Alley cropping in the moist savanna of West-Africa: I. Restoration and maintenance of soil fertility on 'terre de barre' soils in Benin Republic

The potential of alley cropping systems supplied with a limited amount of f ertilizer to restore crop productivity on a degraded site and to maintain c rop productivity on a recently cleared, non-degraded site on 'terre de barr e' soils in Southern Benin was investigated from 1994 to 1996. Leucaena leu cocephala, Senna siamea and Gliricidia sepium were used as hedgerow species . Maize yields of the no-tree control plots dropped from the initial (1990) 401 kg ha(-1) and 2181 kg ha(-1) on the degraded and non-degraded sites, r espectively, to 109 kg ha(-1) and 1346 kg ha(-1) in 1996, even with applica tion of a minimal amount of mineral fertilizer. The alley cropping systems produced on average (mean of three treatments and three years) 107% more gr ain than the initial 1990 values on the degraded site and 11% less grain th an the initial 1990 values on the non-degraded site. Especially the Senna a nd to a lesser degree the Leucaena treatment yielded consistently more grai n than the control. The Senna trees contained a larger amount of N and prod uced more wood during the first pruning on the degraded site (155 kg N ha(- 1) and 14.0 ton fresh wood ha-(1)) than on the non-degraded site (49 kg N h a(-1) and 6.6 ton fresh wood ha(-1)) most likely because of differences in subsoil fertility, as indicated by the higher clay, exchangeable bases, and N content between 60 and 125 cm cm. N accumulation and wood production by the Leucaena and Gliricidia trees was similar in both sites (82 and 36 kg N ha(-1) and 4.6 and 9.3 ton fresh wood ha(-1), respectively). When a limite d amount of fertilizer is available, Senna appears to be the best choice as hedgerow species on sites with a relatively fertile subsoil. For other soi ls, a N-2-fixing species may be a better choice.