The diversity of fertilization practices affects soil and crop quality in urban vegetable sites of Burkina Faso

In this study we wanted to know whether diversity in fertilization practices between and within urban vegetable sites exists and how does this diversity affect soil and crop quality. We sampled soils (at 0–15 and 15–30 cm depth) and lettuce leaves (Lactiva sativa) at four vegetable growing sites in Ouagadougou called Boulmiougou, Hopital, Tanghin and Wayalguin. We determined the pH, total carbon (C), nitrogen (N), phosphate (P), and inorganic P contents, and available P and heavy metals (Cd, Cu, Pb, Cr, Zn, and Ni) contents in these soils. We evaluated also the lettuce leaves yields and we analyzed them for their N, P, K, Ca, Mg, Na and Cd, Cu, and Zn contents. We also interviewed farmers about their fertilization practices. We observed a large diversity of fertilization practices between and within sites. We found that site identity explained a very large fraction of the variability of fertilization practices, soil chemical properties and lettuce leaves production and element contents. The effect of the site was explained by the proximity to nutrient sources, the location of the site in the landscape, the presence of farmers’ association, and farmers’ skills. Solid waste and waste water were major sources of nutrients at the sites located inside the city (Hopital, Tanghin, and Wayalguin) while at Boulmiougou which is located in the periphery, manure and mineral fertilizers were the preferred sources of nutrients. The proximity of the site Tanghin to a major road led to the highest soil Pb content which was attributed to atmospheric pollution. The higher lettuce leaves yields in site Boulmiougou was attributed to good technical skill of farmers. The use of solid waste and waste water led to N, P and metals accumulation in soil. Zn and Cu content in the lettuce leaves were negatively correlated to soil pH, while Cd content in lettuce leaves was correlated to the use of water soluble P fertilizers.

► We investigated how the diversity of fertilization practices between and within urban vegetable sites affect soil and crop quality.
► We sampled soil and lettuce leaves in urban vegetables gardens and we collected information about farmer's fertilization practices.
► Site identity determined the fertilization practices and soil and crop quality.
► The use of solid and liquid waste increased soil nutrient and metal contents while use of mineral fertilizers increased soil available P, decreased soil pH and increased crop metal uptake.