Fertilizer management and soil type influence grain zinc and iron concentration under contrasting smallholder cropping systems in Zimbabwe

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.


Soil fertility
High organic carbon averaging 0.7%. High soil pH(CaCl2) of up to 5.4. High content of exchangeable bases (calcium-Ca and magnesium-Mg), available P and effective cation exchange capacity (eCEC) of around 4.6 cmol(c) kg -1 . Soils are usually red or grey coloured with a relatively high clay content.

Farmer management
Most agronomic practises including planting and weeding are done on time. Crops on the majority of fields are fertilized with high amounts of mineral and/ or organic nutrient resources.

Crops grown
Often allocated to maize intercropped with field bean (Phaseolus vulgaris) or in rotation with groundnut Crop response to fertilization Fields with high crop growth and yield response to external fertilization (both organic and inorganic).

Moisture content
Higher moisture content due to better infiltration rates and water holding capacity facilitated by the high soil organic matter contents.
Farmers preferentially allocate organic nutrient resources (cattle manure, woodland leaf litter, compost) and high rates of mineral fertilizer. Quantities of fertilizer applied vary from farm to farm depending on which resource group the farmer belongs to (see Mtambanengwe and Mapfumo 2009). 20.0-172.5 kg mineral N ha -1 applied as compound D (7N:14P2O5:7K2O) and ammonium nitrate (AN-34.5% N).
Upto 14 scotch carts ** of cattle manure is often applied in combination with mineral fertilizer application.

Soil fertility
Low organic carbon ranging from 0.3-0.4%. Low soil pH of around 4.5. Low exchangeable bases (Ca and Mg) and a low eCEC content averaging 2.9 cmol(c) kg -1 .
Sub-optimal rates of mineral fertilizer often applied to the maize crop, with no mineral 0-172.5 kg mineral N ha -1 applied as compound D Light coloured with high sand content. Fields are often allocated to grain legume crops for soil fertility restoration.

Farmer management
Farmers do not often apply organic nutrient resources (e.g. compost, cattle manure, woodland leaf litter) and do not invest much labour and time towards production on these fields. Planting and/ or weeding is often done late, and fields are characterized by high weeds infestation due to infrequent weeding.

Crops grown
Often allocated to "women crops" such as groundnut, cowpea (see Mapfumo et al. 2001) and soybean. Small grains are also grown on such nutrient depleted soils.

Crop response to fertilization
Usually crop failure with no external fertilization. Low crop response to external addition of mineral and/ organic fertilizer. Crops have yellowish leaves, stunted and poorly established (uneven crop stand).

Moisture content
Low moisture retention resulting in sudden evidence of moisture stress in crops after a rainfall event.
fertilizer use on some of the fields. Organic nutrient resources are often not applied to these fields.
Cattle manure is either not applied or applied after several cropping seasons at lower rates of about 2-5 scotch carts ha -1 .