Marvellous Microbes
Healthy soils aren't built in a season — but tracking and encouraging the right biology is a smart step in the right direction.
In the quest to build resilient, productive soils, more farmers and graziers are turning their attention beneath the surface to soil microbes. Among the most telling indicators of biological soil health is the fungal to bacterial ratio (F:B ratio). This simple yet powerful metric can reveal much about soil condition, land-use history, and how well your farming and land management systems are supporting plant and pasture health.
What is the Fungal:Bacterial Ratio?
Soil is home to billions of microbes in every teaspoon — primarily bacteria and fungi. These microorganisms perform essential services:
- Bacteria are fast-acting decomposers, breaking down simple compounds like sugars and proteins.
- Fungi, especially beneficial types like mycorrhizae, are more involved in decomposing complex organic matter (like lignin and cellulose), enhancing soil structure, and forming symbiotic relationships with plant roots.
The F:B ratio describes the relative abundance of fungi to bacteria. Each type of land use has a characteristic balance. For example:
Soil Type/Use Typical F:B Ratio
Bare/disturbed soil <0.1
Conventional Cropping 0.1 - 0.3
Pasture 0.5 - 1.0
Forest/Perennial systems 2.0 - 5.0
A low F:B ratio may signal over-tillage, high nitrogen inputs, or soil degradation.
A higher ratio typically indicates stable, carbon-rich soils with good structure — conditions that favour nutrient cycling, water retention, and long-term productivity.
* Fungal-dominated soils tend to support perennials, shrubs, and trees, while bacterial-dominated soils suit annuals.
* A balanced F:B ratio improves nutrient availability, root disease resistance, and carbon sequestration.
* Fungi play a vital role in stabilising soil aggregates, which enhances aeration, water infiltration, and reduces erosion.
Measuring the F:B Ratio – Enter the Microbiometer®
Traditionally, assessing soil microbes meant expensive lab-based DNA tests. The Microbiometer® offers a cost-effective, on-farm tool to measure total microbial biomass and estimate the fungal:bacterial ratio in under 20 minutes using a smartphone app.
- It works by extracting microbes from soil and using a colourimetric method to estimate biomass via the phone app. [NB: Colorimetry the science of measuring and quantifying colour]
- The Microbiometer distinguishes between fungal and bacterial biomass based on the particle size and optical density of the suspension.
This means you can track microbial shifts over time as you implement new practices — a crucial feedback loop in regenerative farming. Frequent retesting will provide you with the data necessary to identify if your soil management practices are working.
Bio stimulants are products that stimulate soil microbial activity, not by directly adding nutrients, but by encouraging microbial proliferation or signalling. Examples include humic substances which stimulate fungal growth and improve aggregation; seaweed extracts that enhance root exudation, supporting beneficial fungi; compost teas and vermi-liquids contain living microbes and microbial food that can rebalance communities; an fungal inoculants such as mycorrhizal spores or Trichoderma that directly increase beneficial fungi.
However, to truly shift the F:B ratio long-term, these inputs must be supported by good management, such as reducing tillage to protect fragile fungal hyphae; cover cropping to provide year-round root exudates; increasing organic matter as fungi thrive on complex carbon; and reducing synthetic nitrogen which tends to favour bacteria over fungi.
The F:B ratio is more than a number — it’s a biological compass for your soil. Aiming for a higher fungal presence can lead to more resilient, nutrient-efficient, and climate-smart farming systems.
Tools like the Microbiometer offer practical, real-time insights into your soil biology, and when paired with regenerative practices, can help steer the balance in favour of fungi. By comparing Microbiometer results over time, you’ll easily identify if your management approach is successful. An increase in F:B ratio and microbial biomass indicates the effectiveness of your management practices.
Click HERE to view a general interpretation guide.
The MicroBIOMETER® Starter Kit
Cost $150.00 approx. Compatible with iPhone and Android.
How to use a Microbiometer
Soil microbes produce a natural binding substance that connects soil particles and forms stable aggregates. A Microbiometer extracts these microbes from the bound soil, allowing the heavier particles to settle while the soil-colored microbes remain suspended in the fluid. This suspension is then sampled and placed on a test card, which is scanned using the smartphone app. It is like soil testing in the palm of your hand!
The app measures the colour intensity of the microbes against a colour comparator on the test card, providing lab-grade results.
Taking samples:
A soil probe or auger is ideal for taking soil samples. If you don’t have a probe, you can use a sharp spade, long knife, or trowel — just be sure sample size is consistent.
- Soil: Collect field moist soil samples at a depth between 2-5 inches depending on root length, being sure to take samples at consistent depths.
- Compost Pile: Collect 8-15 samples, depending on the size of the pile, at different depths. Combine samples together and mix thoroughly. Do not take samples from the surface or wet areas.
- Trees: Collect 4 to 8 samples, depending on the circumference of the tree, surrounding the base of the tree in the root zone. Combine samples together and mix thoroughly.
- Johnson-Su Compost Bioreactor: Collect 8-15 samples, depending on the size of the bioreactor, at different depths. Combine samples together and mix thoroughly. Collect samples only at the mature stage.
- Turf: Collect multiple samples at a depth between 2-3 inches. Combine samples together and mix thoroughly.
Click here for more testing and sampling tips.
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