Influence of sand gradation on compaction of loess soils

Abstract

Assessing the interaction among soil organic carbon (OC), Atterberg limits and soil texture, especially sand gradation, in maximum bulk density (BDMAX) can improve prediction of soil compactibility. Our objectives were a) to compare the effect of total OC, complexed OC (COC) and silt + clay (S + C) content on maximum bulk density in cultivated (CULT) and uncultivated (UNCULT) soils, and b) to evaluate how useful it may be to group soils according to Atterberg limits and the sand uniform coefficient (CU) in order to improve BDMAX prediction. Sixteen pairs of CULT and UNCULT soils were sampled in the province of San Luis, Argentina. The standard Proctor test was used to determine BDMAX. The BDMAX was related to OC and S + C in both managements. The BDMAX was better explained by S + C in CULT (R2 = 0.52) and by OC in UNCULT (R2 = 0.62). The COC did not improve prediction compared to OC. The plastic limits of soil, also called Atterberg limits, determine a range of soil moisture contents in which soil has a plastic consistency. Consequently, non-plastic soils are those lacking plastic consistency. In all non-plastic CULT soils, BDMAX was not associated with S + C. However, when these soils were grouped according to CU, the BDMAX was linearly related to S + C, both in well-graded sands (R2 = 0.47) and in poorly graded sands (R2 = 0.27). On the other hand, the relationship between BDMAX and S + C in plastic soils was negative and linear (R2 = 0.96). In coarse-textured soils, soils with a higher CU (well-graded sands) were more easily compacted than soils with a lower CU (poorly graded sands). Therefore, according to this model, in CULT soils with the same S + C content, compaction is 8–10% higher for soils with well-graded sands than those with poorly graded sands.