Recognition of the importance and vulnerability of soils has a long history. In the Oeconomicus, one of the earliest known works on economics, Xenophon in 400BC wrote: “To be a successful farmer one must first know the nature of the soil.”
Since then there has been recognition that exploitation of soils without remediation is unsustainable. Figures as diverse as Karl Marx, who said: “Progress in increasing the fertility of the soil for a given time, is progress towards ruining the lasting sources of that fertility” (1867) to Franklin Delano Roosevelt, who said: “A nation that destroys its soil destroys itself” (1937), have recognised the importance of soil.
Significant European and global initiatives to protect soils started with The Council of Europe’s Soil Charter in 1972, the World Soils Charter (FAO 1982) and the World Soils Policy (UNEP 1982) followed. Other related UN policy instruments such as the Convention on Biological Diversity (1992) and the Convention to Combat Desertification (1994) also link to soil conservation.
In 2002 the European Union published a communication ‘Towards a Thematic Strategy on Soil Protection.’ This led to a proposal for a Soil Framework Directive in September 2006, which failed in its first passage through the European Parliament.
In the directive it was recognised that soils are a non-renewable resource, and, indeed, an active, dynamic system. The principles included in the directive included the establishment of a common framework to protect soils, prevention of soil degradation, and restoration in general.
At the same time, an impact assessment analysed threats to soil quality. It identified eight threats including organic matter decline, compaction and biodiversity decline and it estimated potential costs of the order of billions of Euros.
In the UK, no statutory protection exists for soils, although a measure of protection is provided indirectly from other legislation for planning or pollution prevention. The UK government’s Department for Food, Environmental and Rural Affairs, the Environment Agency and the devolved administrations have developed their own strategies for soil protection. These recognise soil as a non-renewable, multifunctional natural resource rather than just a medium for food and fibre production.
Soil properties cannot be considered in isolation and in fact it can have significant impact at many levels. There can be political consequences across different countries’ borders – erosion can damage irrigation, hydro and infrastructure systems, contaminants can affect neighbouring countries.
Within a country the state of soil affects the food security of the area – either resulting in reduced yields, or the necessity for the import of fertilisers which could create a dependency chain.
The soil is very much part of the ecosystem in an area, and forms a key part of the ecosystem services provided from that area. However, in order to understand the any contribution to a system, it is necessary to have both a baseline and a measurement process.
The ecosystem services approach is now influencing most recent policy documents on soils. However, to do this effectively and to understand all the ramifications, the state of soil must be known – measured – and ideally it must be known for a period of time to establish a baseline e.g. in order to say if the soil has degraded, there must have been a previous measurement for comparison.
This article looks at some of the long term indicators and measurements required to give a complete picture of the soil in the UK. It is based on the work at the Centre for Ecology & Hydrology. Many of the techniques have been used in the UK Countryside Survey work.
The Countryside Survey provides scientifically reliable evidence about the state of the British countryside. It is unique in Europe in providing such information at a countrywide level. The most recent survey was completed in 2007 and can be compared with surveys of 1978, 1984, 1990 and 1998.
There are two main elements to Countryside Survey: using satellite data to create a digital map of different land cover across the UK, and field surveys. The field surveys are designed to give a snapshot of the state of Britain by examining a number of critical environmental indicators across nearly 600 sites, chosen to represent all land types and habitats.
As sampling the entire country would be prohibitively expensive, the sampling strategy is based on a statistical approach, which gives a calculated estimate for all the indicators.
A key requirement for the Countryside Survey analysis is that it is quality assured, and it is important to note the difference between this and the peer-review process of scientific literature. Essentially, peer review will justify the methods and analysis, but will not provide calibration, verification or, in some cases, repeatability.
The QA process, using documented procedures and laboratories that are ultimately calibrated to national measurement systems, resolves this verification issue e.g. reference soil samples are used to check measurements and equipment calibration. Use of QA, to accredited standards, rather than peer review means that the measurements from one period to another, or from one geographical region to another, can be compared in confidence and it allows for the back correction of previously analysed samples. This is essential to provide a baseline to the measurement.
One of the indicators used in Countryside Survey is the state of soil. This is defined through a number of measure methods. The remainder of this article gives a summary of the methods – for a complete description of the methods see the referenced Countryside Survey report1.
Analysis methods – Bulk density
This is a measure of the soil’s physical structure – it gives a measure of the pore space in the soil. The pore space is the soil structure that contains the moisture, air and biota. Increased bulk density means reduced macropore volume and microbial biomass e.g. increased soil degradation, thus it is a key measure of soil quality. It is also necessary in the estimation of the soil carbon, as the values are needed to convert from percentage soil organic matter to a per unit volume measurement. The measurement can only be performed on fresh soil, and the calculation is as follows:
Bulk Density = ((Dry weight core (105oC)(g) – stone weight(g))/(Core volume (cm-3) – stone volume(cm-3))
Measurement of bulk density and comparison of previous measurements allows an estimate of the degree of change of compactness.
Soil organic matter and carbon content
Soil organic matter and carbon content is an essential measurement for the determination of soil carbon content. This measurement is necessary for policy validation and verification and relates not only to the Kyoto protocol, but also the Common Agricultural Policy sections relating to protecting soil and environmental health. It is one of the key indicators of soil quality.
Soil organic matter content can be relatively easily measured by using the Loss-on-ignition method. Here the sample is air-dried at 105oC before being combusted at 375o C for 16 hours. The percentage difference between the pre and post combusted sample is the Loss-on-ignition. Complementary to this method, CEH used an elemental analyser to measure the total amount of carbon in a sample; however, this does not allow the separation of value for the purely organic carbon.
Soil pH is one of the most common soil measurements. The measure is relevant to a number of policy areas e.g. EU and national policy on acidification reduction, and allows input into the Long Range Transboundary Air Pollution protocols. Within Countryside Survey it has been measured since 1978, and is a key verification of the effectiveness of policies or the relocation of industry to other countries. The acidity of the soil will also significantly affect the type of vegetation that can flourish. Measurement of pH in water is a well established technique. However, there are difficulties with it related to the ionic strength of the suspension – particularly the dilution effect where the pH value depends on the soil:water ratio. Measurements in a solution of 0.01M CaCl2 provide more consistent results. In the Countryside Survey, both methods were used: deionised water is added to a sample of dried soil to give a ratio of 1:2.5 by weight. After the pH is measured, the CaCl2 is added and pH re-measured.
Weakly bound phosphorus is a useful measure of the fertility of agricultural soils. It could be related to the policies and soil quality measurements required under the Common Agricultural Policy. However, the measurement of phosphorus is not straightforward across a wide range of soils as exists with the Countryside Survey.
There are several different extractants that can be used. For agricultural soils Olsen’s method is widely used, however this is not so suitable for semi-natural and woodland ecosystems. Other methods do not give results that are directly comparable, so a mixture of methods across different soil types would be hard to compare and give a national picture. Olsen’s method was used as even though it is not considered optimal for particular soil types, it is the optimum for the range of soils considered.
Mineralisable and total nitrogen
There are different measures of soil nitrogen status, linked to plant species composition across a range of habitats, geographical and soil types. The measurement of mineralisable nitrogen links different measures of soil nitrogen with plant species composition. Policy decisions regarding emissions of nitrogen compounds are usually informed by a critical load approach e.g. a quantitative estimate of an exposure to one or more pollutants, below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge (UNECE 2004).
The current established methods of calculating the critical load are limited because they do not allow a timescale to be identified.
Soil nitrogen content influences plant species preferences. These can be indicated using a fertility score, but a direct link to soil nutrient measurements is difficult. In the analysis here the fertility score is linked to combined measurements of Carbon/nitrogen ratio with soil/habitat classification, pH, the amount of extractable nitrate and ammonium, and the net mineralisable/immobilisation of nitrate and ammonium. The measurements define a fraction of nitrogen available to plants, will link soil models with biodiversity responses, and will allow the separation of nitrogen pollution compared to other drivers of biodiversity change. The measurements would also allow assessment of the nutrient cycling process as may be required for the forthcoming UK National Ecosystem Assessment2.
The method involves misting the sample with a solution that represents UK rain, incubating the sample for four weeks and extracting a subsample using 1 molar potassium chloride.
Requirements for biological indicators are set out at national levels. In the UK this is done through the Soil Action Plan, and the Soil Indicators Consortium is developing the indicators from monitoring, to determine how the UK is meeting national policy requirements and complying with international laws and agreements for protecting the environment.
The interactions of soil biota are critical to soil functions and are a key component of the ecosystem. Thus any national measurement of the quality of soil must include a biodiversity component. However, little is known about soil biodiversity and how exactly it interacts with soil functions – although it drives and contributes to many, such as carbon storage and nutrient supply – and what the baseline should be. This implies robust analysis of relevant soil biodiversity may be problematic. However, in the UK, measurements of soil invertebrates were made in the 2000 and 2007 Countryside Survey: this allows for a comparison of the change.
The method of extracting the invertebrates from the soil uses a dry Tullgren extraction. Over a period of five days, a light source is used to drive soil fauna from the samples into collection bottles. Then the invertebrates are categorised to major taxa and counted.
Metals have significant ecological effects in both aquatic and terrestrial ecosystems. Indeed, there are a number of policy initiatives at national and European wide scales that require monitoring, modelling and mitigation of the deposition of metals.
Much of this is in support of the Long Range Transboundary Air Pollution Protocol on heavy metals. In the UK, this has prompted the funding, through the Department for Environment, Food and Rural Affairs, of critical load modelling and monitoring through various national networks such as the Metal Deposition network.
The deposition of metals directly affects the quality of soil, and the problem is that once in the soil it is difficult to remove heavy metal contamination. Therefore it is necessary to understand the rate of heavy metal accumulation in the soil to assist in soil management. What is unique with this particular survey is that concentrations of heavy metals can be linked to biota and vegetation.
The methods used to examine the soil samples were inductively coupled plasma mass spectrometry (ICP-MS) and inductively couple plasma optical emission spectrometry (ICP-OES), of which ICP-MS has better sensitivity and reliability, and provided data for a greater range of metals.
The most abundant and diverse group of microbes in the soil is bacteria. They underpin the food web and are critical to nutrient cycling. However, little is known about their exact classification, their functional roles and the interactions in the ecosystem, or even about the drivers that alter their state.
Most work has been done with culturing of samples, but recent advances in molecular methods have meant that extraction of nucleic acids from samples will give a clear picture of the extent of microbial diversity.
In the CEH Countryside Survey work, the method used was terminal restriction fragment length polymorphism (tRFLP). The work has shown that most soil bacteria belong to one of two lineages: alphaproteobacterial, whose function associations include nitrogen fixation and acidobacterial, whose function in soil is unknown. It has been suggested the ratio of these two main groups will be related to the nutrient status of the soil. Therefore, it is necessary to measure bacterial biodiversity to understand the effects on soil functions and how that may be altered by shifts in composition.
The above methods all use soil sampling as a prelude to analysis. Every effort is made through multiple sampling at multiple sites to provide a picture of the subsurface and spatial properties of the soils. In addition to these, independent methods that do not depend on soil sampling could give complementary information, particularly relating to texture and moisture.
Specifically, recent developments in georeferenced electromagnetic induction (EMI) mean that non-invasive, spatially exhaustive information can be obtained about soil properties at a landscape level. The EMI response signal can be shown to be related to soil texture and soil moisture, and to the hierarchy of plant communities. Although this technique was not used in the 2007 Countryside Survey, this non-invasive measurement will provide a valuable tool for the analysis of soils.
The measurements above represent the application of a diversity of knowledge and skills, and valuable information is derived from the individual measurements. It allows policy questions such as ‘Is there robust evidence of a decline in soil biodiversity?’ or ‘What is the soil carbon stock?’ to be answered, using quality assured evidence.
But, as discussed at the beginning of this article, what is equally important is the combination of the data to give a holistic view of the state of the soils, allowing the understanding of trends and principles. This understanding can be incorporated into other systems – such as ecosystem modelling – to reduce uncertainties and further improve understanding.
CEH uses the analyses to feed into a number of models at different levels e.g. WHAM calculates chemical speciation of elements in waters and soils, and has been used to calculate soil critical limits for heavy metals; JULES (Joint UK Land Environment Simulator) which models surface energy balance, soil heat and moisture status and has been used to assess uncertainties in climate change predictions.
It is through these types of models, the long term expertise in collecting data, understanding trends and principles, and the integration of disparate datasets that organisations such as CEH can provide the evidence that policymakers and land managers need to be able to resolve their questions, to verify policies and to help formulate new policies.
This article is a summary of the work done by many CEH scientists. The methods described here are only some of the techniques in use at CEH for soils measurement and analysis – for example soil moisture techniques have not been mentioned.
Most of the methods described have been applied by the scientists working on the Countryside Survey. The full Countryside Survey reports with full details of the methods, protocols and results can be found at http://www.countrysidesurvey.org.uk/
But the essence of the article is that these methods, with appropriate Quality Assurance, with appropriate periodic, long term monitoring, can be used together to give a cost effective picture of the soil quality across a nation, and provide a benchmark for the quality of soil that we need to have to protect our ecosystems, our food supply security and our soils.
We may not be able to prevent soil degradation, but through measurement surveys and methods we can monitor and try to provide the evidence for policies and directives that will minimise it.
1 CS Technical Report 3/07 Soils Manual: http://www.countrysidesurvey.org.uk/pdf/reports2007/CS_UK_2007_TR3.pdf 2 http://uknea.unep-wcmc.org/ 3 ‘Determining Soil-Tree-Grass relationships in a California Oak Savanna Using Eco-Geophysics’, D A Robinson et al, www.vadosezonejournal.org, v 9, n 1, Feb 2010.
Published: 10th Dec 2010 in AWE International