| dc.description.abstract | Land clearing of a tropical rainforest implies cutting and felling of existing vegetation by manual or mechanized techniques, with or without windrowing, followed by burning in situ or in windrows. The objective of this study was to evaluate effects of land clearing at three levels of initial soil wetness on soil physical properties, and on growth and yield of maize and cowpea for a tropical Ultisol in southern Nigeria. One of three main-plot treatments measuring 6 × 40 m was cleared mechanically in February, April and June 1987, respectively. These months corresponded to the dry, moist and wet seasons, giving three different initial levels of soil wetness. Each main plot was divided into six sub-plots, 6 × 5.5 m each, with buffers in between. Maize (Zea mays) and cowpea (Vigna unguiculata) were grown in a sequence with uniform management in all sub-plots. Sub-plots were subjected to two tillage methods, i.e. tilled and no-till systems. Soil wetness at 0.05 m depth at the time of clearing was 0.11, 0.16 and 0.20 m3 m−3 in the February, April and June clearing, respectively. In July 1987 at 0.05 m depth, soil bulk density was significantly higher in the February (by 28%), April (by 35%) and June (by 41%) clearing than that of the forested control. The increase in bulk density in the February clearing compared with the forested control was significant to 0.35 m depth. However, this difference persisted to 0.65 m depth for the April and June clearings. The steady-state infiltration rate measured soon after clearing was 62.0, 2.2 and 3.3 cm h−1 in the February, April and June clearings, respectively, as compared with 200 cm h−1in the forested control. Tillage reduced soil bulk density in the tilled layer and increased the infiltration rate. Bulk density of tilled plots increased with time after cropping began. In contrast. however, bulk density in no-till plots decreased after cropping began. Infiltration rate increased with cropping duration in all plots irrespective of the tillage method. Maize grain yield was not affected by clearing at either level of initial soil wetness during 1987 when yields were low. In 1988, however, grain yields of maize were lower in the February and April clearings than in the June clearings. Cowpea (Vigna unguiculata) yield was also significantly lower in the February-cleared than in the April- or June-cleared plots. Yield observations were, however, too variable to draw valid conclusions. Furthermore, crop yields were also affected by factors other than alterations in soil properties caused by land clearing. The data suggest that dry-season clearing is better for soil physical properties than wet-season clearing. However, the soil surface must be kept covered by the felled biomass to protect soil against high temperatures and intense rains |
