Source: Science for Environmental Policy, 15 November 2017, based on research paper Guimaraes, R.M.L., et al (2017). Opportunities and future directions for visual soil evaluation methods in soil structure research. Soil and Tillage Research,
Soil structure - the spatial arrangement of soil particles and pores (empty spaces) - provides physical habitat for soil organisms, and controls many functions associated with ecosytsem services, for example by storing water and allowing plants to grow. Soil's ability to withstand and recover from stresses (stability and resilience) is also an important aspect of soil structure, as it determines the risk of compaction which inhibits plant growth and other life. Soil degradation in the EU could cost up to 38 billion Euros, according to the EU's Thematic Strategy for Soil Protection. Measures to protect soils and the functions they perform are part of the Seventh Action Programme.
VSE offers a method of assessing the quality of soil structure, which is not static (such as soil texture), but changes due to external influences, including weather, penetration of plant roots and human activity, such as tillage or driving of vehicles. VSE is mainly used by agricultural advisors and farmers to assess and inform soil management techniques. Requiring no special equipment, it is a simple and cheap tool.
Despite the use of manuals and scales in the evaluation process, one limitation of VSE is considered to be subjective interpretation, as compared to assessing the detailed information offered by computed tomography (CT) imaging, for example. Nevertheless, the researchers behind this paper argue that VSE can provide a crucial tool for monitoring soils, and provide an in-depth review of opportunities and future directions for the technique, based on a 2014 workshop attended by scientists from the ISTRO (International Soil Tillage Research Organization).
Many different VSE methods have been proposed. These techniques generally assess the depth of natural and anthropogenic soil layers, the spatial arrangement and size distribution of soil particles, the strength of the soil, its visible porosity, and, sometimes, colour and earthworm population, amongst other variables. Some methods incorporate a soil-quality index, such as the commonly used ‘Visual Soil Assessment’ method. The different features assessed offer information on how well different plants will grow in the soil, its potential as biological habitat and how nutrients will be cycled in the soil. For example, features such as cloddy structure and surface ponding give clues as to how water drains from the soil. Some VSE studies have shown significant correlations between soil structure and crop yield, while others provide useful tools to assess soil recovery after heavy compaction.