Soil microbial abundance: what does CFU/g say about your crop?

What does a CFU test measure?

CFU stands for colony-forming units. In the lab, a diluted sample is spread on a growth medium. Microorganisms that can grow under the chosen conditions form visible colonies. These are counted and converted to, for example, CFU per gram of soil.

It’s a useful method for a specific question. But it is not a count of all microbial life in a sample. Some microorganisms don’t grow on the medium used, or not at the chosen temperature and incubation time. A colony can also grow from more than one clumped cell. And a bacterial culture result doesn’t automatically describe fungi and other groups of microorganisms.

In research comparing different counting methods for soil bacteria (opens in a new tab), results varied depending on the method and how the sample was handled. The practical lesson: always read a CFU result together with the test method.

From 10⁸ to 10¹⁰ CFU/g: what does that difference mean?

Mathematically, 10¹⁰ is a hundred times as much as 10⁸. For a grower, interpretation only starts there.

Did the samples come from the same field and the same depth? Were they taken at the same crop stage? Was it soil from the root zone or soil further from the roots? Was the same laboratory method used? And are both results expressed in the same way?

If those conditions differ, a comparison can be misleading. Even with well-matched samples, a higher number doesn’t tell you which organisms increased or what they contribute to the crop. There is no general CFU/g threshold at which soil suddenly becomes “healthy”.

A series of measurements within the same system can be interesting, though. It lets you follow whether a value changes after an adjustment in crop management. Assess such a trend alongside soil condition, root development and crop data. USDA soil health guidance (opens in a new tab) also stresses that individual test values need context and comparison over time.

The root zone matters more than one total number

Microorganisms play a role in processes around organic matter, nutrients and roots. But their presence alone doesn’t tell you how those processes are going in your crop.

Directly around the root, in the rhizosphere, conditions change constantly. Roots release compounds; irrigation, oxygen, temperature, pH and nutrition influence which microorganisms can function there. The existing microbial community plays a part too.

That’s why two samples with a similar total count of culturable microorganisms can still be very different. For a crop decision, questions like these are often more useful than “how do I get the highest CFU number?”:

• How are the roots developing in the growing medium?
• Are moisture, oxygen, pH and nutrition in the root zone in order?
• Which microbial function do you want to support in a targeted way?
• Does a selected microorganism suit this crop and this system?
• How will you assess whether the application adds value within the crop program?

Want to know more about that targeted approach? Read how microbial biostimulants are used in professional crops.

CFU/g in soil is not the same as CFU/mL on a label

The supplied label for BIOMICROBES 08 states a total minimum concentration of ≥ 1 × 10¹⁰ CFU/mL. That value describes the concentration of viable microorganisms in the product under the stated storage conditions.

A result in CFU/g describes something else: the culturable microorganisms found with a particular method in a tested soil sample. After application, a product is also spread through water, soil or substrate. The microorganisms then enter an existing root environment.

So a label value of 10¹⁰ CFU/mL doesn’t mean you will measure 10¹⁰ CFU/g of soil after application. The product value is relevant to the quality and concentration of the formulation. The agronomic question is whether the selected organisms are applied in the right place and can function there under suitable conditions.

How do you make a measurement useful for your operation?

Start with the purpose of the test. Do you want to record a baseline, follow a change or compare two growing areas? Then agree with the laboratory which test fits that question.

For a meaningful comparison, it helps to work consistently:

1. Sample the same zone. Record whether you are testing soil near active roots, a specific soil layer or a specific part of the substrate.
2. Keep the timing comparable. Crop stage, recent irrigation and field work all affect the starting situation.
3. Use the same analysis method. Different culture media or sample handling can give different counts.
4. Note what changed in the crop. Think of fertilization, irrigation, temperature, substrate or microbial applications.
5. Look at the crop too. Root development, uniformity and relevant crop data give the lab result its meaning.

A single result can be a reason to look further. A carefully built series is more helpful for making decisions.

From measuring to targeted application

When the root zone needs to function better, the approach doesn’t start with chasing an arbitrary CFU number. First you need to understand what’s happening in the system. A root zone that is low in oxygen or constantly too wet, for example, calls for attention to growing conditions. A microbial product should then fit the crop, the application and the existing program.

That is where the value of a selected formulation lies. You deliberately choose which microorganisms you introduce, instead of just hoping that a higher general microbial count also represents the microbiology you want. Application and rate always follow the current label and must be aligned with irrigation and any water treatment.