Soil quality indicators

Soil quality is defined as the ability of soil to perform a specific function within a managed or natural ecosystem that is essential to people and the environment.

Our soil scientists use a combination of chemical and biological indicators to try and establish whether the application of waste to land has a harmful effect on soil quality.

The indicators used by our scientists are:

Soil acidity (pH)

Soils have a wide range of pH values. These levels influence physical, chemical and biological soil processes.

Soil acidification is a natural process causing soil to become more acidic. This process usually takes hundreds, or even thousands, of years to occur; however, atmospheric deposition of acidic compounds from burning fossil fuels accelerates this process enormously. Most Scottish soils are naturally acidic and have a lower capacity to buffer any additional acidity.

Agricultural soils are therefore often limed regularly to keep soil pH in the most favourable status for plant growth. This also limits leaching of pollutants to ground water. However, atmospheric deposition can affect natural habitats making them more acidic and affecting important soil functions.

Carbon

Soil organic matter consists of all organic material derived from former living things such as plant litter, dead roots and dead soil organisms, as well all organic products obtained from the decomposition process. Organic carbon is the carbon part of soil organic matter and makes up about 50% of soil organic matter.

Organic matter influences a wide range of soil properties and increasing the organic matter content of soil is regarded as ‘a good thing’ because of its influence on many soil functions, in particular from a climate change point of view, as carbon is stored in the soil rather than in the atmosphere. Changing land use or land management practices may cause soil to release carbon into the environment, either in the form of greenhouse gases or as dissolved or particulate carbon into water courses.

Total nitrogen and carbon to nitrogen ratio

Nitrogen is an important nutrient that is essential to the reproduction and growth of all organisms. Nitrogen has been applied to soil for many years to enhance agricultural production. However, the loss of nitrogen from agricultural soils is of great environmental concern, either via leaching (usually nitrate) leading to water quality problems, or via gaseous emissions (ammonia, nitric and nitrous oxide) which can have knock-on effects on atmospheric pollution and the greenhouse effect.

In nitrogen-poor semi-natural ecosystems, elevated soil nitrogen can result in a loss of biodiversity and increased dominance of non-indigenous or exotic species.

Most soil nitrogen is contained within the soil organic matter and only a very small fraction is plant available (mineral) nitrogen. The relative concentration of carbon to nitrogen is a good soil quality indicator as this will determine how any mineral nitrogen present in the soil will be utilised. A high carbon to nitrogen ratio will result in any mineral nitrogen available in the soil (either through mineralisation of organic matter or by addition of nitrogen fertilisers) being locked up in the soil biomass by soil micro-organisms rather than being available for plants – potentially resulting in crop failure due to lack of available nitrogen. A low carbon to nitrogen ratio may mean there is an excess of available nitrogen in the soil which can be leached to water courses or emitted as nitrous oxide, thus harming the wider environment.

Extractable phosphorous

Phosphorous is an important plant nutrient and is essential in the process of photosynthesis (the process by which plants use light to generate sugar), However, only a fraction of all phosphorous in the soil is available to plants – and it is this fraction that is measured.

Much of the phosphorous added to soil to promote plant growth can be locked up in the soil and over time substantial amounts can build-up. Subsequent erosion of soil particles with phosphorous attached can result in significant diffuse pollution problems if they make their way into watercourses.

Some organic wastes applied to land can contain very high amounts of phosphorous, e.g. mushroom compost and sewage sludge.

Extractable potassium and magnesium

Potassium and magnesium are essential nutrients for plant growth and low amounts will limit plant growth. Potassium controls root growth, improves water use efficiency as well as resistance to diseases and pests, and increases the size and quality of fruits, grains and vegetables. Magnesium is part of the chlorophyll in all green plants and is essential for photosynthesis. It also helps activate many plant enzymes needed for growth.

Healthy plants need a balance of all nutrients. Waste often contains one nutrient in excess or may be very limited in another one.

Potentially toxic elements

Potentially toxic elements including zinc, mercury, chromium, cadmium, copper, nickel and lead enter the soil naturally as a result of soil forming processes such as the weathering of minerals. However, they are also added to the soil in the application of organic materials, sewage sludge, composts and inorganic fertilisers as well as though atmospheric deposition and run-off.

Many metals are essential in very small quantities; deficiencies of these can result in a reduction in crop yield or in adverse health effects in animals (copper, zinc). However, in high concentrations, the same elements may damage soil fertility, while the accumulation of others (e.g. cadmium and lead) in the food chain can damage human health. Therefore, safe limits for some metals in soils have been set for the application of sewage sludge to agricultural land and these are used to check compliance with legislation.

Microbial biomass carbon

Most processes in soil are driven by micro-organisms, the most abundant being bacteria and fungi.

Micro-organisms are the dominant component of soil biomass. They are the main drivers for the turnover of soil organic matter, release of nutrients and degradation of organic pollutants.

The biomass of all soil micro-organisms reacts to changes of quality and quantity of soil organic matter, soil acidity and heavy metal content of the soil. In general, microbial biomass increases with rising organic carbon contents as soil organic carbon is the main food and energy supply for the micro-organisms.

Earthworms

Earthworms are the largest soil invertebrates. They are considered to be the main soil engineers and changes in their number and community structure can affect several soil characteristics, such as porosity, aeration, water holding capacity, density, recycling and distribution of organic matter and nutrients. In addition, earthworms are an important food source for birds and mammals (e.g. moles, hedgehogs and shrews). In Scotland there are about 20 earthworm species that can be divided into four groups:

• epigeic species (litter dweller) which live in and feed on litter. They create no burrows;
• endogeic species (shallow earth dweller) which dig extensive branching systems of temporary burrows mainly in the topsoil and feed on organic matter found in the soil;
• anecic species (deep earth dweller) live in semi-permanent vertical burrows up to one metre in depth and feed on organic matter that they collect from the soil surface and carry into the mineral soil below (bioturbation);
• compost worms which live in compost heaps and are used in wormeries.

Earthworm quantity, biomass and the kind of species present in a soil is dependent on soil properties (e.g. pH or texture) as well as land use and management. Ploughing destroys earthworm burrows, kills earthworms and reduces their food supply for part of the year. Earthworm numbers are therefore reduced. On the contrary, waste application provides additional organic material and earthworm numbers may increase because of an abundance of food if the waste contains no harmful substance.

Earthworms may be under threat because of the introduction of the New Zealand flatworm in the early 1960s, and further species since. Flatworms appear to mainly eat earthworms without any preference for one particular worm species.