SOIL NUTRIENT DEPLETION BY AGRICULTURAL PRODUCTION IN SOUTHERN MALI

The degree of soil mining by agricultural production in Southern Mali is assessed by calculating nutrient balances: differences between the amount of plant nutrients exported from the cultivated fields, and tho se added to the fields. Export processes include extraction by crops, losses due to leaching, to erosion, and to volatilization and denitrif ication. Inputs include applications of fertilizer and manure, restitu tion of crop residues, nitrogen fixation, atmospheric deposition of nu trients in rain and dust, and enrichment by weathering of soil mineral s. Nutrient balances are calculated for N, P, K, Ca, and Mg. Both pess imistic and optimistic estimates are given. The resulting figures indi cate, even when the most optimistic estimates are used, large deficits for nitrogen, potassium and magnesium. For the region as a whole, the calculated deficits are -25 kg N/ha, -20 kg K/ha, and -5 kg Mg/ha. Fu rther, acidification is to be expected, in particular in areas where c otton is grown. The deficits are caused by traditional cereal crops, b ut also by cotton and especially by groundnut. The latter two crops ar e fertilized, but insufficiently. It is important to note, that the ne gative figures are not automatic recommendations for application of a specific amount of additional fertilizer. For phosphorus and calcium t he balance of the region as a whole appears to be about in equilibrium , but locally large variations may occur. Erosion and denitrification are important causes of nutrient loss, accounting respectively for 17 and 22% of total nitrogen exports. Atmospheric deposition and weatheri ng of minerals in the soil are still important nutrient inputs that co ntribute as much as nutrients as organic and mineral fertilizer combin ed. Nutrient depletion is very large in comparison to the amount of fe rtilizer applied. Drastic options, such as doubling the application of fertilizer or manure, or halving erosion losses, even if feasible, wo uld still not be enough to make up for the calculated deficits. The an nual value of withdrawn nutrients, if related to prices of fertilizers , varies between 10,000 and 15,000 FCFA/ha (40-60 US $/ha). Since the estimated average gross margin from farming in this area is 34,000 FCF A/ha (123 US $/ha), soil mining appears to provide an amount equal to 40% of farmers' total income from agricultural activities.