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Maize yields on farmers' fields in Mali are usually less than 2 t/ha despite the availability of improved varieties that yield up to 8 t/ha under optimal management. The low yield is partly due to low soil fertility and lack of site-specific fertilizer recommendations. Studies were carried out in three different agro-ecological zones of Mali, namely the Manding Plateau, Koutiala Plateau and Upper Bani-Niger, to determine the response of maize to different combinations of nitrogen (N), phosphorus (P) and potassium (K) fertilizers. The objective was to determine agronomicaly and economically optimum rates of NPK for use by the smallholder producers in different agroecological zones of Mali. The experimental design was a randomized complete block with three replicates. The treatments were18 fertilizers rates each subdivided into two subplots for a hybrid maize variety (Tieba) and an open pollinated variety (Sotubaka). A diagnostic treatment including Zn, Mg, and B was compared with the different levels of NPK to evaluate the effect of trace elements on maize yield and yield components. In terms of grain yields, the best was the diagnostic treatment across the three agro-ecological zones. Increasing application rates to 200 kg N/ha + 80 kg P/ha did not significantly increase grain yields over application of 120 kg N/ha + 30 kg P/ha. Agronomic use efficiency of N and the value cost ratios (VCR) were negative for all treatments at Kolombada, where rainfall received during the growing season was very low (567-674 mm) relative to its historic average of 800-1000 mm. The VCR was highest with 90 kg/ha N + 10 kg/ha P at Bougouni and with 30 kg/ha P at Samanko although VCR was less than the cutoff point of 2 in all cases. At Kolombada, fertilizer use was unprofitable regardless of the amount of nutrients applied.

In Mali, the deficiency of agricultural soils in phosphorus is one of the limiting factors in crop production. They respond well to phosphorus fertilization, but the high price of imported chemical fertilizers limits their use by the producers. However, Mali has large deposits of natural phosphate in the Tilemsi Valley (PNT) in northern Mali. Faced with this situation, the Government encouraged the use of local fertilizing resources including the PNT. Despite the satisfactory results of several years of research and popularization, the PNT was poorly adopted by the producers because of its powdery appearance, which made it difficult to apply in the field, its low solubility in the soil, which made its effect noticeable on crops in the second year of its application and its brown color reminiscent of that of earth made the producers believe that it had no fertilizing value. Faced with these problems relating to the physical appearance and the low solubility of the PNT, the laboratory work of GREAT QUEST Fertilizer SA resulted in the development of simple formulations based on enriched granular PNT 27% P 2 O 5 (average content) and 35% P 2 O 5 (high content) for direct application and NPK complex formulations based on granulated enriched PNT 35% P 2 O 5 mixed mainly with urea, potassium, and sometimes with other nutriments like sulfur and boron (NPK 15-15-15; NPKSB 14-18-18-6-2) with a solubility in citric acid of 71.1%. The results of four years of experimentation of enriched granular PNT in the research stations and in the peasant environment showed a greater or equal effect than the other popular fertilizers. This project aims to improve phosphate nutrition and crop production by using granulated formulations of enriched PNT 27% P 2 O 5 and 35% P 2 O 5 .

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