Micronutrient deficiencies in African soils and the human nutritional nexus: opportunities with staple crops

Abstract

A synthesis of available agronomic datasets and peer-reviewed scientific literature was conducted to: (1) assess the status of micronutrients in sub-Saharan Africa (SSA) arable soils, (2) improve the understanding of the relations between soil quality/management and crop nutritional quality and (3) evaluate the potential profitability of application of secondary and micronutrients to key food crops in SSA, namely maize (Zea mays L.), beans (Phaseolus spp. and Vicia faba L.), wheat (Triticum aestivum L.) and rice (Oryza sativa L.). We found that there is evidence of widespread but varying micronutrient deficiencies in SSA arable soils and that simultaneous deficiencies of multiple elements (co-occurrence) are prevalent. Zinc (Zn) predominates the list of micronutrients that are deficient in SSA arable soils. Boron (B), iron (Fe), molybdenum (Mo) and copper (Cu) deficiencies are also common. Micronutrient fertilization/agronomic biofortification increases micronutrient concentrations in edible plant organs, and it was profitable to apply fertilizers containing micronutrient elements in 60-80% of the cases. However, both the plant nutritional quality and profit had large variations. Possible causes of this variation may be differences in crop species and cultivars, fertilizer type and application methods, climate and initial soil conditions, and soil chemistry effects on nutrient availability for crop uptake. Therefore, micronutrient use efficiency can be improved by adapting the rates and types of fertilizers to site-specific soil and management conditions. To make region-wide nutritional changes using agronomic biofortification, major policy interventions are needed.

Keywords: Biofortification; Fertilization; Human nutrition; Micronutrients; Profitability; Soil fertility management; Sub-Saharan Africa.

Fig. 1

Data entries (n) obtained and used in the analysis of quality effects and profitability of micronutrients in sub-Saharan Africa covering seven crops: maize (Zea mays L.), wheat (Triticum aestivum L.), rice (Oryza sativa L.), cowpea (Vigna unguiculata L.), pearl millet (Pennisetum glaucum), finger millet (Eleusine coracana Gaertn.) and sorghum (Sorghum bicolor L.) and selected micronutrient elements (not all combinations present). * = 178 of these were also used for analysis of effects. Background map: Food and Agriculture Organization of the United Nations. FAO GEONETWORK. Global Administrative Unit Layers (GAUL) (GeoLayer). (Latest update: 04 Jun 2015)

Fig. 2

Selenium concentrations in maize grain and stover under different ranges of Se application rates in maize (Zea mays L.). The mid-line is the median. The box indicates interquartile range, while the whiskers show non-outlier range. The red lines show the lower critical limit of Se for humans

Fig. 3

Effects of NPK fertilizer, and secondary and micronutrients on concentration of copper in maize grain (Zea mays L.) as observed in sub-Saharan Africa (Nigeria and Malawi). Error bars are bootstrap confidence intervals of means. Number of observations varied from 153/156 among the treatments. Control = no fertilizer added, NPK = fertilizer containing nitrogen (N), phosphorous (P) and potassium (K), NPK+ = fertilizer containing NPK and one or more micronutrient elements

Fig. 4

Boxplots showing zinc (Zn) concentration in maize (Zea mays L.) grain and the associated yields following nutrient omissions. Nutrient followed by zero means that the nutrient was omitted. FP = farmer practice (not fertilized)

Fig. 5

Grain zinc (Zn) concentrations in maize (Zea mays L.) at different soil Zn test values for different farms in Zimbabwe. Each data point represents an individual farm. Broken line indicates the similar maximum grain Zn concentrations, while the continuous line indicates trend for the lowest concentrations

Fig. 6

Effects of fertilizers including secondary and micronutrients on quality of ear leaves of maize (Zea mays L.) as observed in sub-Saharan Africa (Nigeria and Togo). Error bars show bootstrap confidence intervals of means. Control = no fertilizer added, NPK = fertilizer containing nitrogen (N), phosphorous (P) and potassium (K), NPK+ = fertilizer containing NPK (and one or more micronutrient elements)

Fig. 7

Concentrations of plant zinc for different crops at different soil Zn values for OFRA study locations across sub-Sahara Africa. All the samples are derived from treatments applied with N, P and K. The crops are maize (Zea mays L.), cowpea (Vigna unguiculata L.), pearl millet (Pennisetum glaucum), finger millet (Eleusine coracana) and sorghum (Sorghum bicolor L.)

Fig. 8

Distributions of net benefits and the associated cumulative percentages for combined secondary and micronutrients (combined), S, Zn and Cu as observed in SSA for maize. N = 44 for combined, 95 for S, 72 for Zn, 39 for Cu and 44 for gypsum. Black vertical line indicates zero benefit value when no benefits or losses are incurred. Few points where maize yield was > 10 t ha⁻¹ were considered as erroneous and therefore omitted as this is not common in the region

Fig. 9

Range of net profits observed with a combined secondary and micronutrients, b S and c Zn as observed in SSA. Red lines indicate zero benefit value when no benefits or losses are incurred. Data used are for all crops