| dc.description.abstract | Fertilization of staple cereal crops in smallholder farming systems of sub-Saharan Africa has mostly focused on supply of nitrogen (N), phosphorus (P) and potassium (K), despite the widespread deficiencies of micronutrients that include zinc (Zn). Cereal crops grown on granitic sandy soils occurring widely in Southern Africa have poor Zn nutrition. This study focused on evaluating the potential of different soil fertility management regimes in enhancing maize (Zea mays L.) productivity, grain nutritional quality and nutrient uptake on nutrient-depleted soils on smallholder farms of Zimbabwe. A preliminary soil fertility survey and on-farm experiments were conducted in different agro-ecological regions (NR) in smallholder farming areas of Wedza (NR II: 750 - 800 mm yr-1) and Makoni (NR III: 650-750 mm yr-1) districts in eastern Zimbabwe. The survey was conducted during the 2008/09 season to evaluate the effect of farmers’ soil fertility management practices on soil Zn status and maize grain nutrient concentrations. Ethylenediaminetetraacetic acid (EDTA) extractable soil Zn from the different farmers’ fields ranged from 0.5 – 2.43 mg kg-1 with the highest concentration associated with application of woodland leaf litter to maize. In both study areas, combined use of organic nutrient resources and inorganic fertilizers produced maize grain yields >2.1 t ha-1, sharply contrasting the non-fertilized treatment which yielded <0.8 t ha-1. Up to 46% and 64% increase in grain Zn concentration was measured against the control in Makoni and Wedza respectively. Co-application of inorganic fertilizers with cattle manure or leaf litter, and legume-cereal rotations significantly influenced Zn uptake (p<0.01), with uptake of up to 48.5 g Zn ha-1 measured in Wedza. Maize grain from farmers’ fields was characterized by low Zn concentrations and poor Zn bioavailability as indicated by exceptionally high phytic acid to Zn (PA:Zn) molar ratios of 150. The PA:Zn ratio is a widely used criterion to predict bioavailability of Zn in humans and should not exceed 15. Field experiments conducted between 2008 and 2011 evaluated the effect of different Zn fertilizer formulations on maize yields and subsequently grain Zn concentration. Results showed that application of basal and foliar Zn fertilizers, solely or in combinations increased maize grain yields, out yielding the non Zn treatment. Significant maize grain Zn uptake differences (p<0.01) among treatments were observed during the two seasons, with highest uptake of ~114 g Zn ha-1 attained after application of a basal NPK fertilizer at 11 kg Zn ha-1. The Zn concentrations measured in maize grain during the 2008/09 and 2009/10 seasons ranged from 14.3 – 30.3 mg kg-1 and 18.7 – 39.0 mg kg-1 respectively, with the highest concentrations being realized after co-application of basal and foliar Zn fertilizers. There was a significant and positive linear relationship between EDTA- extractable soil Zn and maize grain Zn concentration (R2>0.80). An assessment of maize grain yield and quality benefits of Zn following application of organic nutrient resources and mineral fertilizers at different levels indicated that co-application of organic and inorganic fertilizers increased maize yields by 5.5-fold compared to the unfertilized treatment. Maize produced under leaf litter in combination with mineral NPK and Zn had the highest grain Zn concentration of up to 33.0 mg kg-1. Overall results indicated that mineral fertilizers as currently applied by farmers promote Zn mining and are likely to result in long-term severe deficiencies in soils. Integrated soil fertility management practices as currently presented to smallholder farmers are limited by lack of micronutrients which are required to improve available soil Zn, yields and nutritional quality of staple maize grains. While application of Zn- based fertilizers can be an entry point for enhancing grain quality of staple maize, farmers who apply sole mineral fertilizers have a higher demand for Zn than those who use combinations of organic nutrient resources and inorganic fertilizers. | en_ZW |
