SOIL DEGRADATION IN THE TROPICAL LOWLANDS OF SANTA-CRUZ, EASTERN BOLIVIA

This paper examines the soil degradation processes that are threatenin g the productivity of the mechanized annual cropping areas in the Sant a Cruz lowlands of tropical eastern Bolivia. The dominant process is t hat of machinery-induced and natural compaction, which has resulted in an estimated 50 per cent of the soils in the Central Zone being moder ately to severely compacted, causing serious root restriction and the loss of both transmission and water-storage pores. Degradation has mad e the soils increasingly susceptible to moisture stress due to the com bined effect of (i) restricted rooting as a result of compaction and t he hardsetting characteristics of the soils, (ii) reduced rainfall inf iltration due to the loss of transmission pores and surface crusting, and (iii) a decrease in available soil moisture caused by the loss of storage pores, the incorporation of wind-blown fine sand deposits, and soil organic matter losses due to accelerated decomposition rates. Th e loss of transmission pores has also made the soils more prone to wat erlogging in periods of high rainfall. The degradation tendencies of t hese soils are exacerbated by the greater variability of seasonal rain fall during the last 20 years that has led to a greater frequency of e xtremely high or extremely low rainfall events than hitherto.