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Zinc (Zn) deficiency remains a public health problem in Malawi, especially among poor and marginalized rural populations, linked with low dietary intake of Zn due to consumption of staple foods that are low in Zn content. The concentration of Zn in staple cereal grain can be increased through application of Zn‐enriched fertilizers, a process called agronomic biofortification or agro‐fortification. Field experiments were conducted at three Agricultural Research Station sites to assess the potential of agronomic biofortification to improve Zn concentration in maize grain in Malawi as described in registered report published previously. The hypotheses of the study were (i) that application of Zn‐enriched fertilizers would increase in the concentration of Zn in maize grain to benefit dietary requirements of Zn and (ii) that Zn concentration in maize grain and the effectiveness of agronomic biofortification would be different between soil types. At each site two different subsites were used, each corresponding to one of two agriculturally important soil types of Malawi, Lixisols and Vertisols. Within each subsite, three Zn fertilizer rates (1, 30, and 90 kg ha−1) were applied to experimental plots, using standard soil application methods, in a randomized complete block design. The experiment had 10 replicates at each of the three sites as informed by a power analysis from a pilot study, published in the registered report for this experiment, designed to detect a 10% increase in grain Zn concentration at 90 kg ha−1, relative to the concentration at 1 kg ha−1. At harvest, maize grain yield and Zn concentration in grain were measured, and Zn uptake by maize grain and Zn harvest index were calculated. At 30 kg ha−1, Zn fertilizer increased maize grain yields by 11% compared with nationally recommended application rate of 1 kg ha−1. Grain Zn concentration increased by 15% and uptake by 23% at the application rate of 30 kg ha−1 relative to the national recommendation rate. The effects of Zn fertilizer application rate on the response variables were not dependent on soil type. The current study demonstrates the importance of increasing the national recommendation rate of Zn fertilizer to improve maize yield and increase the Zn nutritional value of the staple crop. [ABSTRACT FROM AUTHOR]

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 (n=475) 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 Tot), available (DTPA (diethylenetriamine pentaacetate) extractable; Zn DTPA), soluble (dissolved in 0.01 M Ca(NO3); Zn Soln) and isotopically exchangeable Zn, using the enriched stable Zn isotope 70Zn (Zn E). 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 Tot ranged from 14.1 to 291 mg kg -1 (median = 100 mg kg -1), whereas Zn DTPA in the majority of soil samples was less than 0.5 mg kg -1 , indicating widespread phyto-available Zn deficiency in these soils. The labile fraction of Zn in soil (Zn E as % Zn Tot) 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 70Zn was explained by soil pH. Similarly, 86 % of the variation in Zn Soln was explained by soil pH. Zinc distribution between adsorbed Zn E and Zn Soln was controlled by pH. Notably, Zn isotopic exchangeability increased with soil pH. This contrasts with literature on contaminated and urban soils and may arise from covarying factors, such as contrasting soil clay mineralogy across the pH range of the soils used in the current study. These results could be used to improve agronomic interventions to tackle Zn deficiency in SSA. [ABSTRACT FROM AUTHOR]

The prevalence of micronutrient deficiencies including zinc (Zn) is widespread in Malawi, especially among poor and marginalized rural populations. This is due to low concentrations of Zn in most staple cereal crops and limited consumption of animal source foods. The Zn concentration of cereal grain can be increased through application of Zn‐enriched fertilizers; a process termed agronomic biofortification or agro‐fortification. This trial protocol describes a field experiment which aims to assess the potential of agronomic biofortification to improve the grain Zn concentration of maize, the predominant staple crop of Malawi. The hypotheses of the study are that application of Zn‐enriched fertilizers will create a relatively small increase in the concentration of Zn in maize grains that will be sufficient to benefit dietary supplies of Zn, and that the effectiveness of agronomic biofortification will differ between soil types. The study will be conducted at three sites, Chitedze, Chitala, and Ngabu Agricultural Research Stations, in Lilongwe, Salima, and Chikwawa Districts respectively. These three sites represent locations in the Central and Southern Regions of Malawi. At each site, two different sub‐sites will be used, each corresponding to one of two agriculturally important soil types of Malawi, Lixisols, and Vertisols. Within each sub‐site, three Zn fertilizer rates (1, 30, and 90 kg/ha) will be applied to experimental plots using standard soil application methods, in a randomized complete block design. The number of replicates at plot level has been informed by a power analysis from pilot study data, assuming that a minimum 10% increase in Zn concentration of grain at 90 kg/ha relative to the concentration at 1 kg/ha is of interest. Grain mass (yield), stover mass, and both stover and grain Zn concentrations will be measured at harvest. A second year of cropping will be used to establish whether there are any residual benefits to grain Zn concentration. The potential for Zn agronomic biofortification will be communicated to relevant academic and government stakeholders through a peer review journal article and a briefing paper. [ABSTRACT FROM AUTHOR]

Metal bioavailability and phytotoxicity may be exaggerated when derived from studies based on amending soils with soluble metal salts. It is therefore important to evaluate soil tests for their consistency in estimating plant uptake and phytotoxicity in both field-contaminated and freshly-spiked soils. This study aimed to compare the effects of zinc (Zn) on plant growth in soils (i) recently spiked with soluble Zn and (ii) historically amended with biosolids. The objective was to reconcile methods for determining bioavailability in both cases by testing a range of 'quantity-based' and 'intensity-based' assays. Soils with a range of Zn concentrations, from an arable farm used for biosolids disposal for over a century, were further amended with Zn added in solution, and were incubated for one month prior to planting with barley seeds in a glasshouse pot trial. The majority (67–90%) of the added Zn remained isotopically exchangeable after 60 days. Zinc in the solution phase of a soil suspension was present mainly as free Zn2+ ions. Cadmium bioaccumulation factors were inversely proportional to Zn concentration in the soil solution confirming that greater Zn availability suppressed Cd uptake by plants. Measurements of soil Zn ' quantities ' (total, EDTA-extractable and isotopically exchangeable) and ' intensity ' (solution concentration and free ion activity) were correlated with Zn uptake and toxicity by barley plants. Correlations using Zn intensity were much stronger than those using quantity -based measurements. The free Zn2+ ion activity appears to be a consistent driver for plant uptake and phytotoxic response for both metal-spiked soils and historically contaminated soils. Surprisingly, soil Zn accumulation of up to 100 times the current regulations for normal arable land only produced a mild toxic response suggesting that constituents in biosolids (e.g. organic matter and phosphates) strongly restrict metal bioavailability. Unlabelled Image • Free metal ion activity is the driver for plant uptake and phytotoxicity. • Greater Zn availability suppressed Cd uptake by plants. • Ratios of Zn:Cd in barley strongly reflected those in the soil solution. • Only mild phytotoxicity was found in biosolids-amended soils with up to 2000 mg Zn/kg. • E.U. rules on biosolid use in agriculture seem appropriate in terms of phytotoxicity. [ABSTRACT FROM AUTHOR]