Zinc in some New Zealand soils and factors affecting its availability and uptake by plants

Twenty surface soils (0-150 mm depth) from Southland and Canterbury, New Zealand were examined for their zinc status, and plant availability of applied and native soil zinc. The soils collected were from a range of parent materials, states of soil development and drainage status. Some of the factors affecting the availability of soil zinc to plants were also investigated. The ability of different extractants to extract soil zinc decreased in the order EDTA > DTPA > HCI > Ca(NO₃)₂ > NH₄COOCH₃. The amounts of EDTA-extractable zinc concentrations in these soils ranged between 0.7 to 25.5 µg Zn g⁻¹ soil and the total zinc concentrations in these soils ranged between 38.1 to 113.8 µg Zn g⁻¹ soil. Using a chemical fractionation scheme it was found that only a small proportion of total soil zinc is present in exchangeable and organically-bound forms. A major proportion of soil zinc is present in oxide and residual fractions. In soils originating from a single parent material (greywacke alluvium), the amount of exchangeable zinc decreased with increasing soil pH. Higher amounts of total and organically-bound zinc were present in imperfectly and poorly drained soils compared with well drained soils of the same age group. In the short-term most applied zinc was found in the exchangeable and organically-bound forms, however, with time these forms of zinc appeared to be gradually transferred to the amorphous iron oxide fraction. Highly significant correlations were found between EDT A, DTPA and NH₄COOCH₃- extractable soil zinc concentrations and plant zinc concentrations and plant zinc uptake. Zinc extracted with these reagents appeared to be derived predominantly from exchangeable and organically-bound zinc. The critical soil zinc concentrations for these soils below which zinc deficiency symptoms in crops, or response to applied zinc, can be expected to occur lies below 0.7 µg Zn g⁻¹ soil for the EDTA-extract, 0.6 µg Zn g⁻¹ soil for the DTPA-extract, and 0.4 µg Zn g⁻¹ soil for the NH₄COOCH₃-extract. The availability and uptake of applied and native soil zinc by plants depends on several factors. In soils with high pH, high CEC, high clay content, and high organic matter levels, a large proportion of applied zinc remained in an unavailable form. These types of soils need comparatively large amounts of applied zinc to produce a crop response. Phosphorus application to a soil with a native soil zinc concentration well above the critical soil zinc level had no great effect on yield or zinc uptake by plants. However, application of phosphorus to a soil with a high Olsen P level and low available native soil zinc concentration caused a large reduction in plant zinc concentration. Application of phosphorus had a long term effect on the level of exchangeable and organically-bound zinc. Zinc in these soil fractions gradually decreased due to it being transformed into the amorphous iron oxide fraction following the phosphorus application. Lime application, up to pH 7.2, to a highly acid soil of moderate zinc content decreased plant zinc concentration, but had little effect on zinc uptake due to a yield increase resulting from increased phosphorus availability. However, lime application, up to pH 7.2 to a soil with a pH around 6.0 had little effect on either yield or zinc uptake. Application of lime decreased plant zinc availability by decreasing native soil zinc concentrations in the exchangeable and organically-bound form. However, with applied zinc, the lime application decreased the level of exchangeable zinc and increased the level in the organically-bound form. Lime application, up to pH 7.2, is unlikely to cause any significant interaction effect on zinc, if the native soil zinc concentration is well above the critical soil zinc level.