The Change in Soil Physical Conditions Resulting from Forest Clearance in the Humid Tropics

Abstract

The soil changes which result from the clearance of tropical rainforest have been the focus of research for a number of years. To date most of the research has focused on the levels of soil nutrients following clearance. The conclusion has been that nutrient levels are commonly very low, which, together with the low pH levels and the possibility of high levels of exchangeable aluminium, has highlighted the need for very careful management of these soils if satisfactory yields of agricultural crops are to be achieved. Few studies have investigated soil physical properties in these soils either before, during or following clearance of the rainforest cover. This paper considers the changes in soil physical properties following clearance by traditional slash and burn methods and by mechanical/ bulldozer methods, with an emphasis on work in the Amazon Basin of South America. The changes in soil physical properties which result from mechanical clearance are substantial and, in some cases, subsequent growth of annual and perennial crops may be impossible without the introduction of remedial cultivation operations. Clearance in the tradi- tional manner of slash and burn leads to relatively small changes in soil physical conditions, but additionally the ash and organic matter is an important component of the initial post clearance fertility. It seems likely that we must reconsider the widespread use of mechanical forest clearance methods, perhaps replacing them with a combination of traditional slash and burn and mechanical methods. Where mechanical clearance alone is used, some form of remedial cultivation may be necessary if reasonable crop yields are to be achieved.