First, consider soil from the perspective of the organisms that live in it, and then from the perspective of someone who makes decisions that affect the environment of the organisms. Now ask the question - what is soil fertility?
The definition of soil fertility depends on your perspective. For example, the fertility of a wheat field and a forest may appear different, but they may be equally fertile in terms of plants that grow in each place. In order to explain the background to soil fertility, it is useful to know how soil types arise from different types of rock. This is important for predicting the likely level of fertility of soil for a particular land use (e.g. forest plantation or vineyard).
The range of organisms that live in soil is vast. Soil biological diversity is far greater than the diversity of organisms that live above the soil (including plants and larger animals). In soil, there are many types of animals (e.g. earthworms and single-celled amoebae), plants (e.g. single-celled algae and root systems) and microorganisms (the most obvious being fungi and bacteria, but there are also other forms of microbial life in soil). Numerous techniques have been devised for extracting organisms from soil so they can be identified, counted or studied in the laboratory.
Usually, specific methods have to be used for various groups of organisms because they differ greatly in size and characteristics, and because they are not uniformly distributed in soil. The matrix of the soil is primarily made up of particles of sand, silt, clay and rock fragments containing minerals and organic matter, and this may occur as independent particles, or as aggregates of particles at the scale of millimetres. Depending on the extent of aggregation, the soil habitat may be very suitable or less suitable for growth and activity of organisms.
Soil is a heterogeneous and dynamic environment. It can change in response to the activities of the organisms themselves, variations in the weather, and physical and chemical disturbances (e.g. in response to ploughing, vehicle activities, erosion events and fertiliser application).
Soil organisms frequently experience sudden changes in their habitat that may either increase or decrease the suitability of their immediate environment for their growth and survival. An example of such a change is water infiltration.
Many soil organisms are very active only when conditions are suitable, but suitable conditions may be rare. However, they may remain relatively inactive for most of the year. Although most of the time conditions are unsuitable for their growth and reproduction, many survive in a relatively inactive (dormant or quiescent) state during these periods if they are protected from predators or desiccation (e.g. inside an aggregate of soil particles).
Some organisms move considerable distances in soil, but most remain roughly in the same place unless the soil is wind-blown or moved by vehicles or water (e.g. across the soil surface or vertically down the soil profile). The mobility of organisms depends on their structure. Obviously, earthworms and many other soil animals are fairly mobile compared with bacteria. Bacteria are dispersed when the soil is physically disturbed or moved around by water or wind. Similarly, although fungi are generally restricted to discrete pockets in soil (e.g. in micro-sites), some can be transported by water and wind. A number of fungi form networks that extend for many metres underground or near the surface of soil.
Soil organisms carry out a number of important processes that may:
• help soil develop from the original parent rock material,
• contribute to the aggregation of soil particles,
• enhance the cycling of nutrients,
• transform nutrients from one form to another,
• assist plants in obtaining nutrients from soil,
• degrade toxic substances,
• cause plant disease,
• reduce plant disease, and
• assist or hinder water penetration into soil.
Thus, organisms, alone or in groups, contribute in a variety of ways to the condition of soil.
The challenge for land managers is to select management practices that manipulate soil conditions to take advantage of beneficial biological processes and minimise the possibility of detrimental processes.