Visual Soil -Field Assessment Tool (VS-FAST)

The VS-FAST methodology is based on the Visual Soil Assessment (VSA) approach. Like the VSA approach, the VS-FAST methodology describes and evaluates the morphological condition of soils in the field. While VSA has been used mainly to identify soil constraints for general or specific land uses and adapted management techniques, VS-FAST has been specifically developed to assess changes in soil conditions over time, to assess the nature and causes of and quantify the extent, severity and impact of land degradation.

VS-FAST places emphasis on the assessment, both qualitative and quantitative, of soil physical condition conducted during field visits. Where the focus of VSA is on soil visual descriptors (such as soil depth, structure, texture, colour, eathworms, roots), VS-FAST also includes simple soil measurements, in relation to slaking and dispersion, soil pH, water infiltration and the labile fraction of soil carbon.

Slaking and dispersion refer to the stability of soil aggregates when water is added to the soil and, while commonly laboratory tested, VS-FAST includes a field procedure developed by Field et al. (1997). The differentiation between slaking and dispersion is important, since the products of slaking can reconstitute larger aggregates, whereas dispersion into primary particles is irreversible and results in undesirable, massive structure.

The soil pH measures the concentration of hydrogen ions in the soil solution. Extreme values, expressed in either strongly acidic or strongly alkaline soils, are associated with negative impacts on soil fertility and structural stability. Soil pH can be measured in the field with acceptable accuracy, using a field test kit and the Universal indicator.

Water infiltration, measured through the characteristic ‘saturated hydraulic conductivity’,  impacts soil productivity, soil moisture regime and  soil erodibility. It is best measured in the field using a 100 (length) x 100 mm (diameter) ring (in metal or PVC with sharpened tip), a container holding 50 mm of water and a watch.

Most functions associated with soil quality are strongly influenced by soil organic matter. Loss of the small portion termed ‘active (or labile) organic carbon’ has  deleterious effects on soil aggregate stability, bulk density, water infiltration, nutrient levels, and risk of erosion. VS-FAST promotes the use of a field kit method, developed by Weil et al. (2003), based on the determination of carbon oxidised in a dilute solution of potassium permanganate (KMnO4).

Guidelines are provided for scoring each of these characteristics and weighting/integrating the scorings into two measures of soil quality, one based on visual observations  and the other based on the soil measurements. For recording the VS-FAST information and data, score-cards are included in the methodology. The scoring system is based on the scoring procedure of Sheperd (2000), also used in the VSA tools.