Nitrogen and phosphorus dynamics in tropical soils of Mali, West Africa

Abstract

Low soil fertility is one of the major biophysical constraints affecting African agriculture. Phosphorus and nitrogen are the two most common limiting nutrients. Before fertility amendment recommendations are made a soil's natural nutrient availability should be assessed. In 1998, soil samples were collected at Cinzana, Mali, West Africa for the purpose of documenting the seasonal dynamics of soil nitrogen and phosphorus in two soils after nine years under five crop management systems. The cropping systems are: continuous grain (sorghum (Sorghum bicolor) or millet (Pennisetum glaucum)), continuous grain with stalk residue returned to the field every second year, grain in rotation with cowpea (Vigna unguiculata), grain in rotation with a sesbania (Sesbania rostrata) green manure crop, and grain in rotation with a dolichos (Dolichos lablab) green manure crop. Nitrogen availability graphs showed an inorganic nitrogen flush early in the rainy season in both soils. Extractable N concentration in a loamy sand soil peaked around 10-15 mg N kg⁻¹ soil or 15-22 kg N in the upper 10 cm of soil ha⁻¹. The inorganic N concentration declined four weeks after the onset of the rainy season. Extractable N concentration in the clay soil reached a maximum of 22-34 mg N kg⁻¹ soil or 33-51 kg N in the upper 10 cm of soil ha⁻¹. In the clay soil the high N concentrations associated with the early season flush lasted eight weeks after the onset of the rain. Further improvement of cereal grain yield may not be possible by rotation with sesbania and dolichos green manure or cowpea without additional nutrient input. Nitrogen and P return through rotation crops and crop residue is low. Phosphorous Bray-1 measurements fluctuated by 1.43 mg P kg⁻¹ in the clay soil in 1998. Soil P availability in the clay soil was not influenced by the crop management systems. Bray-1 P measurements in loamy sand soil five months after Tilemsi phosphate rock application were surprisingly significantly lower than before application. Findings from this study can be used to make future crop management recommendations in the Cinzana, Mali, region.