It has been known for some time that the ratio of fungi to bacteria varies in different types of soil and in different plant communities. Forests have a higher ratio of fungi while grassland and agricultural soil is higher in bacteria. This has led to the idea that plants grow best when the fungal to bacterial ratio is matched to their needs. Gardeners should not only monitor their fungi to bacteria ratio, but also modify the soil to change it.
In this post I will look at the science support this idea.
Fungal to Bacterial Ratio
Fungi are relatively large in comparison to bacteria. When we talk about the fungal:bacterial (F:B) ratio we are really talking about the biomass of these two organisms, not the actual number.
Many different types of soils have been analyzed for this ratio.
- Coniferous forest: 100 to 1,000
- Deciduous forest: 5 to 100
- Weeds and Grasslands: 0.1 to 1
- Agricultural fields 0.1 to 1
A study that looked at 47 sites in Russia, including various biomes, found that “a predominance of fungal over bacteria activity with F:B ratios always higher than one (4.9 on average). Natural sites were characterized by higher F:B ratios (on average 5.6) compared to agricultural ones (on average 3.5).”
Forests have very healthy soil, while agricultural soil is less healthy. This has led to the idea that having a higher relative fungal population results in healthier soil.
Dr. Ingham and the Soil Food Web School
Dr. Ingham did some of the early work measuring the fungal to bacterial ratios. She then developed the concept that each plant has a preferred ratio, and that gardeners and farmers should create this ‘optimal’ ratio for the plants they are growing.
How do gardeners do this? It all starts with measuring the soil and determining the current F:B ratio.
Dr. Ingham’s Soil Food Web School provides courses that will teach you how to identify and count microbes under a microscope. The company claims that this knowledge will allow you to calculate the F:B ratio.
Many of these concepts have been documented in the book Teaming With Microbes and I have done a detailed review of that book.
Facts About the Fungal to Bacterial Ratio
Scientists agree that different soils have different ratios. It is also accepted that agricultural practices such as tilling and using synthetic fertilizer tend to favor bacterial growth.
The ratios can be modified by changing the way we manage the soil. Adding organic matter increases the total microbe population. Using organic matter with a high C/N ratio favors fungal growth and low carbon organic matter favors bacterial growth, although one study looking at wood chips used as a mulch or tilled into soil found no change in the F:B ratio. Tilling reduces fungal growth. Adding nitrogen to soil changed soil biomass from fungal-dominant to bacterial-dominant.
What science does not agree on is the notion that plants grow best in soil with a particular F:B ratio.
What Came First the Chicken or the Egg?
We accept the fact that soil in forests has a high fungal to bacteria ratio. But does that mean trees need to grow in such a high ratio? What came first first, the forest or the fungal dominated soil? That is the key question to answer.
If we take agricultural soil and modify it to increase the ratio of fungi, will trees grow better? Is the modification required to grow trees?
Or is this a case where trees will grow just fine in agricultural soil and over time their presence modifies the soil to favor fungal growth?
How Does Science Measure the F:B Ratio?
There are numerous ways to quantify the amount of fungi and bacteria in soil, but the results from these tests vary greatly. Even light microscope has significant flaws. The testers tend to over count fungi because of their size and because it is very difficult to distinguish living and dead fungi. Conversion from counts to mass is also a problem.
Even the scientists don’t agree on the best method for measuring the ratio, ” Because of all the different methods and conversion factors for calculating F:B ratios, it is questionable to compare different studies. ”
Some Common Examples
Grasses and trees grow in very different F:B ratios. When I first heard this I immediately pictured a modern day park. Lots of healthy old trees surrounded by grass. Both the trees and grass grow well and these parks are located on all kinds of soil. How can these two plant types grow so well together if they require different ratios?
Take a well used agricultural field and allow nature to take over. What happens? Weeds cover the soil fairly quickly. Soon after that, shrubs and pioneer trees start to grow. Eventually the conifers show up. If the trees and especially the conifers required high fungal soils, why would they grow? The agricultural soil and the soil under weeds are bacterial soils, completely unsuitable for trees, if the F:B ratio is important.
In a YouTube video, Dr. Ingham points out that brassicas need a bacterially dominated soil and tomatoes need a more fungal soil. But gardeners have been growing these crops side by side for years.
What about hydroponics? Plants seem to grow quite well without any soil, or microbes.
Common sense examples don’t support the need for special fungal to bacterial ratios.
What Does Science Say?
I did a search for studies that looked at plant growth at different F:B ratios. I did not find a single one. If you find one, please add the link in the comments below.
I checked for plant lists and their preferred F:B ratio, and found none from the scientific community. A seller of microbes did have a very short list but provided no reference to the source and claimed that, “Most orchards need a high ratio”. One of the comments in the Research Gate discussion mentioned below said the opposite.
The Rodale Institute makes the statement, “weeds require a soil with lots of bacteria”. Have they never seen land after a forest fire? It is quickly covered in weeds taking advantage of the increased light.
Someone did ask the question on Research Gate, “What are the optimal fungal to bacterial biomass ratios for different crops?”
No one pointed to such a list and one scientist said, “we have yet to quantify the optimum load of soil bacteria and fungi in relation to production levels”, which implies there is no list.
One study looked at the F:B ratio in terms of pH and found that fungi biomass did not change much as pH changed from 4 to 8, but that bacterial growth was very affected by pH. Bacteria are more prominent at high pH. Acidic soil has a higher F:B ratio than alkaline soil. Coniferous trees prefer to grow in acidic soil, as do fungi – can soil pH explain the higher F:B ratio in coniferous forests?
There is also a lot of research work trying to understand the relation between F:B and sequestering of carbon, and how F:B influences natural ecosystems. But it seems that even in this area there is not yet agreement about the importance of the ratio.
It is clear that studying the right ratio for plants has not been done, nor is it a current priority. We don’t know how changes in the fungal to bacterial ratio affect plant growth nor do we have any insight into the optimum levels for any specific plant. Any group that promotes the importance of the F:B ratio for growing plants is completely ignoring current science.
Number vs Ratio
We do know that soil with higher levels of microbes tends to grow healthier plants. Higher levels of organic matter will increase the microbe biomass, making the soil healthier, but it also adds other benefits like better moisture retention, higher nutrient levels and better aggregation.
Adding organic matter, reducing tilling and compaction all increase the number of microbes. That is different than trying to control the ratio.
Are Fungal to Bacterial Ratios Important for Plant Growth?
We don’t know how the ratio affects plant growth. We do know that there is no current science to support the idea that plants need a special ratio. There is also no support for the notion that gardeners should modify the ratio for better plant growth.
I suspect that the soil environment and the plants that grow there, affect the fungal bacterial ratio. Plants drive fungal growth, not the other way around.
The reason forests have a higher ratio is that trees produce difficult to digest molecules like lignin. This is not food for bacteria, but it can be used by fungi, who then out compete the bacteria, changing the ratio. Grasslands produce much less lignin and much better bacterial food and therefore bacteria do better, reducing the ratio.
Plants can grow with any ratio. Grasses grow in forests if there is more light, and trees do just fine in low ratio grasslands.
There is no science to support the idea that gardeners or farmers should try to manipulate the fungal to bacterial ratio. There are no accepted lists of the optimum ratio for plants. Here is what one report concludes, “Comparing a given soil to an idealized ‘optimal’ soil microbial community will never be useful, because an ‘optimal’ soil microbial community simply does not exist.”