Modern agriculture has placed greater emphasis on the development of sustainable farming systems. This has led to greater interest in farm management practices that promote the biological aspects of soil fertility.

To help farmers in this regard, many approaches to soil biology testing have been developed, which can be classified into tests for population analysis, biological activity, and indirect indicators.

Population tests look at the types and numbers of organisms present in the soil. Whilst they give a current picture of the soil biology, it is good to remember that soil populations are very dynamic and subject to rapid changes. For example, biological populations existing before planting can be very different from those at harvest time. It is advisable to consider test results in terms of seasonal variations and time trends.

There are many population tests in use or under development. These include nematode analysis (an indicator of nutrient cycling and microbial diversity), fatty acid methyl ester analysis (which calculates ratios of bacteria to fungi, where high values are associated with nitrate/ammonia build-up), food web analysis, detritus diversity and earthworm counts.

Biological activity analysis gives an idea of the activity level of soil organisms. Microbes respond rapidly to environmental factors such as temperature, soil pH, moisture, organic matter content, cultivation, and contamination by chemicals and toxic heavy metals. With so many determinants, it is not easy to isolate particular soil problems. But, when compared to information from other fields, the data can indicate what needs to be done to improve soil quality, especially organic matter content.

Tests for biological activity include soil respiration rate (to measure amounts of carbon dioxide released from soil into the atmosphere), assays for specific enzymes, and nitrification rates.

Indirect indicators assess factors that, when present in sufficient amounts, suggest good soil biological fertility. Most tests measure carbon content at various levels — soil organic carbon, particulate organic carbon, dissolved organic carbon (to determine amount of carbon available in biological form), and microbial biomass organic carbon (or amount of carbon in microbes). Ground cover as a percentage of total area is also an indirect indicator of soil health. Lack of ground cover suggests salt or acid spots, compacted surfaces or crusting.

It is recognised that the thousands of soil organisms give different contributions to healthy soil. The roles of some microorganisms have already been demonstrated. Others have very subtle roles, making it difficult to assess their value. Most biological tests require special equipment and trained technicians. Many farmers might find the following tests very important for their management decisions:

Total soil respiration. Since this measures the amount of carbon produced by the total biological population and the respiration (converting organic carbon into carbon dioxide) activity of microorganisms, it gives an idea the rate of organic matter degradation.

Microbial biomass nitrogen. Organic matter is a rich source of nutrients, including nitrogen. This test measures the amount of biologically active nitrogen in the soil. It is better to look at values over a time series: a consistently high level provides a safe indication of good soil quality.

Earthworms. Earthworms influence, and are also influenced by, other soil organisms. They initiate the first stage of breaking down litter, which provides food to microorganisms responsible for further degradation. They are highly sensitive to changes (both short- and long-term) in the soil ecosystem. The farmer will find it easy to estimate earthworm numbers in the field (expressed in grams per cubic metre) without need for special equipment to obtain a reasonable, albeit rough, measure of soil quality.

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