CHANGES IN A SANDY SAHELIAN SOIL FOLLOWING CROP RESIDUE AND FERTILIZER ADDITIONS

The use of crop residues as surface mulches has been shown to have a beneficial effect on the growth of pearl millet [Pennisetum glaucum (L.) R.Br.] in the Sahelian zone of West Africa. This study was conducted to discern the mechanism(s) responsible for yield increases resulting from crop-residue additions in a field trial located on a sandy soil at the ICRISAT Sahelian Center near Niamey, Niger. Soil chemical and physical properties were examined to a depth of 120 cm after 5 yr of application of millet residue as a surface mulch, P and N fertilizer, or a combination of fertilizer and residue. Annual residue application resulted in a higher exchangeable-base content, lower Al saturation, and slightly higher available-P values than the control. The use of fertilizer resulted in an increase in available P and exchangeable Ca. The combination of residue plus fertilizer resulted in greater enhancement of soil fertility parameters than the use of these inputs alone. Differences in soil chemistry were operative in only the top 20 cm of the soil profile, however, the surfaces of the plots receiving residues were 15 to 20 cm higher than the surfaces of the control and fertilizer-only plots. The surfaces of the residue plots also had lower clay contents than the surface soils in the nonresidue plots. The increase in soil fertility following the application of millet residue as a surface mulch was due to two mechanisms: (i) the recycling of nutrient elements to the soil following termite and microbial decomposition of the residue, and (ii) the entrapment of eolian materials, which generally have better fertility characteristics than the subsoil, or protection of the more fertile surface soil from the erosive effects of the strong winds that are common in the Sahel.