Zinc fractions and solubility in a tropical semi-arid soil under long-term cultivation

There is an increasing incidence of Zn deficiency in savanna soil under intensive cultivation. The sizes of Zn pools and Zn²⁺ solubility were studied in a savanna soil under long-term cultivation and varying management practices which included fertilization with NPK, farmyard manure (FYM), NPK + FYM and a control which received neither NPK nor FYM for 50 years. An uncultivated natural site was sampled as a reference for assessing the effect of management history on Zn fractions and solubility. Total Zn ranged from 38 to 63 mg kg^-1 in the natural site, and from 28 to 57 mg kg^-1 in the cultivated fields. The FYM-fertilized field maintained total and extractable Zn on par with the natural site. Cultivation and management history did not affect the concentration of diethylenetriaminepentaacetic acid- (DTPA) extractable Zn, water-soluble Zn (SOL-Zn), exchangeable Zn (EXCH-Zn) and organically complexed Zn (OM-Zn). For the natural site, residual Zn (RES-Zn) accounted for 48% of total Zn, whereas Mn-oxide-bound Zn (MnOX-Zn) accounted for between 40% and 61% of total Zn in the cultivated fields. Cultivation caused the depletion of RES-Zn especially in FYM- and FYM + NPK-fertilized soils. Solubility studies indicated that some mechanism involving Zn and Fe, but independent of pH, appeared to control Zn²⁺ concentration in soil solution consistent with the reasonably constant values of pZn²⁺ + 2pFe³⁺ in soil solution.