Improvement of the physical fertility of a degraded Alfisol with planted and natural fallows under humid tropical conditions

Topsoil (0-15 cm) hulk density, aggregate stability soil dispersibility, wa ter retention and infiltration were measured between 1989 and 1996 on an Al fisol under rehabilitation in southwestern Nigeria. The planted leguminous species were Pueraria phaseoloides, Senna siilnrrir, Leucaena leucocephala, , Acacia leptocarpa and A. auriculiformis. plots with natural fallow and ma ize/cassava intercropping were included. Level (minimum) and mound tillage with hoes was adopted for the cultivated areas under study after 4 and 6 ye ar fallow periods. Under fallow, the soil bulk density decreased from 1.56 to 1.11 t m(-3). The continuously cropped treatment (level tillage) had sig nificantly higher bulk density than the fallowed subplots after 6 years. Me an soil penetrometer resistance ranged from 75 to 157 kPa for fallowed plot s and from 192 to 295 kPa for the continuously cropped (level tillage) subp lot. Surface soil water contents were similar for all the treatments during the soil strength measurements. Although soil aggregates were generally of low stability and not well formed, they were improved by fallowing. Soil structural improvement by planted fallows was similar to that by natur al fallow, but the trees were more promising for long-term fallow (>6 years ) than the herbaceous P. phaseoloides. However, the improvement in soil str ucture after 4 or 6 rear falloff could not be maintained in subsequent crop ping. Furthermore, the significant improvement in soil bulk density caused by A. auriculiformis and natural fallow was more rapidly lost on the cultiv ated subplots compared with other fallow treatments. Thus, soil structure r ecovery under a fallow does not imply a sustained improvement when stress i s applied to this soil. Post-fallow soil management options such as residue incorporation and tillage to ameliorate compaction or soil strength mill b e necessary to enhance the improvements by fallow species.