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Lack of moisture limits crop production in semi-arid west Africa but poor soil fertility is a more serious constraint in the long run. Work done by IFDC and ICRISAT showed that phosphorus is the most limiting nutrient although response by millet to nitrogen when moisture and P are non-limiting can be substantial. A summary of the results of field trials in Niger showed that the sufficiency level for P estimated by regression analysis on the basis of Bray 1 extractable P is 7.9 JAg Pig of soil for 90% of maximum yield of millet. Application of 15-20 kg P/ha was usually adequate for optimum yields. Matam phosphate rock (PR) from Senegal, Tilemsi PR from Mali and Tahoua PR from Niger which are medium reactive were found to be suitable for direct application while partial acidulation (50% with sulfuric acid) of the less reactive phosphate rocks resulted in products with similar agronomic effectiveness as commercial superphosphates. Tests conducted by farmers showed that millet yields can be increased by more than 250% by the use of fertilizers. The need for constant monitoring of the crop needs for sulfur and potassium under continuous cultivation was highlighted.

Mineral fertilizer use is increasing in West Africa though little information is available on yield response in farmers' fields. Farmers in this region plant at low density (average 5,000 pockets h a-1, 3 plants pocket-1), which can affect fertilizer use efficiency. A study was conducted with 20 farmers in Niger to assess the response of pearl millet [Pennisetum glaucum (L.) R. Br.] to phosphorus and nitrogen fertilizers under farm conditions. In each field, treatments included control, single superphosphate (SSP) only, SSP plus N (point placed near plant), and either SSP or partially acidulated phosphate rock (PAPR) plus N broadcast. N and P were applied at 30 kg N ha-1 and 30 kg P2O s h a-1. Farmers were allowed to plant, weed, etc., as they wished and they planted at densities ranging from 2,000 to 12,000 pockets h a-1. In the absence of fertilizer, increasing density from 2,000 to 7,000 pockets h a-1 increased yield by 400%. A strong interaction was found between fertilizer use and density. Farmers planting at densities less than 3,500 pockets h a-1 had average yields of 317 kg grain hawhile those planting at densities higher than 6,500 pockets h a-1 showed average yields of 977 grain h a-1. Though phosphate alone increased yields significantly at all densities, little response to fertilizer N was found at densities below 6,000 pockets h a-1. Significant residual responses in 1987 and 1988 were found to P applied in high-density plots in 1986. Depending on fertilizer and grain prices, analysis showed that fertilizer use must be be combined with high plant density (10,000 pockets h a-1) or no economic benefit from fertilizer use will be realized.