Microbial biomass (MB) is the best single indicator of soil health (Doran, 2000). Microbes feed and protect plants, build soil structure which prevents erosion, increase water holding capacity, and build soil organic matter (SOM). MB is low in any situation that is harmful to plant growth (and vice versa) and protects against pathogens, thereby reducing the need for pesticides. MB can predict success before plant outcome. The Fungal:Bacterial ratio (F:B) of the MB provides crucial information regarding colonization by Arbuscular Mycorrhizal Fungi (AMF), and the recycling metabolic processes of saprophytic fungi (SpF).
Soil stewards all over the world are seeking to understand the microbial levels in their soil and the ratio of fungal to bacterial life. The higher the microbial biomass, the more nutrients will be available to plants naturally, decreasing or eliminating the need for chemical fertilizers. Higher fungal to bacterial ratios are critical for building soil structure that prevents erosion and runoff off of pollutant chemicals while building moisture holding capacity of the soil and sequestering carbon.
Soil health is fast becoming one of the most important factors in agriculture and in the growing efforts to improve the earth’s stock of agricultural land. Farmers, industry, and environmentalists are looking for cost-effective and reliable ways to measure soil health and to assess impacts of progressive changes to soil and harvest management.
Testing soil in homogeneous sections at similar stages of the growth cycle can set a baseline for microbial biomass and fungal to bacterial ratio. That baseline can be used to assess how different stewardship practices are impacting the soil and allow for refinement to soil management plans and show soil health improvement over time. While every soil steward’s situation is unique, microBIOMETER® can help measure, follow, and assess efficacy of improvement to soil health.
Soil microbes are tightly bound to and often covered in soil making them very hard to evaluate by microscopy. The special magic of microBIOMETER® is the extraction powder and whisking process that separates most of the microbes from the soil. And during the 20 minute settling time allows the soil particles to precipitate leaving the extraction fluid >95% microbial.
This allows microBIOMETER® to examine 100 – 1000 times more microbes than any other method. When you apply extraction fluid to the membrane in the test card the colored microbes are captured on the surface of the membrane. A cell phone picture of the card is analyzed by the app and the intensity of the color of the microbes indicates their quantity – this is the basis for all laboratory colorimetric tests. We discovered that the fungi in soils are a slightly different color than bacteria, and so the app is able to distinguish between bacteria and fungi.
Click here to see a full video tutorial of microBIOMETER® soil testing.
Arbuscular Mycorrhizal Fungal (AMF) are dependent on the plant for their food, therefore, they die when the plant dies. Lucky for us before they die they form spores that can live a long time in the soil.
When we have looked at the soil from vineyards in winter it is filled with fungal spores. Pictured here of some of the types of AMF spores. The size of these spores can vary from microscopic to visible.
The spore starts growing when it receives a chemical message from a nearby plant. It has a day or two to reach the plant, enter the root and build a little space called an arbuscule where it can get food from the plant. If it fails at this, the fungi dies. This is why we like to plant seeds with AMF. The plant feeds the fungi because the fungi send out long hair like structures called hyphae that bring minerals and water back to the plant. In fact, scientists have recently shown that the fungi and the plant actually barter with one another, i.e. when phosphorus is low, the fungi gets more food for delivery of phosphorus.
microBIOMETER® measures both fungi and fungal spores as well as bacteria. The lab methods of PLFA and Carbon Fumigation do not adequately measure spores. Standard microscopy does also not adequately measure fungi.
The graph pictured here from the USDA website depicts the ratio of fungi to bacteria as a characteristic of the type of system it is in. An excerpt from the article:
“Grasslands and agricultural soils usually have bacterial-dominated food webs – that is, most biomass is in the form of bacteria. Highly productive agricultural soils tend to have ratios of fungal to bacterial biomass near 1:1 or somewhat less. Forests tend to have fungal-dominated food webs. The ratio of fungal to bacterial biomass may be 5:1 to 10:1 in a deciduous forest and 100:1 to 1000:1 in a coniferous forest.”
If you are measuring soil attached to the roots colonized by mycorrhizal fungi, your ratios should be much higher than is shown for agricultural soil. Also the saprophytic fungi population increases when there is a lot of litter for digestion, so you would expect to see different ratios at different times of the year and under different conditions.
The graph pictured below based on USDA website information shows the expected fungal to bacterial ratio for various plants.