Critical Moisture Content, Bulk Density Relationships and Compaction of Cultivated and Uncultivated Soils in the Humid Tropics

Soil compaction affects soil fertility through increasing bulk density and soil strength. It also decreases infiltration rate, total porosity and amount of water stored in the root zone for crop use. In this study, we evaluated the optimum moisture contents (OMC) in relation to maximum dry density (MDD) and compaction of cultivated and uncultivated soils. The study was carried out on four land use types viz: uncultivated: Velvet tamarind (Dialium quineese), rubber plantation (Hevea brasiliensis) and cultivated: 5-year fallow and 10-year continuous cultivated soil to maize crop. Proctor test for the maximum dry density-moisture content relationship was carried out, including some hydraulic and structural properties of the soil, and their effect soil compaction. Results showed that optimum moisture content (OMC) for compaction relate to the maximum dry density (MDD). In which case, dry density increased with water content to a maximum and decrease as moisture content increased above the optimum. Soil organic matter (SOM) content and particle size distribution highly affected the MDD and OMC. The MDD and OMC were: 1.92 g cm-3 and 10.4%, 1.95 g cm-3 and 11.2%, 1.91 g cm-3 and 12.3%, and 1.87 g cm-3 and 12.8% for velvet tamarind, 5-year fallow, CC and rubber plantation soils respectively, at 0-15 cm depth. Changes in field bulk densities at similar depths were in the order of velvet tamarind < rubber < 5-year fallow < CC. There were highly significant (p < 0.01) relationships between MDD, total porosity, Ksat and SOM and negative relationships between these parameters and OMC. Thus, continuous cultivation increased MDD and reduced OMC for compaction. Two- season fallow periods with legume could improve soil hydraulic properties and maintain the MDD of sandy soils at minimum.