Zinc in Soils

Zinc has a complete 3d¹⁰4s² outer electronic configuration and, unlike the other d block micronutrients such as such as manganese, molybdenum, copper, and iron, has only a single oxidation state and hence a single valence of II. The average concentration of zinc in the crust of the Earth, granitic, and basaltic igneous rock is approximately 70, 40, and 100 mg kg^-1, respectively (38), whereas sedimentary rocks like limestone, sandstone, and shale contain 20, 16, and 95 mg kg^-1, respectively (39). The total zinc content in soils varies from 3 to 770 mg kg^-1 with the world average being 64 mg kg^-1 (40).

There are five major pools of zinc in the soil: (a) zinc in soil solution; (b) surface adsorbed and exchangeable zinc; (c) zinc associated with organic matter; (d) zinc associated with oxides and carbonates; and (e) zinc in primary minerals and secondary alumino-silicate materials (41). There is evidence that Zn²⁺ activities in the soil solution may be controlled by franklinite (ZnFe₂O₄), whose equilibrium solubility is similar to that of soil-held zinc over pH values of 6 to 9 (42,43). The mineral will precipitate whenever zinc concentration in the soil solution exceeds the equilibrium solubility of the mineral and will dissolve whenever the opposite is true. This process provides a zinc-buffering system.

Zinc may be associated with soil organic matter, which includes water-soluble and organic compounds. Zinc is bound via incorporation into organic molecules, exchange, chelation, or by specific and nonspecific adsorption (41).

Zinc is associated with hydrous oxides and carbonates via adsorption, surface complex formations, ion exchange, incorporation into the crystal lattice, and co-precipitation (41). Some of these reactions fix zinc rather strongly and are believed to be instrumental in controlling the amount of zinc in the soil solution (44). Zinc is complexed with CaCO₃ in alkaline (pH 8.2) soils in the western half of Texas where most of the pecans are grown in the state (45-47). Soil-incorporated ZnSO₄ at 91 kg per pecan tree did not bring the zinc content of the soils to an adequate level because the zinc was transferred from the sulfate form to sparingly soluble ZnCO₃ (48).

Five rates of ZnSO₄ and three rates of S were supplied to pecan trees in March 1966 in a single application to soil (deep Tivoli sand, pH 8.2; mixed thermic, Typic ustipamments) in Dawson county, Texas (south plains) (49). In the absence of applied sulfur, adding of ZnSO₄ in excess of 20 kg per tree was required to raise zinc concentrations in leaflets in June or September 1966 above the minimum optimum of 60 mg kg^-1. Additions of sulfur reduced the amount of ZnSO₄ required to reach 60 mg kg^-1 to 18.8 kg per tree with 4.5 kg S per tree and to 16.2 kg per tree with 11.9 kg S per tree. Leaflets collected in September 1967 contained more than 60 mg Zn kg^-1 if ZnSO₄ was applied in March 1966 at rates greater than 4.8 kg per tree. However, in 1967, at any given rate of ZnSO₄ (above 1.4 kg per tree), leaflet zinc concentration was reduced by the addition of sulfur, but the concentrations of zinc in the leaflets remained above the minimum optimum level. This study indicates that leaflet zinc of pecan trees in calcareous soils can be increased by soil applications of ZnSO₄, but that a larger increase will occur if S is applied with ZnSO₄. On the other hand, soil applications seemed impractical considering the fact that with a planting of 86 trees per ha, an application of 120 kg of ZnSO₄ ha^-1 would be required. In acid soils of the southeastern United States, high rates of soil-applied zinc may be responsible for the elusive mouse-ear symptom in the acid soils of the southeastern United States (50). These results agree with Sommers and Lindsay (51), who reported that in soils with high concentrations of heavy metals, nickel will compete with zinc for chelation in acid soils and that cadmium and lead will do the same in alkaline soils.