Predicting Amounts of Rock Phosphate Needed for Crop Production in West African Soils

Sidibe Diarra, Amniata
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The direct application of rock phosphate (RP) can be a good alternative to water-soluble P fertilizers. However, despite the extensive work done on RPs, there are no models to predict amounts of RP required. A study including both laboratory and field experiments was conducted to develop an algorithm for estimating amounts of rock phosphate (RP) needed for crop production in West African soils on a site-specific basis. Soil properties affecting RP dissolution and sorption processes were studied in laboratory incubation and incorporated into an algorithm. The accuracy of the method used for estimating RP dissolution (ANaOH-P) was assessed. Field experiments were conducted at Konobougou, Mali and Keur Madieng, Senegal to test the predictions made by the algorithm. Another incubation study was conducted to evaluate the limitation of RP dissolution in the Keur Madieng soil. Results showed that amounts of RP for maximum yield could be predicted using plant critical P level, KCI-extractable acidity, Ca saturation, effective cation exchange capacity and clay content. The presence of water-soluble P in the RP might affect the accuracy of ANaOH-P method as a measure of the amount of P dissolved from RP. The ANaOH-P extraction can, however, be used as an estimate of the amount of P released from the RP material. The accuracy of the RP algorithm could not be conclusively tested because of the large variability in the field. However, there are indications that the RP algorithm could be overestimating RP amounts for maximum millet yield. A unique Bray 1-P critical level varying between 7 and 11 mg kg-1 was estimated for both soluble P fertilizers and RPs. The limitation of the RP dissolution in the Keur Madieng soil was due to the lack of soil acidity. The results showed that the algorithm has a good potential to estimating quantitatively the amounts of RP needed for crop production. However more studies are needed for refining and generalizing the algorithm. The results indicate that RP dissolution needs to be improved in the sandy soils of West Africa.
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