TILLAGE AND MULCHING EFFECTS ON PHYSICAL-PROPERTIES OF A TROPICAL ALFISOL

The mechanization of field operations like seeding, spraying and harve sting in continuous zero-tillage may lead to a severe compaction of th e surface layer of coarse textured tropical soils, especially when mul ch is sparse or missing. Therefore, a 2 year (1982-1984) field experim ent was initiated on an Alfisol in Nigeria to study the effect of till age, mechanization and mulch on soil structure and physical properties . Three zero-tillage treatments and a plough treatment were compared. The disk-plough and one of the no-till treatments were highly mechaniz ed: all the field work was performed with tractors and machines, and c onsisted of secondary bush clearing, crop cultivation and harvest. On the other two no-till treatments, the impact of machine load was reduc ed, either by hand harvesting or by performing all field operations ma nually. These four tillage-traffic systems were either treated with mu lch or left unmulched. There were four growing seasons, with maize (Ze a mays L.) as a test crop. After 2 years of zero-tillage the bulk dens ity (BD) and penetration resistance (PR) were significantly greater on plots with high mechanization compared with hand treated plots. Plots with hand harvest but otherwise mechanized were in between. Because o f the hard-setting nature of the soil, the ploughed plots with and wit hout mulch exhibited a dramatic change in PR and BD during the season. On no-till the infiltration transmissivity (A) was greater and BD and PR were less in the mulched compared with the unmulched treatments. T he gravel content of the topsoil was negatively correlated with BD and positively correlated with A. Geostatistical analysis revealed that w ithin the experimental area there was a similar spatial distribution o f gravel content and A after the first season. Because of the superimp osing effect of gravel on BD, which cannot be accounted for by conside ring the gravel content per se, BD was adjusted by means of covariance analysis for evaluation of the treatment effects already mentioned. I t was concluded that mechanization of a no-till system on sandy Alfiso ls may only be successful in the long run if appropriate measures like mulching, crop rotation and fallow systems are applied to regenerate soil structure and to enhance macroporosity.