On-farm evaluation of ridging and residue management options in a Sahelian millet-cowpea intercrop. 1. Soil quality changes

Abstract. In the Sahel, promising technologies for agricultural intensification include millet stover mulching and ridging. A four year on-farm experiment was set-up in order to assess the effect of various combinations of these two technologies on soil chemical and physical quality in a millet ( Pennisetum glaucum (L.) R. Br.) –- cowpea ( Vigna unguiculata (L.) Walp.) relay intercropping system. Treatments included bare surface, ridging, a surface applied banded millet stover mulch (2 t ha^–1) and a banded millet stover mulch (2 t ha^–1) buried in ridges. The latter three treatments were implemented exclusively in the cowpea rows, with an annual rotation between the millet and cowpea rows. Both the bare and ridge plots experienced a rapid loss of organic carbon, acidification and the development of extensive surface crusts but no increase in bulk density or penetration resistance. In the year of application, mulching improved soil quality in the cowpea row with respect to pH, organic carbon and exchangeable K⁺ and Mg⁺⁺ content, penetration resistance and bulk density, and it reduced the decline in exchangeable Ca⁺⁺ and total N content. In the year following mulch application, a general decline in soil chemical quality was observed in the millet row, except for organic carbon content, and a positive residual effect was observed on penetration resistance and bulk density. As a rule, the effects of mulching in the year of application tended to be stronger in the ridged treatment with buried residue than in the banded surface mulch. In the year following application, this tendency was reversed. For the purpose of reducing soil degradation by nutrient mining and wind erosion, a banded surface mulch therefore appeared more effective than buried mulch.