By Victoria Butler: September 2009
Supervisors: Dr Nick Voulvoulis & Dr Martin Head
This project aims to classify soil types by the parameters set in the OPAL Soil and Earthworm guide, and assess the guide's suitability.
Soil is a non-renewable resource, which faces increasing levels of degradation. The extent of soil degradation in the UK is not yet fully understood, due to limited data. There are several soil classification systems in use in England, for example, the Soil Classification of England and Wales and the British Soil Classification System.
However, none of these are concerned specifically with the role of soil in supporting biodiversity and how this is impacted by human activities. Existing classification systems usually rely on the results of soil surveys and laboratory analyses of soil samples, the cost of which can limit the amount of data collected.
The OPAL Soil and Earthworm Survey was designed so that soil sampling could be carried out by people with no prior knowledge or experience. This allows a large number of samples to be taken across England, whilst at the same time raising awareness of the importance of soil protection.
- Assess how data on soil and earthworms collected by the public can be used to provide valuable information about the condition of soils in England.
- Develop a soil classification system using indicators from the OPAL survey.
- Evaluate the effectiveness of the classification system as a screening tool to identify sites where further investigation into soil conditions should be prioritised.
Assess OPAL survey indicators
Indicators of soil properties can be physical, chemical or biological. Ideal indicators are simple and inexpensive to obtain and interpret. The OPAL survey results provide indicators of soil characteristics such as water retention capacity, soil particle size, soil pH, contamination and compaction.
Select the criteria by which the samples are classified
There are many potential criteria by which soils can be classified. It was decided that the most useful classification system, which makes best use of the indicators from the OPAL survey, is one that gives an indication of the soil’s ability to support life, along with its level of human impact. These two criteria are referred to as ASL (Ability to Support Life) and AIP (Anthropogenic Impact and Pollution).
Develop a scoring system
Each relevant indicator was given a positive, negative or neutral score, depending on the extent to which the soil fulfils the function of supporting life or how severely it has been impacted by human activity. Although an individual observation or test result might indicate more than one possible soil characteristic, each sample is an aggregation of many indicators. This allows an evaluation to be made of the soil’s condition as a whole.
It is likely that the total scores for both ASL and AIP are influenced by factors such as the weather, time of year, type of land use and underlying geology. Therefore, adjustment factors were added to samples which achieved a high score for one of the criteria, but scored poorly in the other. The adjustment factors are used to exclude as far as possible any variation due to variables that are not directly related to soil properties. They are also used to add weight to a score if non-soil variables increase the level of certainty.
Group the samples according to their scores
The transformation of the scores into soil types is a two-stage process, which requires the calculation of the midpoint between the maximum and minimum possible scores. Each set of OPAL survey results was transformed into two scores and the samples grouped according to their scores. Next, the samples falling into the 'medium' group were split into two. This was achieved by applying the adjustment factors to the respective scores, then summing the total scores. The maximum and minimum possible scores were calculated and the midpoint determined.
Samples scoring higher than the midpoint were classed as Type B and those scoring below, Type C. The “high” group became Type A and the “low” group became Type D.
Assess the effectiveness of the classification system
- Soil types were compared with known concentrations of mercury (150 samples)
- To determine whether soil properties had an effect on earthworm numbers, scores for earthworms found were removed from the ASL scores and the two were compared
- Proportions of different soil types were compared for London and England overall
- Distribution of soil types was mapped using GIS.
The classification system was applied to the results of 2,708 samples. The number of samples falling into each soil type is as follows:
Type A – 1779
Type B – 591
Type C – 255
Type D – 83
- No relationship was found between mercury concentrations and ASL score, AIP score or soil type
- No significant relationship was found between earthworm numbers and ASL score
- There was a marked increase in the proportion of unfavourable soil types in London and a reduction in Type A soils, compared to England overall
- Maps of soil type distributions did not show any discernable patterns
From the mercury comparison test conducted, the classification system appears to be limited as a screening tool. As a tool for comparing the state of soils over a large area, the classification system was relatively successful. Further examination of geographical distributions of soil types may give valuable information on levels of soil degradation in different regions or cities. In order to map soil types effectively, a greater number of samples is required.
Download the full project summary (PDF, 339KB)