Contents lists available at ScienceDirect Soil & Tillage Research journal homepage: www.elsevier.com/locate/still Influence of sand gradation on compaction of loess soils Juan de Dios Herrero a, ⁎, Juan Cruz Colazo a , Daniel Buschiazzo b , Juan Galantini c a Instituto Nacional de Tecnología Agropecuaria. San Luis, Argentina b INCITAP (Instituto de Ciencias de la Tierra y Ambientales de La Pampa, CONICET), Argentina c CERZOS (UNS-CONICET)-Departamento de Agronomía, Universidad Nacional del Sur, Argentina ARTICLEINFO Keywords: Bulk density Texture Organic carbon ABSTRACT Assessing the interaction among soil organic carbon (OC), Atterberg limits and soil texture, especially sand gradation, in maximum bulk density (BD MAX ) 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 (C U ) in order to improve BD MAX 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 BD MAX . The BD MAX was related to OC and S + C in both managements. The BD MAX was better explained by S + C in CULT (R 2 = 0.52) and by OC in UNCULT (R 2 = 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, BD MAX was not associated with S + C. However, when these soils were grouped according to C U , the BD MAX was linearly related to S + C, both in well-graded sands (R 2 = 0.47) and in poorly graded sands (R 2 = 0.27). On the other hand, the relationship between BD MAX and S + C in plastic soils was negative and linear (R 2 = 0.96). In coarse-textured soils, soils with a higher C U (well- graded sands) were more easily compacted than soils with a lower C U (poorly graded sands). Therefore, ac- cording 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. 1. Introduction Soil compaction is one of the major soil degradation processes worldwide (FAO, 2015). Excessive soil compaction reduces air and water movement in the soil profile, decreases nutrient availability for plants and limits the root growth of crops, thereby affecting crop yields (Hanza and Anderson, 2005; Chamen et al., 2015). Maximum bulk density (BD MAX ), a parameter derived from the Proctor test (Proctor, 1933), has been widely used to study soil compactibility and the factors that affect it in soils under different managements (Zhao et al., 2008; Blanco-Canqui et al., 2009). Soil compactibility is related to different soil properties; one of the relationships most extensively reviewed has been that with organic carbon (OC) (Soane, 1990; Hanza and Anderson, 2005; Gregory et al., 2015). Generally, BD MAX decreases when OC is higher (Aragón et al. (2000); Díaz-Zorita and Grosso, 2000), as a result of the stability and strength of aggregates (Ball et al., 2000). In order to find optimal or desirable OC values in relation to physical properties, Dexter et al. (2008) found that, in soils under pastures with a high OC content, bulk density (BD) was more closely related to clay than to OC content; however, in cultivated soils with low OC values, they reported the opposite trend. In this sense, these authors defined the complexed OC (COC), finding the maximum correlation with BD when the OC:clay ratio was 1:10. However, Johannes et al. (2017) did not find an op- timum value of BD MAX and COC correlation; instead, they verified the maximum correlation with BD when the total OC was considered. These authors attributed the differences to the choice of soils with a wide range of OC and a good structure. Probably, this behavior would be different in sandy soils with low OC content and it should be verified. With respect to the relationship of BD MAX and soil texture, contra- dictory results can be found in the literature. Aragón et al. (2000) re- ported that BD MAX decreased with higher silt content in Mollisols of Argentina. Reichert et al. (2009) found that BD MAX decreased according to silt plus clay (S + C) content in Oxisols and Alfisols. In a wide tex- tural range, Nhantumbo and Cambule (2006) reported a quadratic re- lationship between BD MAX and S + C, increasing up to 40% of S + C, https://doi.org/10.1016/j.still.2019.104414 Received 18 March 2019; Received in revised form 3 September 2019; Accepted 6 September 2019 ⁎ Corresponding author. E-mail address: dediosherrero.juan@inta.gob.ar (J. de Dios Herrero). Soil & Tillage Research 196 (2020) 104414 0167-1987/ © 2019 Elsevier B.V. All rights reserved. T