Microorganisms as indicators of soil quality

If we are to monitor soil quality, we need indicators of trends and changes. In order to choose relevant indicators, we must understand the factors controlling soil quality. Soil quality is dependent upon the physical, chemical and biological soil components. The interaction between these components is complex. The chemical and physical components have been more fully investigated than the biological. The methods used for the abiotic components are more or less highly developed and the results can be directly related to the soil properties. This is not the case with the biological component, for which the methods are still in their infancy and the results often difficult to interpret. The key is to find those components which are important for soil quality and which respond rapidly to changes in it. However, the complex nature of soil quality only allows it to be assessed if all three components are taken into account and evaluated simultaneously. Doran and Parkin (1994) proposed an index system with a `minimum data set` but a disadvantage with such a system is that information may be hidden during the process of evaluation.

The maintenance of soil quality and fertility are important aspects of a sustainable farming system. An increased recirculation of organic residues may give a number of environmental benefits, such as decreased need for land filling and recirculation of nutrients. However, there is also a risk of an increased load of heavy metals and xenobiotics. If we are to increase the circulation of organic residues, such as sewage sludge, between the urban and rural areas, we must ensure that the quality and fertility of our soils are not negatively affected in a long-term perspective. Soil microorganisms are an important factor when determining the impact of anthropogenic activities on soil quality. Microorganisms live in intimate contact with their environment and respond quickly to changes. In add ition, soil microorganisms are responsible for a great number of important processes involving the mineralisation and immobilization of plant nutrients. Together with physical and chemical parameters this makes them suitable as indicators of changes in soil quality and fertility.