The benefits of good soil biological health and links to soil organic matter are well established. Healthy soil is typically alive with billions of soil fauna comprising bacteria, fungi, and invertebrates such as earthworms, mites, collembolans, and nematodes. These biodiverse organisms underpin essential ecological processes including decomposing and stabilising organic matter, nutrient cycling, controlling pests and diseases, and improving soil structure, which are critical for sustainable crop and pasture production.

Measuring soil biological health

There is increasing interest in the use of biological measures for soil health to help gain insight into sustainable land management and the performance of plant-soil systems. Biological techniques should be considered where existing, established measures are insufficient to explain the system behaviour. It is also worth noting that soil biology represents an area of active and growing scientific research.

Soil organic carbon content is often used as an indicator of soil biological health, however the chemical and physical condition of soil also needs to be considered when interpreting such results. High soil organic carbon levels may not necessarily indicate high biological activity and favourable conditions for plant growth. For example, highly acidic soils can have high soil organic carbon levels but low biological activity and hence slower turnover of organic carbon. The microbial biomass and rate of turnover of soil organic carbon (i.e. biological activity) may provide a better indication of soil health than total organic carbon. Observations of rapid breakdown of plant residues can indicate high levels of biological activity, soil carbon turnover, and nutrient recycling to plants. This is the premise behind the Soil Your Undies citizen science program.

A range of soil biological health measures are possible. These measures often relate to the simple questions: ‘what soil microbes are there?’, ‘what can they do (i.e. functional potential)?’, and ‘what are they doing (right now)?’ Presently many soil biology tests would be used by researchers only, however over time we are likely to see more soil biological tests offered by commercial providers.

Information on the entire composition of bacteria, fungi, and other groups of organisms can be obtained through DNA-based microbial community diversity profiling techniques. DEW is piloting the use of these techniques to examine differences in soil microbial communities across different land use and management practices.

SARDI’s Molecular diagnostics Centre offers a laboratory test service to farmers, consultants and researchers to quantify the presence of recognised soil pathogens and disease-suppressive organisms, where particular organisms may be of heightened concern in the context of the crop types being grown.

In agriculture, there is growing interest in beneficial, symbiotic fungi (e.g. mycorrhiza) and bacteria, including so-called plant growth-promoting rhizobacteria and plant growth-promoting fungi.

Functional activity tests examine the rates of transformation by microbes of key compounds and molecules, for example, involving carbon, nitrogen and phosphorus, and the quantities of enzymes involved in their associated transformation pathways.

Another DNA-based technique termed ‘shotgun metagenomics’ can look at all DNA found in soils to piece together the functional potential, and entire community of organisms (bacteria, fungi, invertebrates, plants, viruses) that are present in particular soil environments.

Monitoring and trends

The study of soil biology still represents an area of active scientific research and there is insufficient data to support monitoring and assessment of trends at a broad scale. However, it is known that soil microbial communities are shaped by a variety of factors including soil type and properties (e.g. pH, salinity, drainage), climate (moisture availability, temperature), nutrient/feedstock forms and availability, vegetation type and diversity, and historic and current land use and management practices. To reduce potential confusion over what may be driving results, it is common for soil biological results to be compared across a limited set of related samples (e.g. within similar soils, land type and climate).