Cropping system effects and seasonal dynamics of extractable phosphorus in a semi-arid soil of the Sahel, West Africa

Phosphorus (P) is often the most limiting nutrient to crop production in the Sahel. Crop rotation and residue management, as well as time of soil sampling, may influence extractable soil P. The objective of this research was to determine the influence of five long-term crop-management systems and temporal soil sampling on Bray 1 P concentrations in a clay soil in 1998, near Cinzana, Mali. This information could be used to predict an availability range for single-sample soil measurements or to adjust laboratory-measured soil P according to date sampled. Bray I P concentration ranged from 2.7 to 4.2 mg P kg(-1) soil on an oven-dry (105 degrees C) basis in soil collected from 27 May until 3 November 1998. In addition, comparison of extractable soil P between crop-management systems did not reveal a significant increase in P caused by residue return, crop rotation, or green manure compared to the traditional continuous sorghum [Sorghum bicolor (L.) Moench] cropping system. Crop-management system did not influence sorghum P uptake, and little P (0.3-0.6 kg ha(-1)) was returned in residue. Continued cropping without larger inputs of fertilizer P will further deplete soil fertility in the Sahel.