Contrasted effects of no-till on bulk density of soil and mechanical resistance

Compaction caused by traffic, results in degradation of soil structure with possible agronomic and environmental consequences. On the other hand, no-till systems have a great impact on soil structure and can improve their mechanical resistance. This property is described by compression curves using two parameters: the precompression stress P-c and the compression index C-c. Both mainly depend on bulk density, water content and soil texture. We aim to study the effect of farming practices on bulk density and mechanical resistance and also on the relation between those two properties in order to define diagnostic tools on the lasting effect of practices. Unsaturated soils were used, since cohesion is mainly attributed to capillary forces, themselves structure-dependent. Equilibrium states of soil structure corresponding to a fixed matric potential value, are then compared at various bulk densities. Pedo-tranfer functions (PTF) were defined by the relationship between P-c, C-c and the initial void ratio (e(0)). Soils originated from two long-term experimental sites: Versailles and Boigneville (France), where till and no-till systems are being compared. PTF have been established from remolded samples taken in till systems using a large range of eo values. The effect of practices has been measured through mechanical properties of undisturbed soil cores. Uniaxial compression tests were performed in laboratory in drained conditions. In till systems, the global shape of compression curves varies for remolded cores from an S-shaped model to a rounded shape when e(0) decreases, followed by a bi-linear one. C-c appeared simply proportional to e(0) in all soils due to the progressive water saturation of soil core at inflexion point, when e(0) decreases. For intact cores coming from till systems, this PTF should reveal heterogeneities and variability in soil structure. According to this PTT, there is no effect of practices in field. On remolded samples and till system, Pc was mostly constant in Versailles while P-c did strongly increase when e(0) decreased in Boigneville, due to progressive saturation of porosity. Effects of no-till were contrasted among soils and shape of curves. In Boigneville, mechanical resistance deduced from bi-linear curves increased as it decreased with S-shape curves and lead to a global increase in e(0) values. The opposite was observed in Versailles soils probably due to the opposite change induced by no-till in soil-water-air interaction among soils. The original approach developed is therefore relevant to study other soil, climates, land uses and plants. (C) 2010 Elsevier B.V. All rights reserved.