Field-scale analysis of water and nutrient use efficiency for vegetable production in a West African urban agricultural system

Urban agriculture increasingly supplies food and non-food services to the rapidly growing West African cities. However, with its typically heavy use of fertilizers and uncontrolled use of water, it bears severe risks of soil and groundwater pollution. This study was carried out in Bobo Dioulasso (Burkina Faso, West Africa) at two commercial gardening sites. It aimed at performing a detailed characterization of farmer’s water and nutrient management strategies. Four vegetable crops were monitored (tomato, cabbage, carrot and lettuce). Water inputs (rainfall and irrigation), nutrient inputs (organic and mineral fertilizers and nutrients in irrigation water) and crop uptake were monitored over a 1–2-year period. In addition, pan lysimeters allowed monitoring drainage. Depending on the site, 3–8% of the input water was lost by drainage, i.e., as much as 293 mm/year at one of the sites. During the dry season, when the farmer has full control over the water supply, water application exceeded plant requirements by as much as 40%, which reveals inefficient water use. Up to 800 kg N ha⁻¹, 140 kg P ha⁻¹ and 500 kg K ha⁻¹ were applied for a single crop cycle. With few exceptions, crops tended to be strongly over-fertilized, except for K at one of the two sites. Nutrient supply exceeded crop requirements by 109 to 2,012 kg N, 66 to 450 kg P and 0 to 393 kg K per year. These results, in combination with the large observed rates of drainage, are indicative of a high risk of nutrient leaching and groundwater contamination. The partial factor productivity of nutrients tended to be low. These results therefore suggested that there is scope to substantially reduce nutrient application rates in these systems without adverse effects on yields, which would be beneficial for groundwater resources and improve the economic returns.