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Stellenbosch University (2018)

Evaluation of a peri-urban smallholder farmers’ soil amendment practices on soil quality and crop growth, yield and quality

Gobozi, Thamsanqa Khanya Sikho

Titre : Evaluation of a peri-urban smallholder farmers’ soil amendment practices on soil quality and crop growth, yield and quality

Auteur : Gobozi, Thamsanqa Khanya Sikho

Université de soutenance : Stellenbosch University

Grade : Master of Science (MS) 2018

Peri-urban, smallholder farmers surrounding Cape Town, which are the main producers of fresh vegetables in the region, are generally not producing at their optimum level due to lack of agronomic support, marginal sandy soils and socio-economic constraints. The aim of this study was to evaluate the soil amendment practices of an organic smallholder farmer from Raithby, near Stellenbosch, in comparison with potential alternative organic and chemical amendments on soil fertility and vegetable crop growth, yield and quality and economic profitability. During the first winter field trial, the farmer’s routine soil amendment practice of adding 10 t/ha of commercially bought compost was compared with three alternative organic amendment practices and a commercial chemical fertilizer programme on broccoli production. Two on-farm produced composts, composted plant and animal waste (CW) and composted waste containing 20% biochar (CB), and the commercial compost (CC) were applied at typical smallholder application rate of 10 t/ha. The CW was also applied at broccoli N requirement equivalent to 22 t/ha (CWCR). These organic treatments were compared with a control soil (C) and a chemical fertilizer (CF) programme designed specifically for broccoli. There were no significant differences in soil quality at planting or at harvest (pH, EC, ECEC, plant available macro or micro-nutrients) or broccoli head nutrient content between treatments. However, the CF significantly (p<0.05) increased soil mineral N compared to all other treatments, whereas, CB significantly (p<0.05) enhanced soil C. Application of CF significantly (p< 0.05) increased broccoli yields (88% increase compared to CC) which was correlated with the higher soil mineral N, followed by CW (28% increase compared to CC). Application of CC, CB and CWCR resulted in non-significant changes in yield compared to the control, which was attributed to too much C being added to soil compared to N. Compared to the farmer’s routine amendment practice (CC), the CF resulted in the greatest income increase (455%) followed by CW at 10 t/ha (151%). During the second summer field trial, the effect of two composts, i.e., university compost (UC) and farmer’s compost (FC), two commercial organic fertilizers, i.e., OF1 (blood and bone meal based) and OF2 (chicken manure based), and commercial chemical fertilizer (CF) programme was evaluated on green bean production. The commercial organic and mineral fertilizers were applied at green bean N requirement rate of 158 kg N/ha. Whereas, the two composts were inadvertently applied at different N application rates relative to the commercial fertilizers (UC was added at 8.9 ton/ha ( 49 kg N/ha), while FC was added at 17.8 ton/ha ( 181 kg N/ha) due to a commercial laboratory providing incorrect elemental analysis of the composts prior to the field trial. All compost and fertilizer treatments significantly (p< 0.05) increased soil Bray II P contents above the critical value 25 mg/kg at planting except FC. The commercial organic fertilizers increased soil EC by a factor of 2-3, which resulted in lower bean plant survival. There were no significant differences in bean nutrient content between treatments, except for OF1 which contained significantly lower Mg content. Application of CF significantly (p< 0.05) increased (56% compared to control) green bean yields which was associated with a significantly (p< 0.05) higher (168% compared to control) cumulative soil mineral N, while the FC applied at 17.8 t/ha produced the second highest increase (37% compared to control) which was associated with higher (5%) number of plants that survived to harvest and the order was consistent in terms of economic feasibility. The availability of mineral N was the main driver of crop yields and size of economic yield per plant in this study. Composts, especially commercial composts with low inherent N content, are not reliable sources of mineral N for intensive crop production. The commercial organic fertilizers, although better sources of mineral N, were prohibitively expensive and decreased plant survival. The organic smallholder farmer is likely to generate more income when he produces his own compost using animal and plant waste and applily the on-farm produced compost at N requirement of the crop in production rather than buying composts or organic fertilizers. The study also indicated that the farmer would generate much higher income, especially in winter when organic N mineralisation is slowest, if he would use a chemical fertilizer programme for both model crops.

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