Copper Availability in Soils

Parent material and formation processes govern initial copper status in soils. Atmospheric input of copper has been shown to partly replace or even exceed biomass removal from soils. Kastanozems, Chernozems, Ferrasols, and Fluvisols contain the highest levels of copper, whereas Podzols and Histosols contain the lowest levels.

Chelation and complexing govern copper behavior in most soils (9). For most agricultural soils, the bioavailability of Cu²⁺ is controlled by adsorption–desorption processes. Permanent-charge minerals such as montmorillonite carry a negative charge. Variable-charge minerals such as iron, manganese, and aluminum oxides can carry varying degrees of positive or negative charges depending on soil pH. Therefore, adsorption and desorption of Cu²⁺ is affected by the proportion of these minerals in soils (105). Adsorption of Cu²⁺ in variable charged soils is pH-dependent. Adsorption of Cu²⁺ in soils is often coupled with proton release, thereby lowering soil pH. Organic matter in soils has a strong affinity for Cu²⁺, even at low Cu²⁺ concentrations. Copper adsorption capacity of a soil decreases in the order of concentration of organic matter + Fe, Al, and Mn oxides + clay minerals (105). In the Zhejiang providence of China, a Quaternary red earth soil (clayey, kaolinitic thermic plinthite Aquult, pH 5.39, 9.03 g organic C kg^-1) absorbed a higher percentage of Cu²⁺ added as Cu(NO₃)₂ than an arenaceous rock soil (clayey, mixed siliceous thermic typic Dystrochrept, pH 4.86, 6.65 g organic C kg^-1) (105).

The solubility of copper minerals follows this progression: CuCO₃>Cu3(OH)₂(CO₃) (azurite) >Cu(OH)₂>Cu₂(OH)₂CO₃ (malachite)>CuO (tenorite)>Cu Fe₂O₄ cupric ferrite + soil-Cu. Increasing carbon dioxide concentrations decreases the solubility of the carbonate minerals. The solubility of cupric ferrite is influenced by Fe³⁺ and is not much greater than soil copper. Copper will form several sulfate and oxysulfate minerals; however, these minerals are too soluble in soils and will dissolve to form soil-Cu (100). Application of rare earth element fertilizers (23.95% lanthanum, 41.38% cerium, 4.32% praseodymium, and 13.58% neodymium oxides) increased the copper content of water-soluble, exchangeable, carbonate, organic, and sulfide-bound soil fractions, but not the Fe-Mn oxide-bound form (101).

Copper availability is affected substantially by soil pH, decreasing 99% for each unit increase in pH (40). In soil, Cu²⁺ dominates below pH 7.3, whereas CuOH⁺ is most common at about pH 7.3 (40). The concentration of total soluble copper in the soil solution influences mobility, but the concentration of free Cu²⁺ determines the bioavailability of copper to plants and microorganisms (106). In an aquatic system, Cu²⁺ is the dominant form below pH 6.9, and Cu(OH)₂ dominates above that pH. Treatments of 87, 174, 348, and 676 mg CuSO₄ kg^-1 to an alfisol soil (Oxic Tropudalf) in Nigeria significantly acidified the soil and reduced total bacterial counts, microbial respiration, nitrogen and phosphorus mineralization, short-term nitrification, and urease activity relative to untreated soils (107).

Copper ions are held very tightly to organic and inorganic soil exchange sites (9), and CuOH⁺ is preferably sorbed over Cu²⁺. The greatest amounts of adsorbed copper exist in iron and manganese oxides (hematite, goethite, birnessite), amorphous iron and aluminum hydroxides, and clays (montmorillonite, vermiculite, imogolite) (9). Microbial fixation is also important in copper binding to soil surfaces (9). Although Cu²⁺ can be reduced to Cu⁺ ions, copper is not affected by oxidation- reduction reactions that occur in most soils (40). In neutral and alkaline soils, CuCO₃ is the major inorganic form, and its solubility is essentially unaffected by pH (108). The hydrolysis constant of copper is 10^-7.6 (109).

Copper forms stable complexes with phenolic and carboxyl groups of soil organic matter. Most organic soils can bind approximately 48 to 160 mg Cu g-1 of humic acid (9). These complexes are so strong that most copper deficiencies are associated with organic soils (40). Addition of composts (biosolids, farmyard manure, spent mushroom, pig manure, and poultry manure) increased the complexation of copper in a mineral soil in New Zealand, and addition of biosolids was effective in reducing the phytotoxicity of copper at high levels of copper addition (106). At the same level of total organic carbon addition, there were differences among these manure sources for copper adsorption (106). In this same study, a significant inverse relationship occurred between copper adsorption and dissolved organic carbon, indicating that copper forms soluble complexes with dissolved organic carbon. Addition of sewage sludge-bark and municipal solid waste compost at about 1000 kg ha^-1 (containing 126 to 510 mg Cu kg^-1 dry matter) to a vineyard soil in Italy did not affect total soil or ethylenediaminetetraacetic acid (EDTA)-extractable copper but did decrease diethylenetriaminepentaacetic acid (DTPA)-extractable copper (110). The copper content of grape (Vitis vinifera L.) leaves, musts, and wine were not affected by compost treatment over a six-year period but were affected by the nearly 15 to 20 kg Cu ha^-1 applied through fungicidal treatments (110). Differences in copper accumulation by bean were observed in response to added poultry manure (1% by mass). After 2.0 mM Cu kg^-1 as CuSO₄ was added to a Brazilian agricultural soil, bean plants accumulated 40.5 mg Cu kg^-1 dry weight without manure additions, but plants grown on soil amended with poultry manure accumulated only 16.9 mg Cu kg^-1 dry weight (77).

Kabata-Pendias and Pendias (9) report that copper is abundant in the soil solution of all types of soils, whereas Barber (97) notes that soil solution copper is rather low. According to Kabata- Pendias and Pendias (9), the concentration of copper in soil solutions range from 3 to 135 �g L^-1. Soils of similar texture do not have the same copper concentration (30). The most common forms of copper in the soil solution are organic chelates (9). Deficiencies are common on sandy soils that have been highly weathered, on mineral soils with high organic matter, and on calcareous mineral soils (111). Although Kubota and Allaway (69) generalized that crop yield responses to copper usually occur only on organic soils, Franzen and McMullen (112) reported that spring wheat yield significantly increased in response to 5 lb of 25% copper sulfate acre^-1 (5 kg ha^-1) on a low organic matter, sandy loam in North Dakota and not on soils with more than 2.5% organic matter. Removal of copper from soils by plant growth is negligible compared to the total amount of copper in soils (9). An average cereal crop removes annually about 20 to 30 g ha^-1, and forest biomass annually removes about 40 g ha^-1 (9).

Copper extraction from soils can differ by extraction method. Ethylenediaminetetraacetic acid has been shown to preferentially extract micronutrients associated with organic matter and bound to minerals (113). Copper extraction from soils in India was highest for 0.5 N ammonium acetate + 0.02M EDTA, followed in order by 0.1 N HCl, a DTPA extraction mix (0.004M DTPA, 0.1 M triethanolamine, and 0.01 M CaCl₂ at pH 7.3), and 0.05 N HCl + 0.025N H₂SO₄ (102). In these Indian soils, soil solution fractions contained 0.38% of the total soil copper, exchangeable forms accounted for 1.00%; specifically absorbed, acid-soluble and Mn-occluded fraction accounted for 4.47%; and the amorphous Fe-occluded and crystal Fe-occluded fraction accounted for 9.94% (102). Increasing strengths of ammonium acetate (0.1, 0.3, 1 M) alone was a poor copper soil extractant; however, the addition of 1M NH₂OH.HCl in acetic acid to the sequential extraction procedure removed 60 to 65% of the total soil copper and further extraction with 30% H₂O₂ in 1M HNO₃ removed another 20%, which was likely associated with the organic soil fraction (103). The remaining soil copper is termed residual (the difference between extractable and total soil Cu) and is often approximately 50% of total soil copper (97). In contrast, Miyazawa et al. (77) report no differences in copper extraction from a sandy dystrophic dark red latosoil in Brazil by Mehlich-1 (0.05 N HCl + 0.025 N H₂SO₄), 0.005M DTPA, pH 7.3 (15.0 g triethanolamine [TEA] + 2.0 g DTPA + 1.5 g CaCl₂.2H₂O), and 1M NH4OAc, pH 4.8. Atomic absorption spectrophotometry or colorimetry has been shown to work well in the analysis of ammonium acetate extraction methods (114).

Application of copper usually is not required every year, and residual effects of copper have been reported up to 12 years after soil application (115). Contamination of soils by excess copper occurs mainly by overapplication of fertilizers, sprays, and agricultural and municipal wastes containing copper and from industrial emissions (9). Copper hydroxide is the most widely used fungicide- bactericide for control of tomato diseases (116). Due to the intense use of foliar-applied, copper-containing chemicals, about 25% of tomato leaf samples from greenhouses in Turkey contained over the maximum accepted tolerance level of 200 mg Cu kg^-1 (31). Due to overuse of copper- containing pesticides and fertilizers, 8.1% of 210 greenhouse soil samples in Turkey were shown to contain greater than 200 mg Cukg^-1, the critical soil toxicity level (31).

Localized excess soil copper levels occur in close proximity to industrial sites, but airborne fallout of copper is not substantial. Kabata-Pendias and Pendias (9) reported that atmospheric deposition of copper in Europe ranged from 9 to 224 g ha^-1 year^-1. The average copper concentration of unpolluted river waterways was approximately 10 �g L^-1, whereas polluted water systems contained 30 to 60 �g L^-1 (88). After soils were irrigated for one season with copper-enriched wastewater from a family-owned copper ingot factory in Jiangsu providence, China, copper levels increased sevenfold from 23 to 158 mg kg^-1 compared to other soils in the region (116).

Runoff from tomato plots receiving 10 kg of 77% copper hydroxide solution ha^-1 season^-1 contained significantly more copper if polyethylene mulch was used between the rows instead of a vegetative mulch of vetch (Vicia villosa Roth.) (118). Incidentally, the particulate phase of the runoff contained 80% more copper than the dissolved phase. On a calcareous Fluvisol in Spain, Chinese cabbage (Brassica pekinensis Rupr.) accumulated 90% more copper under a perforated polyethylene, floating-row cover than plants in the bare-ground treatment. The floating-row cover increased the air temperature by 6.3�C and the root zone temperature by 5.2�C at a 5-cm depth and 4.3�C at a 15-cm depth (119).