distribution of Cu among these fractions. Most organically bound Cu was held by specific adsorption mechanisms and Mehlich-3 strongly extracts this fraction. Mehlich-3 extraction patterns were indicative of Cu binding strength in the different soils. Since Mehlich-3 strongly extracts specifically bound Cu and only poorly removes oxide Cu, this test may underestimate Cu availability on sandy soils with low organic matter content. However, Mehlich-3 provides an acceptable test for soil Cu.
Soil sorption characteristics were studied for Cu, Pb, and Zn added, both alone and simultaneously. Copper sorption energy was higher than both Pb or Zn, however, sorption capacity followed the order Pb > Zn > Cu. High Pb and Zn sorption was in part due to precipitation reactions especially at high initial solution concentrations. Zinc was bound to soil mostly by weak electrostatic forces. Copper and Pb were bound at specific sorption sites and by complex multi-site bonding mechanisms possibly involving organic substances. These specific and multi-site mechanisms account for metal ions removed from solution at low concentration. Even at low metal concentration, Cu and Pb sorption results in concurrent release of H⁺ and Ca²⁺ at a greater than one to one charge basis.
Soils adjusted to various pH levels were equilibrated with Cu solution and then extracted with a series of dilute acid extractions to determine Cu adsorption and fixation capacities. Copper adsorption and more importantly Cu fixation increased with an increase in soil pH. Soil with a high organic matter content as a result of manure applications adsorbed and fixed more Cu at all pH levels than the control and CuSO₄ soil. The presence of organic matter may have a greater effect on metal sorption characteristics than maintenance of pH ≥ 6.5. Heavy metal sorption was accompanied by concurrent release of H⁺ and Ca²⁺ which represents exchangeable and specifically bound cation nutrients. Soils which receive heavy metal applications from sewage sludge or animal manure would have a short-term increase in plant available nutrients at the expense of long-term reserve capacity. Soil tests for cation nutrient availability on soils receiving heavy metal applications should be adopted to account for these responses.