Effect of ammonium polyphosphate and diammonium orthophosphate on the availability of zinc to wheat on soils varying in calcium carbonate

In a screen house experiment conducted to evaluate the efficiency of granular ammonium polyphosphate (APP) and diammonium orthophosphate (DAP) for wheat on soils of variable CaCO₃ contents, P-Zn relationship was investigated. Shoot Zn concentration, Zn uptake by wheat and DTPA extractable Zn in soils were higher with APP than DAP treatments. Shoot Zn concentration and DTPA extractable Zn progressively decreased with increasing levels of DAP, while Zn uptake increased up to 40 mg P kg⁻¹ and then decreased at 60 mg P kg⁻¹. However, with increasing levels of APP, Zn uptake progressively increased in all the soils, while effect on shoot Zn concentration and DTPA extractable Zn varied with the soil. Availability of Zn to the crop and DTPA extractable Zn generally decreased with increasing CaCO₃ content of soils treated with DAP. Interestingly, on soils of higher organic matter content, the effect of CaCO₃ on Zn availability was counteracted by the application of APP.