Agronomic biofortification of crops with zinc (Zn) can be enhanced under increased nitrogen (N) supply. Here, the effects of N fertilizer on grain Zn concentration of maize (Zea mays L.) and cowpea (Vigna unguiculata L.) were determined at two contrasting sites in Zimbabwe over two seasons. All treatments received soil and foliar zinc-sulphate fertilizer. Seven N treatments, with three N rates (0, 45, and 90 kg ha(-1) for maize; 0, 15, and 30 kg ha(-1) for cowpea), two N forms (mineral and organic), and combinations thereof were used for each crop in a randomized complete block design (n = 4). Maize grain Zn concentrations increased from 27.2 to 39.3 mg kg(-1) across sites. At 45 kg N ha(-1), mineral N fertilizer increased maize grain Zn concentration more than organic N from cattle manure or a combination of mineral and organic N fertilizers. At 90 kg N ha(-1), the three N fertilizer application strategies had similar effects on maize grain Zn concentration. Co-application of N and Zn fertilizer was more effective at increasing Zn concentration in maize grain than Zn fertilizer alone. Increases in cowpea grain Zn concentration were less consistent, although grain Zn concentration increased from 39.8 to 52.7 mg kg(-1) under optimal co-applications of N and Zn. Future cost/benefit analyses of agronomic biofortification need to include information on benefits of agro-fortified grain, complex farmer management decisions (including cost and access to both N and Zn fertilizers), as well as understanding of the spatial and site-specific variation in fertilizer responses.

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.

Cowpea (Vigna unguiculata [L.] Walp.) is an important but under-studied grain legume which can potentially contribute to improved dietary zinc (Zn) intake in sub-Saharan Africa. In this study, surveys were conducted on smallholder farms in Zimbabwe during 2014/15 to determine the influence of diverse soil fertility management options on cowpea grain productivity and nutrition quality. Guided by the surveys, field experiments were conducted to investigate the influence of Zn fertilizer on the productivity and quality of cowpea under integrated soil fertility management (ISFM). Experiments were conducted on two soil-types, namely, sandy (6% clay) and red clay (57% clay) in 2014/15 and 2015/16 where cowpea was grown in rotation with staple maize (Zea mays L.) and fertilized with combinations of Zn, nitrogen (N), phosphorus (P) and two organic nutrient resources, cattle manure and woodland leaf litter. Cowpea grain yields on surveyed farms ranged from 0.3 to 0.9 t ha(-1), with grain Zn concentration ranging from 23.9 to 30.1 mg kg(-1). The highest grain Zn concentration was on fields where organic nutrient resources were applied in combination with mineral N and P fertilizers. Within the field experiments, mean grain yields of cowpea increased by between 12 and 18% on both soil types when Zn fertilizer was applied, from a baseline of 1.6 and 1.1 t ha(-1) on red clay and sandy soils, respectively. When Zn fertilizer was co-applied with organic nutrient resources, grain Zn concentrations of cowpea reached 42.1 mg kg(-1) (red clay) and 44.7 mg kg(-1) (sandy) against grain Zn concentrations of 35.9 mg kg(-1) and 31.1 mg kg(-1) measured in cowpea grown with no Zn fertilizer on red clay and sandy soils, respectively. Agronomic biofortification of legumes is feasible and has the potential to contribute significantly towards increasing dietary Zn intake by humans. A greater increase in grain Zn of cowpea grown on sandy than red clay soils under Zn fertilization illustrates the influence of soil type on Zn uptake, which should be explored further in agronomic biofortification programs.

A two-year study was conducted to assess how mulch influences weed dynamics following imposition of different fertilization treatments under three crop establishment options: (i) conventional; (ii) ripping; and (iii) basin, in a two-year maize-legume rotation. Eight treatments were imposed within each crop establishment option and received maize stover mulch applied at 0% or 30% cover before planting maize (Zea mays) or cowpea (Vigna unguiculata) as test crops. Maize received nitrogen (N) at 35, 90, or 120 kg·ha−1 and phosphorus (P) at 14 or 26 kg·ha−1 applied alone or in combination with 4 or 7 t cattle manure·ha−1, while cowpea received 8 or 17 N·kg·ha−1 and similar P rates to maize. Results indicated that both weed biomass and diversity were influenced more by fertilization than method of crop establishment. On treatments under high fertilizer application rates, or previously planted to cowpea weed biomass ranged between 220 and 400 g·m−2 under mulch and 370–510 g·m−2 (no mulch). Here species richness ranged between 7–16 and was dominated by dicotyledons. This was in contrast to biomass ranges of 75–200 g·m−2 in the low fertilized and control plots, where only one or two grass types dominated. Overall, weed densities were 6% to 51% higher under conventional tillage compared to the two conservation agriculture (CA) options, although the data indicated that mulch significantly (p < 0.05) depressed weed density by up to 70%. We concluded that mulching could be a potential mechanism for reducing weeding labor costs for smallholders and the general environmental and health concerns associated with the use of herbicides in CA systems.

Productivity of grain legumes on sandy soils of southern Africa is critically limited by marginal fertilisation. Effects of co-applying phosphorus (P)-based mineral fertilisers and organic nutrient resources to cowpea (Vigna unguiculata (L.) Walp.) and soyabean (Glycine max L.) were investigated on smallholder farms in eastern Zimbabwe over two years. Over 70% of the surveyed farmers grew cowpea without fertilisation. Fertilisation of legumes with one or more nutrient resources increased shoot biomass productivity by between 20% and 300% relative to the non-fertilised control. Fertilised soyabean and cowpea yielded 2.2 t grain ha−1 and 2.5 t grain ha−1, respectively, translating to more than double the yields of unfertilised controls. In contrast, sole application of either mineral P-containing fertilisers or organic nutrient resources yielded less than 1 t ha−1 legume grain. The effects of combined organic and mineral fertilisation were also reflected in increased C〇2-carbon evolution from soils following growth of the legumes. Under the same soils, net nitrogen (N) mineralisation was highest where cattle manure was co-applied with an NP-containing fertiliser, with at least 85 mg N kg−1 soil released within six weeks. Co-application of organic and NP-containing fertilisers significantly enhance legume grain yields and residual soil N availability, but most smallholder farmers do not currently use this fertilisation strategy.

The Harvest Plus Zinc Project Soil Fertility Consortium for Southern Africa (SOFECSA) CIMMYT