Effect of Organic Amendments on the Heavy Metal Distribution in Vegetable Gardens in Senegal

The major constraints to food production in West Africa are related to the lack of suitable lands. Studies have shown that soil degradation is threatening food security in many developing African countries. As a result, urban agriculture plays an increasing role in food security for its high supply of vegetables needed to improve health. However, production is still low because soils are infertile and low in organic matter. Consequently, farmers incorporate organic amendments, such as animal manure to improve their yields. Within limits, such organic wastes enhance soil fertility and improve the soils physical properties. However, many of these organic amendments contain elevated concentrations of heavy metals and the advantages of using organic waste as fertilizer and soil amendment should be examined with possible environmental and toxicological impacts of these heavy metals. The objective of this study is to assess the effect of organic amendments on the distribution of heavy metals in market gardens. Composted wastes and soils samples were collected from four Senegalese sites. The physico-chemical analyses showed that all sites were sandy in nature, low in carbon content (0.83 to 3.66 %) with pH ranging from 5 to 7.9. The Tessier's sequential extraction procedure for metal extraction followed, showed a general pattern of metals distribution with the highest concentration found respectively in the amorphous Fe-Mn oxide, the organic and the residual fraction. The NY State Department of Environmental Conservation, has guidelines for determining the safety of various land uses based on total soil metal concentrations. The metal levels unrestricted use for agriculture (mg/kg) are 270 for Cu, 1100 for Zn, 0.43 for Cd, 11 for Cr, 72 for Ni and 200 for Pb while the highest concentrations obtained after analysis of our samples are 20 for Cu and 56 for Zn in Pikine site, 0.7, 8.9, 8, and 12 for Cd, Cr, Ni and Pb respectively in Rufisque site. Hence, except for Cadmium, all the metals are found very low compare to these referenced values. In addition, for the US Environmental Protection Agency , heavy metal maximum concentrations in sludge (mg/kg) applied to soils are 85 for Cd, 3000 for Cr, 4300 for Cu, 420 for Pb, 75 for Ni and 7500 for Zn. These values are far greater than those found in the organic wastes used in this study which are 0.4 for Cd, 20.9 for Cr, 22.4 for Cu, 10.3 for Pb, 7.3 for Ni and 107.2 for Zn. (Texte intégral