Visual assessment of soil structural quality across soil textures and compaction levels – Part I: Examination of intact soil cores

Abstract

Soil structural degradation has become very common and leads to a serious decline in soil health. Improving or maintaining soil structure is key to build resilience against drought and flooding, and thus contributes to assuring food and water security. Quantification of soil structural quality is usually accomplished by traditional laboratory-based methods, which are time-, labour- and money-consuming. Visual soil evaluation and examination methods comprise rapid and simple tests that offer a numeric semi-quantitative assessment of soil structure. The common criteria are aggregate size and shape, ease of break or rupture resistance, and inter- or intra-aggregate porosity. In this study, the CoreVESS method, a variant of the Visual Evaluation of Soil Structure (VESS) method originally developed for topsoil assessment, was applied on 250 cm3 undisturbed soil core samples. Resulting soil quality scores (Sq, ranging between 1, good, and 5, poor) were compared with soil quality indicators (SQi) derived from traditional quantitative methods of soil structure analysis. Tested SQi’s included penetration resistance, bulk density, soil hydraulic properties, soil organic carbon and texture (percentage of clay, silt and sand). The objective of the study was to assess and quantify the state of soil structural degradation induced by agricultural operations using a suite of methods of analysis at a regional scale, with a special focus on CoreVESS. Soil core samples were taken in Belgium from 42 cropped fields, at two positions (headland and in-field zone) and from three layers, notably the ploughed topsoil (TOP, 10–20 cm), the compacted subsoil (CSUB, 30–40 cm) and the deeper subsoil (SUB, 60–70 cm), totalling the sample horizons to 252. Test and sampling sites comprised all major soil texture classes within the Belgian soil textural triangle and varied from sand to heavy clay. In-field positions (IN-FIELD) showed significantly better coreVESS-based Sq scores as compared to headland positions (HEAD), with the CSUB layers always exhibiting significantly lower quality than the other two layers. Laboratory-derived soil quality indicator (SQi) values portrayed the same trend, with CSUB layers always indicating the poorest soil quality. Significant differences in SQi’s were also found between soils of acceptable (Sq ≤ 3) and degraded (Sq > 3) structure. Likewise, grouping soils into the same categories resulted in significant differences in the soil water retention and hydraulic conductivity curves in their wet ranges. There were good significant relationships between the SQi values and CoreVESS-based Sq scores, with for example Pearson R correlation coefficients of 0.64 for both penetration resistance and bulk density. Also, Sq scores related well with an SQi-based soil quality index (SQI) value averaged per layer and texture class (Pearson R of 0.60). The study confirmed that agricultural operations and their intensity clearly affect soil structural quality across various soil textures, with CoreVESS being as responsive to variation in soil structure quality as traditional quantitative methods, even when applied in the subsoil. VESS methods thus have a potential for monitoring soil structural quality over a variety of textures in a rapid, intuitive, practical and cheap way, as an alternative for or being complementary to more expensive labour-intensive traditional quantitative methods.