Strength, swelling and compressibility of unsaturated sugarcane soils

Large-scale sugarcane plantation is characterized by successive and disordered traffic of heavy machinery and equipment over the cycle, which can cause damage to the soil structure. As the stress induced by traffic exceeds the soil load-bearing capacity, it results in soil compaction. However, the mechanisms and key variables that govern the compressive characteristics in agricultural unsaturated soils to support those applied loads without plastic deformations (compaction) is not fully understood. To investigate this process, we measured the strength, swelling and compressibility characteristics of unsaturated soils with contrasting texture from sugarcane fields. Undisturbed soil cores were sampled in nine cultivated sugarcane fields, equilibrated at a matric potential of -100 hPa and subjected to confined uniaxial compression tests. Precompression stress (sigma(p)), compression (lambda) and swelling (kappa) indexes were extracted from the soil compression curves. Soil organic C (SOC), degree of compactness, total porosity and particle size fractions were also measured and correlated to the compressive parameters sigma(p), lambda and kappa. Principal components, correlation and multiple regression models were used to the data analyses. Overall, reductions in soil strength and increases in the soil compressibility and elasticity occurred with the increase in SOC, whereas increases in the degree of compactness resulted in higher soil strength and lower soil compressibility and elasticity. Furthermore, our results revealed that silt + clay content considerably increased soil compressibility. In combined scenarios for a range of soil texture, SOC and degree of compactness at matric potential at around field capacity (i.e. -100 hPa), the maximum sigma(p) observed was approximately 200 kPa, which is a much lower load-bearing capacity than the estimated stress for some trucks and trailers used in sugarcane harvest (similar to 600-800 kPa). Our study suggests that soil compaction is eminent in sugarcane fields at matric potential close to field capacity, and can be aggravated in clayey soils with high SOC content and low degree of compactness. Therefore, urgent strategies of traffic control are necessary to prevent or mitigate soil compaction induced by mechanized sugarcane operations.