Zinc interactions, forms and transformations in soil
After reviewing the literature on factors influencing zinc availability in soils, it was decided to investigate further aspects of the well documented zinc/phosphate interaction for the soil/maize system and to study the migration and transformations of inorganic and organic (sludge) fertilizer zinc in soils. A conventional pot experiment with maize to look at zinc/phosphate interaction, with thorough mixing of added fertilizer, indicated that phosphate could alleviate zinc toxicity effects, but did not show whether this was a soil or plant effect. A subsequent experiment in which soils treated with zinc and phosphate fertilizers were spatially separated by nylon net provided evidence that the interaction occurred, at least in part, in plant roots. This suggests limited benefit in the field for separate placement. Under field conditions, it is unrealistic to expect the same degree of soil mixing as used in typical pot experiments. Therefore an experiment with 65Zn was used to study the vertical movement of surface-applied inorganic zinc fertilizer. The same experiment was also used to study the changes in distribution of added zinc between a range of soil fractions (exchangeable, organic, manganese oxide, iron oxide and residual) at three soil pH values. Uptake was higher at lower pH, corresponding to greater amounts of zinc being retained on exchange sites. However, much of the added zinc was found in the manganese oxide fraction especially at higher pH. Zinc mobility during the experiment was almost negligible. Subsequently the soils left at the end of this experiment were incubated for 6 months and then used for a further pot experiment, to gain insight into residual zinc fertilizer effects. This showed a reduction in exchangeable zinc and plant uptake, with increases in zinc associated with organic matter and iron oxides.