By Dr Judith Fitzpatrick, Ph.D.
Sponsored by microBIOMETER®

We are often asked what is a good level of microbial biomass (MB). There is no one answer. It depends on your crop needs, your past soil health practices, and geographic considerations. We do know that increasing your soil’s microbial biomass is an important step in improving your soil and crop health. 

The level of MB you can reach is dependent on soil organic matter (SOM). Soil organic matter is a mixture of Carbon (C), Nitrogen (N), Phosphorus (P), Sulfur (S) and all the other minerals that microbes and plants need.   

There are 2 types of SOM: Stable SOM, often referred to as humic matter; and Fresh SOM. Stable SOM is composed of small particles of well-decomposed organic matter (plant and microbial). This portion of the SOM is protected by soil particles and is difficult for microbes to access.

Fresh SOM is composed of recently decayed organic matter that is not yet tightly sequestered in the soil and so it is more efficient at releasing nutrients to soil than stable SOM. Fertilizers, amendments and litter quickly become fresh SOM. Fresh SOM does not last as long in the soil because it is quickly consumed by microorganism and fed to plants.  Agronomists in intensive organic agriculture often use soils containing lots of fresh organic matter. The microbial biomass of these mixtures can read as high as 2,000 ug MBC/gram of dry soil. As the microbes and plants in this rich soil die, they become fresh SOM and if there is surplus fresh SOM it is converted to stable SOM over time.  

The amount of stable SOM that soil can store depends to a large degree on the type of soil because storage requires mineral surfaces for attachment and aggregates for protection. If your soil is inherently poor at storing SOM or if your soil has low stable SOM because of past practices that removed the prior organic matter from the soil, you can compensate by providing lots of fresh SOM like compost and amendments. However, the key to the efficacy of fresh SOM is that it needs to be nutrient balanced – it needs the correct balance of C,N,P, and S. That is where understanding soil chemistry and using the right additives makes a huge difference in improving your soil health and crop output. 

Soil organisms are essential for keeping plants well supplied with nutrients because they break down organic matter and when they die release these nutrients to the plant. These organisms also make nutrients available in both fresh and stable SOM by freeing them from organic molecules. Some soil bacteria fix nitrogen gas from the atmosphere, making it available to plants. Other organisms, like mycorrhizal fungi, dissolve minerals and make phosphorus more available. If soil organisms aren’t present and active, more soil additives will be needed to supply needed plant nutrients.

Soil organisms are essential for keeping plants well supplied with nutrients because they break down organic matter and when they die release these nutrients to the plant.

You can increase your stable SOM by making sure your fresh SOM is balanced and contains enough microbes to make the nutrients in the soil available to your crops. For calculating how to balance your fresh SOM, see the reference below. Your microbial biomass levels will depend on many factors: your soil type, the stable SOM, fresh SOM, crop health, and time of year. Microbial biomass rises as a plant begins to grow and slowly falls throughout the growing season. We consistently see a significant drop in microbial biomass as a plant goes into flowering; at this time the plant cuts back on exudates and puts all its energy into reproduction. Frequent testing of soil for microbial biomass will allow you to watch the trend and see if you over year you are adding to your soil health or depleting your soil.  You can also use testing to assess how well your amendments and additives are at increasing microbial biomass. 

We highly recommend that you read the review referenced below to better understand SOM.

Coonan, E.C., Kirkby, C.A., Kirkegaard, J.A. et al. Microorganisms and nutrient stoichiometry as mediators of soil organic matter dynamics. Nutr Cycl Agroecosyst 117, 273–298 (2020).

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