Gardeners have come to understand that the microbe population in soil is critical for healthy soil and healthy plants. There is also tremendous chatter on blogs and in social media about “doing the right thing” to increase microbe populations. Companies have packaged microbes to produce garden “probiotics”. Just sprinkle a bit on your soil and watch the magic happen. So I think we can agree that gardeners should take care of their existing microbes and work on increasing their numbers.
More microbes = healthier soil = happier plants
But ….. there is a catch. How does the gardener measure the current microbe population? If you can’t measure it today, how do you know that you have increased the population over time? How do you know which method of soil improvement worked the best?
Without a method for measuring microbes, you are flying blind. In this post I will discuss some lab methods, some DIY methods and some new technology that is just coming on the market.
Can Scientists Measure the Amount of Microbes?
With all the talk about the importance of microbes you would expect the answer to be yes, but it turns out it is very difficult to count microbes.
You have probably seen microbiologists grow microbes in petri dishes and then count colonies. That does work but only for species that grow on the food in petri dish. The majority of soil organisms won’t grow this way.
Dr. Elaine Ingham from the Soil Food Web Organization promotes the use of a microscope for counting organisms in soil, but as I have discussed before this really does not work, especially if done by trained gardeners. Even when done in a lab it has limited value for determining total biomass.
There are many different types of microbes and their numbers are huge. They are mostly very small and like to hide inside soil channels. Measuring their number is difficult.
From a soil health point of view it is more important to measure the mass (weight) of microbes rather than the number because mass accounts for the vast difference in size between organisms.
Scientists have developed several indirect ways for measuring the microbial biomass and you can order these tests from various soil testing laboratories.
The Fumigation-Incubation Method (FI)
Two soil samples are collected. One is untreated and the other is fumigated to kill all living organisms. Living microbes are added to the samples and they start digesting any dead organisms which releases CO2 as part of respiration. The measured amount of CO2 is uses to calculate the mass of dead organisms due to the fumigation. This is an accurate method for some types of soil.
The Fumigation-Extraction Method (FE)
Two samples are collected and one is fumigated. The amount of carbon (FE-C), nitrogen (FE-N) or phosphorus (FE-P) resulting from the dead organisms is used to determine a biomass value. This method works for a large number of soils including wet soils and acidic soils. When chloroform is used for fumigation the method is called CFE.
The Substrate-Induced Respiration Method (SIR)
Two samples are collected and one is fumigated. Sugar is added to the samples and CO2 is measured. The amount of CO2 produced is relative to the amount of microbial respiration caused by the added sugar and is therefore an indirect way of measuring the mass of microbes. The method is simple, fast, and allows for the estimation of the fungal to bacterial ratio (F:B). One problem with the test is that it only measures microbes that are able to consume sugar.
The ATP Activity Method (ATP)
The microbial cells are destroyed in one of two samples, which releases ATP. This is extracted and measured. The amount of ATP is proportional to the mass of microbes killed in the sample.
Measuring Microbes with Earthworms
Earthworms consume microbes that are found on organic matter. An increase in microbes results in an increase in earthworms and therefore counting earthworms is an indirect way of measuring microbial mass. The following is taken from the book Soil Science for Gardeners, by Robert Pavlis.
“Measure out a square foot of soil. Dig down 12 “and remove all of the soil. Spread it out on cardboard or newspaper. Break up the soil and count all of the worms you find. If your soil is healthy, you’ll find at least 10 earthworms per cubic foot (0.092 cubic meters).”
The method does not provide an actual count of microbes or a mass, but the method is easy to do and will show improvements in microbe populations over time.
The Tighty Whitie Soil Test
Microbes consume organic matter and so you can add some cotton to the soil and see how quickly the microbes digest it, a method known as Tighty Whitie.
It is fully described in Tighty Whitie Soil Test – A Brief Review.
The method does not provide accurate numbers but it does give you an approximation of soil health and can be done by any gardener.
The microBIOMETER is a new home/field test that was developed specifically for measuring microbial biomass and the fungal:bacterial ratio. I will be reviewing and testing this product shortly, so I won’t say too much now. The kit comes with an extraction powder that you mix with water. When this is mixed with soil, it extracts the microbes from the soil. You then take the extracted liquid and place it on a test card where it reacts with a dye. Wait 2 minutes for the dye to react and then take a picture of the sample using their cell phone app which converts the image to mass and ratio values.
The app counts stained microbes. It is not clear how it converts these counts to mass? The ratio can be estimated because bacteria and fungi stain different colors. There is no mention of other types of microbes or how these affect either of the results.
Does the microBIOMETER work? The company certainly thinks it does and they do have internal testing to show it does, but they have no third party testing to support their claims. There is a masters thesis research project that uses the kit and it found inconsistent results. I have not been able to find any other research that used the technique and the company acknowledged that their only testing has been done by their personnel.
If we start with an average sample of say a billion microbes per teaspoon, work through the procedure and dilution process, and assume all microbes are extracted, you will put 50,000 microbes on the card in a 1/4″ diameter circle (area = 0.005 inch squared). It is hard to believe a cell phone can be accurate enough to count such a large number in such a small space? It is possible that the extraction is not 100% effective and that the app corrects for this.
Can Gardeners Measure Microbial Biomass?
You can certainly order such a test from a soil lab but they are not cheap. The two above mentioned DIY tests give a very rough idea of soil health but they don’t provide microbial mass values. The microBIOMETER costs about $15 US per test for the initial kit and drops to $9, or $7 if you buy refill kits. This price is reasonable for a home gardener. The key question with this kit is about reliability? It just has not been tested enough to know how well it works.
I do plan on some testing and will report back here when I have the results.