Sugarcane straw effects on soil compaction susceptibility

Abstract

Crop residues may buffer part of compressive stress applied over the soil surface. Thus, indiscriminate crop residue removal would magnify soil compaction in highly mechanized cultivation systems, such as sugarcane in Brazil. Nevertheless, the effect of different amounts of sugarcane crop residue (straw) on soil compaction mitigation still needs to be tested. We conducted a laboratory study to quantify the effects of sugarcane straw rates on soil compaction indicators in response to external applied stress. The experiment involved remolded soil cores with four initial bulk densities: 1.25, 1.35, 1.45 and 1.55 Mg m−3, and five amounts of straw were left on soil surface, equivalent to 0, 5, 10, 15 and 20 Mg ha−1. The soil cores were subjected to uniaxial compression test and seven successive vertical static loads (25, 50, 100, 200, 400, 800 and 1600 kPa) were applied over the soil surface using a pneumatic consolidometer. Soil displacement was measured after each applied load. Based on these values, apparent precompression stress (σp), apparent compression index (CI) and final bulk density were calculated as indications of soil deformation. Because, soil inherent properties should not change, we call the changes due to straw layer addition "apparent" in this study. Our results revealed that large amounts (>10 Mg ha−1) of straw increased subtly apparent soil load bearing capacity (i.e., increased up to 15 kPa in apparent σp; 7%). The bulk density change rate (soil deformation) decreased due to presence of straw under low stress (<100 kPa), and increased under high stress (>400 kPa) levels in loose soil. On the other hand, compaction was slightly reduced in compacted soil, with corresponding reduction on apparent CI values. Nevertheless, despite this buffer effect provided straw, we noticed that increases in surface straw amount are likely insufficient to reduce the risk of compaction (little gain in apparent load-bearing capacity), but a reduction of about 5% on the soil deformation (as apparent σp is overcome) could occurs in compacted soils. We advocate that benefits of leaving sugarcane straw covering the soil go beyond the direct “damper” effect, because straw is source of carbon, which is fundamental for enhancing soil structure stability and biological activity, and thus, reducing soil compaction risks or even alleviating soil compaction restrictions to plant growth. Finally, we encourage future studies to test the straw effects under machinery traffic in field conditions, which could involve the additional effect of shear stress over the tyre-straw-soil interface.