So You Got Your Soil Food Web Test Results, What Do They Mean?

A Soil Food Web test provides insights into the biological makeup of your soil or compost, helping you understand the balance of beneficial microorganisms. The test is performed by diluting the sample in water, and then viewing a drop under the microscope. Data is collected from approximately 25 fields of view (FOV) at 400x magnification. These total biomass and number of organisms per gram is then extrapolated.

Typical Field of View for compost, showing fungal hyphae and spores, testate amoebae, bacteria and organic matter.

Here’s a breakdown of the key data points in a sample report and what they mean.

Test Results from a Compost Pile

1. Bacteria

Bacterial Biomass (µg/g) value represents the total amount of bacterial life in the soil, expressed in micrograms (µ) per gram. Bacteria play an essential role in decomposing organic matter and making nutrients available to plants. The "bio-complete" standards for bacterial biomass level is 135 µg/g , which helps maintain a balanced soil environment. Most samples we see are quite a bit higher in bacteria then this standard (as is the case in the example test result above).

About Standard Deviation & Percentage of Mean

The standard deviation reflects the variability in data across each Field of View (FOV). A lower percentage indicates consistent sampling, whereas a higher percentage may suggest uneven distribution. We advise our client to sample multiple areas and combine these samples to achieve a representative average reflected in the data. Additionally, we go through great lengths to homogenize the sample to keep the Standard Deviation low. If the Percentage of Mean exceeds 70%, the data becomes statistically less reliable. Various factors could contribute to this. If there are few organisms, the distribution will be uneven (for example, observing only one organism out of 25 FOV). Poorly mixed samples can also result in uneven distribution, leading to a high Standard Deviation as a Percentage of Mean. In the example provided, the percentage is approximately 14%, indicating even distribution and reliable data. A Standard Deviation Biomass of 937 µg/g implies that the total Bacterial Biomass 6751 µg/g could vary by +/- 937 µg/g.

2. Actinobacteria

Actinobacteria are a specific group of bacteria that help decompose tougher organic materials like cellulose and chitin. Their presence can indicate soil with good organic matter breakdown.

3. Fungi

Fungi contribute to soil health by breaking down complex organic matter. In this test, we are specifically looking at saprotrophic fungi, which decompose dead organic material and play a key role in nutrient cycling. The ideal fungal biomass level is 135 µg/g or more, which supports a balanced soil ecosystem when bacteria biomass is roughly equal.

4. Fungal-to-Bacterial Ratio

This ratio helps determine whether the soil is more bacterial- or fungal-dominated. Bacteria-dominated soils are common in annual crop systems, while fungal-dominated soils are better for perennial plants and forests. The ideal fungal-to-bacterial ratio is 0.3 to 1 or greater, which helps maintain a balanced and healthy soil ecosystem. In the example provided, bacteria far outnumber fungi, so the ratio is very low, at 0.02 to 1.

5. Protozoa and Beneficial Nematodes

These microorganisms help regulate bacterial and fungal populations. They also release nutrients into the soil by consuming bacteria and fungi, making nutrients more available to plants. The ideal protozoa minimum count is 10,000/g, while beneficial nematodes should be at least 100/g to support a healthy soil ecosystem. This compost has a tremendous amount of testate amoeba.

Detrimental vs. Beneficial Microorganisms

The ratio of detrimental to beneficial microbes influences the resistance of soils and the plants they support against diseases. Generally, a greater presence of beneficial microbes indicates healthier soil with fewer plant health problems. In this evaluation, we classify Root-Feeding Nematodes, Oomycetes, and Ciliates as "Detrimental" organisms because they signal management problems such as anaerobic (low oxygen) conditions resulting from compaction or poorly draining soils. Increasing the number of beneficial organisms can enhance soil structure and decrease the conditions that allow "Detrimental" organisms to prosper. For more about the Soil Food Web, see our Blog Post.

Interpreting the Data

When analyzing your results, compare them to the "bio-complete" standard:

• Bacterial Biomass: 135 µg/g

• Fungal Biomass: 135 µg/g

• Fungal-to-Bacterial Ratio: 0.3 to 1 or greater

• Protozoa: 10,000/g

• Beneficial Nematodes: 100/g

• Ciliates: Less than 5/drop at 1:5 dilution

Plants at different stages of ecosystem succession need different F:B ratios. As an ecosystem transitions from an early succession stage, such as a highly disturbed condition, to a late succession stage, like an old-growth forest, these requirements shift. Adjusting your soil to align with the desired plant type (grassland, annual gardens, orchard, etc.) will enhance plant health and productivity.

• Early Succession:

  • Bacteria dominant

  • Generally alkaline soil

  • Support weed and grass growth

• Mid Succession

  • Balanced F:B ratio

  • pH Balanced

  • Supports most food crops

• Late Succession

  • Fungal dominant

  • Generally acidic

  • Supports shrubs and tree growth

It is important to remember that this assessment is just a tiny slice of data in a constantly evolving natural ecosystem. The results from a single test are interesting, but really the value of this sort of analysis is in repeating the test over time, looking for trends. You can use these trends to guide your management decisions.

Improving Soil Microbiology

To enhance soil microbial life, consider:

• Adding compost and organic matter to increase beneficial microbes.

• Reducing chemical inputs that might harm microbial populations.

• Maintaining soil moisture to support microbial activity.

• Planting diverse cover crops to encourage a balanced microbial community.

Understanding your Soil Food Web test results allows you to make informed decisions to enhance soil health, leading to stronger, more resilient plants and ecosystems. By fostering the right microbial community, you can improve soil fertility naturally and sustainably. We can help you work through this process to bring your land or growing operation into balance and a state of high productivity.