Mulch and soil organic carbon content and their relationship with the maximum soil density obtained in the proctor test

The susceptibility of soils to compaction, measured by the Proctor test, decreases with increasing soil organic matter (SOM) content. For a given energy level, with increasing SOM contents the maximum obtained density decreases and the corresponding critical moisture content increases. Due to its low density, elasticity and deformation susceptibility, straw is potentially able to dissipate applied loads. This study was conducted to evaluate the SOM effect on the soil compaction curve and to evaluate the ability that mulch has to absorb compactive energy in the Proctor test. The compaction test was carried out using soil surface samples (0 to 0.05 m) of a Hapludalf, with sandy loam texture at its soil surface, and an Oxisol, with clayey texture at its soil surface, both with variations in the SOM content. The maximum density, the critical moisture content, the liquid and plastic limits, and the soil organic carbon content were determined. A second test was performed to evaluate the ability of mulch to absorb compactive energy, by compacting Hapludalf samples with the presence of a straw layer on the soil surface, inside a Proctor cylinder, at amounts corresponding to 2, 4, 8 and 12 Mg ha(-1). SOM accumulation reduced the maximum density and increased the critical moisture content, suggesting an increased resistance to soil compaction. In the Proctor test the straw on the soil surface dissipated up to 30 % of the compactive energy and reduced the bulk density, confirming the hypothesis that mulch can absorb part of the compactive energy caused by machine traffic and by animals.