The solubility of native and applied zinc in the soil as affected by liming and type of inorganic phosphate fertiliser

Abstract

The prevalence of Zinc (Zn) deficiency in plants is a common global phenomenon. Soils with inherent low Zn contents are found, but it is often induced by high carbonate or organic matter contents in soils, high soil pH, or heavy phosphate fertilisation and often by a combination of the latter two. The effects of the latter are the most researched due to their negative effects on the mobility and plant-availability of Zn. The negative phosphate–Zn interactions are caused by several chemical factors in soils and physiological factors in plants. The objectives of this study were: a) to examine the impacts of different phosphate fertilizers sources and lime on the solubility of both native and applied zinc in soils based on different chemical fractions. b) To study the impact of these amendments on the diffusion of applied zinc from fertiliser bands. The study was carried out in two highly weathered red apedal soils of the Hutton form, but different textural classes: sandy loam and clay. Three phosphate sources: Mono-ammonium phosphate (MAP), diammonium phosphate (DAP) and dicalciumphosphate (DCP) were co-applied individually with ZnSO4 in simulated fertiliser bands to both soils in their unlimed and limed states. This resulted in eight fertiliser treatments combinations together with controls. A sequential extraction procedure was used to determine the amounts of Zn in different chemical pools. The study showed that a large proportion of the native Zn was in acid extractable Zn fraction representing the Mn + Fe bound fractions while applied Zn was largely in NH2OH fraction. Liming caused very big increases the zinc concentrations in the NH2OH, Mg(NO3)2 and NH2OH*HCl fractions in the extraction sequence in the applied Zn in both the clay and sandy loam soils. These represent the labile (soluble + adsorbed) zinc fraction and the zinc sorbed to sesquioxides. The co-application of both ammonium phosphates and lime in the fertilizer bands increased or decreased sum of fractions depending on the type of phosphate fertilisers. MAP extracted higher native Zn concentrations than DAP and DCP. The application of calcium phosphate (DCP) in the limed soils very strongly reduced both the Zn concentration in this fraction in the band and movement of Zn from the band into the surrounding soil. This shows the very strong impact of the combination of liming/somewhat higher pH and co-application of a calcium phosphate on Zn in the soil.