Mechanical Deformation and Pore Functioning of Tropical No-till Soils.

Soil compaction by wheeling affects the combined effect of soil aeration, moisture and temperature on plant growth and development. This study was conducted to evaluate mechanical properties and air and water permeability of two tropical soils under different uses. Two loading times (600 and 7200s) did not affect the pre-consolidation stress or load-bearing capacity (σ_p) of Oxisols and Alfisols in four soil layers (0.00-0.07, 0.10-0.15, 0.25-0.30 and 0.40-0.45 m). The σp was increased under no-till soil compared either with native forest or grassland, in three surface layers due to farm machinery traffic, and even deeper soil layers were affected by wheeling on no-till soil. Stress applied by wheeling by tractor and grain harvester was greater than σ_p and although mulch was able to reduce stress transmission to deeper soil layers, it was insufficient to avoid further compaction. Traffic affected soil properties in deeper layers on clayey Oxisols, than for the sandier Alfisols, which demonstrates a relation between soil granulometry and stress transmission in the soil. Air permeability (K_a) was greatest for the Oxisols under native forest, particularly in the uppermost soil layer, where soil drying did not increase K_a, whereas no-till soil had low K_a and increased when decreasing matric potential. The highest saturated hydraulic conductivity was observed for Oxisols under native forest (754 mm h^-1) compared to less than 3 mm h^-1 for no-till soil, in the uppermost soil layer. These data demonstrate that: (a) besides the formation of a “’no-till pan”, subsurface compaction is also a concern in no-till soils, (b) no-till soil might be significantly affected by soil compaction even when with high degree of compactness, and (c) proper soil managements systems must be adopted to allow adequate soil functioning and maintain ecological properties related to water and air flow in the soil.