EFFECTS OF CONSERVATION AND CONVENTIONAL TILLAGE SYSTEMS AFTER LAND CLEARING ON SOIL PROPERTIES AND CROP YIELD IN SANTA-CRUZ, BOLIVIA

Mechanized annual cropping in the central zone of Santa Cruz, Bolivia, has caused serious soil degradation, manifested by problems of compac tion, accelerated soil organic matter losses, nitrogen deficiences, an d wind erosion. Overcultivation with disc implements is believed to be the main factor responsible. A long term tillage experiment was estab lished on a Typic Haplustalf in Saavedra 2 years after forest clearing to compare the effects of conventional disc tillage (CT) and three co nservation tillage systems, i.e. chisel plough (CP), no tillage (NT) a nd flexible tillage (FT), on the changes in soil physical and chemical properties, weed infestation and crop yields. Flexible tillage is sim ilar to NT, except that CP or CT may be used if serious problems of so il compaction or weeds arise. Before implementing the CP treatment, re duced tillage (RT) was used for three seasons followed by CT for one s eason. Results after the first 4 years (eight cropping seasons) showed that CP caused least soil physical degradation, with compaction being in the order CP < CT < FT < NT, whereas NT caused least chemical degr adation, resulting in higher soil organic matter and total N contents. Conventional tillage was most effective for controlling weeds, especi ally grassy weeds. Differences in crop yields occurred for only three out of eight seasons, with evidence that compaction was largely respon sible for yield decreases, indicating the importance of CP for this so il. However, the increase in soil degradation over 4 years using mean values from the trial was greater than that among tillage treatments. During the 4 years, in addition to increases in soil compaction, mean infiltration capacity decreased 35-48%, mean equilibrium infiltration rate decreased 77%, average available water capacity decreased 47%, an d mean aeration capacity decreased 30%. Significant decreases in soil organic matter, total N, available P, and exchangeable Ca were found f or some or all tillage treatments during the 6 years since forest clea ring, and estimates of average annual soil organic matter and total N losses were 8.0 and 8.8%, respectively. Thus the overall soil physical and chemical degradation of an Alfisol during 4 years of tillage shor tly after forest clearing was much more pronounced than the difference s between conventional disc tillage and the three conservation tillage practices.