A decade of non-sorted solid urban wastes inputs safely increases sorghum yield in periurban areas of Burkina Faso

Abstract

Non-sorted solid urban wastes (SUW) are used by periurban cereal farmers in Africa. There is however limited information on how these SUW affect soil quality and cereal production and quality. In order to answer this question we identified around Ouagadougou (Burkina Faso) sites cultivated with sorghum (Sorghum bicolor (L.) Moench) that had received SUW for less than 5 years, for more than 5 years and less than 10 years and for more than 10 years. We sampled soils at 0–15 and 15–30 cm depth and we analysed pH, total carbon (C), nitrogen (N) and inorganic phosphate (P) content and P and heavy metals availability. We also measured at some of these sites sorghum production and nutrient and metal contents in sorghum grain and straw. Our results show that the 0–15 cm horizon of the soils that had received SUW for less than 5 years had lower pH, available P and heavy metals contents and produced lower yields than those that had been amended with SUW for more than 10 years. Maximum grain yield was observed in the sites that had been amended for more than 5 years but less than 10 years. There were no clear effects of SUW application time on the heavy metal contents of sorghum grain and straw. The increases in nutrient and heavy metals content observed in the 15–30 cm horizon of soils that had been amended for more than 10 years point out to the risks of element transfer to deeper horizons. Our results suggest that a complete sorting of organic matter from SUW and its further composting as presently recommended, is not necessary. Simply removing dangerous items from the SUW such as plastics, glass and batteries, would be sufficient. Adding this sorted substrate for 5 to 10 years to cereal fields would be sufficient to reach optimal yields, thereafter this substrate should be added to other surfaces.