THE CERRADO BIOMA CONVERSION TO CULTIVATED PASTURES AND FERRALSOLS FUNCTIONING

Abstract

During an IRD-Embrapa project the functioning of ferralsols has been studied with regard to its physics, hydrodynamics and biology and the geochemistry of phosphorus against a background of the transformation of the natural savannah vegetation into cultivated Bracharia spp. pastures. This paper describes the main balances and attempts to clarify the features of the soils under pasture which are affected, and in particular ferralsols. The work has shown that the state of the soil under an old, unproductive pasture is characterised by medium-low organic matter content and very low concentrations of plant-available phosphate ions. The surface horizon usually exhibits a loss of porosity and the soil structure is altered, being more compacted. The hydraulic properties do not seem to be greatly affected, and runoff and erosion are only localised problems resulting from very bad soil management practices used on certain watershed catchment bassin. The diversity and biomass of the soil macrofauna are reduced. However the physical and biogeochemical behaviour recorded indicate that properties favourable to plant growth can be restored by changing the management of the system. On the other hand, we have shown that these soils remain very susceptible to fluctuating rainfall, the available water reserve declining rapidly, particularly when the fine porosity falls. But the cause of the decline in this case is the occurrence of short dry seasons.