CHANGES IN SOIL MICROBIAL ACTIVITY FOLLOWING CONSERVATION TILLAGE PRACTICES IN A SORGHUM FIELD UNDER SUBTROPICAL CONDITIONS

A field experiment was carried out in a degraded sorghum field, under warm subtropical conditions, to determine the influence of conservation tillage practices such as no tillage and reduced tillage (subsoil-bedding and shredbedding), and conventional tillage practices such as mouldboard ploughing, on physical and microbiological soil quality indicators, over a period of three years. An adjacent soil under natural vegetation was used as a standard of local high quality soil. Conservation tillage systems, in particular no tillage, increased crop residue accumulation on the soil surface. Organic matter decreased with increasing tillage at all depths and was 33% greater with no tillage compared with the average of the other tillage treatments in the surface 0 to 5 cm. No tillage produced values of soil organic matter, available P and nitrate similar to soil under natural vegetation. The no tilled soil had values of water soluble C, dehydrogenase, urease, protease, phosphatase and β-glucosidase activities, and aggregate stability higher than tilled soils but lower than the soil under natural vegetation. The enzyme activities and aggregate stability reflected early changes in soil the profile to a greater extent than did physical-chemical and chemical properties. The no tillage system was the most effective for improving soil physical and biochemical quality. In the medium-term, this conservation tillage system is still far from achieving the quality levels of soil under natural vegetation.