EFFECTS OF BULLDOZER AND CHAIN CLEARING ON SOIL PROPERTIES AND CROP YIELDS

The use of bulldozers and straight blades for clearing subtropical forests in eastern Bolivia is believed to result in serious soil degradation and low crop yields. It was hypothesized that chain felling with a tree pusher and rake windrowing would cause less soil degradation and give higher crop yields. An experiment established on a Typic Haplustalf compared the effects on soil degradation and subsequent crop yields of (i) conventional bulldozer felling and windrowing with a straight blade (land clearing control), (ii) bulldozer felling with a straight blade and tree pusher, and rake windrowing, (iii) chain felling with a tree pusher and straight blade windrowing, (iv) chain felling with a tree pusher and rake windrowing, and (v) undisturbed forest (control). All four methods caused significant soil physical degradation as manifest by 10 to 20% increases in bulk density and a 6.2% loss of total porosity at the 0- to 0.20-m depth, a lowering of the soil surface by about 30 mm due to compaction, and a significantly lower 30-min cumulative infiltration (4.36 cm) compared with the forest control (15.08 cm). All four land clearing methods gave significant increases in Ca (from 7.7 to 11.6 cmol(c) kg(-1)) in the 0- to 0.20-m depth as a result of forest burning. Very few significant differences in soil properties, and no significant differences in subsequent yields of corn (Zea mays L.) or soybean (Glycine max. L.), were found among land clearing methods. Thus, under dry to slightly moist soil conditions and with skilled operators, the clearing of a low-biomass, subtropical forest by bulldozer clearing with a straight blade was no more degrading to the soils than chain felling with a tree pusher and rake windrowing, and did not result in lower yields of subsequent crops.