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 compaction, accelerated soil organic matter losses, nitrogen deficiences, and wind erosion. Overcultivation with disc implements is believed to be the main factor responsible. A long term tillage experiment was established on a Typic Haplustalf in Saavedra 2 years after forest clearing to compare the effects of conventional disc tillage (CT) and three conservation tillage systems, i.e. chisel plough (CP), no tillage (NT) and flexible tillage (FT), on the changes in soil physical and chemical properties, weed infestation and crop yields. Flexible tillage is similar to NT, except that CP or CT may be used if serious problems of soil compaction or weeds arise. Before implementing the CP treatment, reduced tillage (RT) was used for three seasons followed by CT for one season. 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 degradation, resulting in higher soil organic matter and total N contents. Conventional tillage was most effective for controlling weeds, especially grassy weeds. Differences in crop yields occurred for only three out of eight seasons, with evidence that compaction was largely responsible for yield decreases, indicating the importance of CP for this soil. 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%, and mean aeration capacity decreased 30%. Significant decreases in soil organic matter, total N, available P, and exchangeable Ca were found for some or all tillage treatments during the 6 years since forest clearing, 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 shortly after forest clearing was much more pronounced than the differences between conventional disc tillage and the three conservation tillage practices.