The numeric visual evaluation of subsoil structure (SubVESS) under agricultural production

Abstract

Subsoil degradation in agriculture is an increasing problem worldwide, particularly due to compaction caused by heavy machinery. Here, we describe a numeric assessment of subsoil structural quality in relation to soil as a crop growth medium and illustrate its utility with results from compaction experiments and from fields under minimum tillage. The scoring scheme resembles the topsoil visual evaluation of soil structure (VESS) (Guimarães et al., 2011) with more emphasis on examination of the profile wall and of soil fragments. The focus is on identification and evaluation of the anthropic ‘transition layer’ immediately below the topsoil, usually >30cm depth. Layers of contrasting hardness and colour were identified and the overall subsoil quality of each layer was scored from separate, sequential assessments of soil mottling, soil strength, visible soil porosity, the pattern and depth of root penetration and aggregate size and shape using a colour diagnostic flowchart. Use of the method enabled identification of extent and severity of compact transition layers in both well-drained and imperfectly drained soils. Porosity and strength assessments were particularly relevant. Reference soils under forest or long-term grassland helped to distinguish whether subsoil structural quality resulted from the natural soil composition or from degradation by land management. The derived scores may be used to judge the requirement for amelioration by subsoil loosening by mechanical inputs (e.g. deep tillage) and/or natural processes (e.g. shrinkage crack formation). The method was also used to identify differences in subsoil structural quality within fields associated with field traffic levels (Oxisol in Brazil) and with moisture status (Luvisol in France). The focus of SubVESS on structure rather than on texture may not permit recognition of effects such as low water holding capacity that influence agronomic potential. In such cases the more comprehensive evaluation of overall agronomic potential by methods such as the ‘profil cultural’ is required.