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Zinc (Zn) deficiency is a widespread nutritional problem in human populations, especially in sub-Saharan Africa (SSA). The Zn concentration of crops consumed depends in part on the Zn status of the soil. Improved understanding of factors controlling the phyto-availability of Zn in soils can contribute to potential agronomic interventions to tackle Zn deficiency, but many soil types in SSA are poorly studied.

Soil samples () were collected from a large part of the Amhara Region of Ethiopia, where there is widespread Zn deficiency. Zinc status was quantified by measuring several fractions, including the pseudo-total (aqua regia digestion; Zn), available (DTPA (diethylenetriamine pentaacetate) extractable; Zn), soluble (dissolved in 0.01 M Ca(); Zn) and isotopically exchangeable Zn, using the enriched stable Zn isotope (Zn). Soil geochemical properties were assessed for their influence on Zn lability and solubility. A parameterized geochemical assemblage model (Windermere Humic Aqueous Model – WHAM) was also employed to predict the solid phase fractionation of Zn in tropical soils rather than using sequential chemical extractions.

Zn ranged from 14.1 to 291 mg kg (median  100 mg kg), whereas Zn in the majority of soil samples was less than 0.5 mg kg, indicating widespread phyto-available Zn deficiency in these soils. The labile fraction of Zn in soil (Zn as % Zn) was low, with median and mean values of 4.7 % and 8.0 %, respectively. Labile Zn partitioning between the solid and the solution phases of soil was highly pH dependent, where 94 % of the variation in the partitioning coefficient of …