Abstract
Micronutrient deficiencies remain prevalent in food systems of southern Africa, although advances in biofortification through crop breeding and agronomy provide opportunities to address these. We determined baseline soil availability of zinc (Zn) and iron (Fe) and the effects of soil type and farmer management on extractable soil Zn and Fe and subsequent concentration in cereal and legume grains under two contrasting agro-ecologies in Zimbabwe. Soil and crop surveys were conducted in Hwedza and Mutasa Districts of Zimbabwe in 2015–16 on 350 locations over different soil types. Fields with different levels of productivity (designated as “most” and “least” productive fields) were sampled using an inherited hierarchical randomized sampling design. Grain Zn and Fe concentration in maize (Zea mays), sorghum (Sorghum bicolor), finger millet (Eleusine coracana) and cowpea (Vigna unguiculata) were generally insufficient for adequate human nutrition. A Linear Mixed Effects (LME) model revealed that diethylene triamine penta-acetic acid- (DTPA) extractable soil Zn concentration and grain Zn concentration were affected primarily by field productivity level. DTPA-extractable soil Zn concentration was more than two-fold greater on the most productive fields (mean 0.8 mg kg−1) than on the least productive fields, with mean grain Zn concentration of 25.2 mg grain Zn kg−1 which was 13% greater than seen on the least productive fields. An interaction effect of field productivity level and total soil Zn concentration on DTPA-extractable soil Zn concentration suggests potential contribution of organic matter management to unlocking unavailable forms of soil Zn. DTPA-extractable soil Fe and grain Fe concentration were primarily affected by soil type and crop type, respectively. The LME modelling approach revealed additional soil geochemical covariates affected DTPA-extractable soil Zn and Fe concentration and grain Zn and Fe concentration within Districts. Future studies can therefore be powered to detect their roles at wider spatial scales for sustainable management of crop Zn and Fe nutrition.
Highlights
The prevalence of micronutrient deficiencies (MNDs) due to inadequate dietary intake remains high
The effect of baseline soil type on maize (Zea mays L.) grain Zn concentration in Malawi was reported by Chilimba et al.[17] and showed that maize grown on vertisols had ~30% greater grain Zn concentration than on other soil types
Using village lists provided by Agricultural Extension Workers (AEWs) in each District, we identified farms located on clayey (20–60% clay) and sandy (6–20% clay) soil types which represent the major soils used for crop production and excluded farms on multiple soil types and/or on other soil types
Summary
The prevalence of micronutrient deficiencies (MNDs) due to inadequate dietary intake remains high. In contrast to steady reductions in risk in Latin America, East and South Asia over the past 50 years, Zn and Fe deficiencies have remained high in sub-Saharan Africa (SSA)[4,6] This is partly attributed to less total food intake and dietary diversity in SSA2,6, poorer soil quality, and fewer options for soil fertility management in smallholder systems[7]. Adult women consuming maize in proximity to vertisols had a median Zn intake of 6.4 mg person−1 day−1, while those near non-vertisol acid soils had a median Zn intake of 4.8 mg person−1 day−1 18,19, which was consistent with predictions based on baseline soil/grain surveys[17] These studies demonstrated that variations in inherent micronutrient levels in different soil types may have implications on human nutrition. The application of organic manures increased biomass and grain yield, translating to more animal feed and greater purchasing power, which could help to alleviate MNDs
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