Microbial Biomass

Soil Texture
% OM

Sand 2.0 55
Loamy Sand 1.5 137
Sandy Loam 1.6 106
Silt Loam 3.2 292
Loam 4.5 358

Biomass ug/g
Range ug/g

0 to 1.0 76 10 to 165
1.0 to 2.0 130 17 to 379
2.0 to 3.0 169 24 to 418
3.0 to 4.0 219 119 to 300
4.0 to 5.0 345 127 to 454
5.0 to 6.0 427 369 to 506
6.0+ 613 421 to 805

The microbial biomass of soil is defined as the part of the organic matter in the soil that constitutes living microorganisms smaller than the 5-10 um3. It is generally expressed in the milligrams of carbon per kilogram of soil or micrograms of carbon per gram of dry weight of soil. Typical biomass carbon ranges from 1 to 5% of soil organic matter. The degradation of organic compounds, such as industrial chemicals and pesticides, can be monitored by following changes in the soil microbial biomass.

Several methods can and are used to estimate the soil microbial biomass population. They can be summarized as follows:

  1. Staining and counting the microbial cells.
  2. The use of physiological parameters such as ATP, respiration, and heat output.
  3. An application of a soil fumigate and measurement.

AGVISE currently employs a soil fumigate method for measuring the biomass of the soil. The soil is fumigated with chloroform to kill the microbial population. After the microbes are killed by fumigation, cytoplasm is released into the soil environment. The soil microbial biomass carbon is extracted with potassium sulfate on both fumigated and unfumigated soil. The carbon content of the extract is tested and the biomass is calculated based on the difference between the carbon content of fumigated vs. the unfumigated soil. We currently measure the carbon by dichromate oxidation, but plan on using our automatic carbon analyzer with an IR detector in the future.

Over the last few years the number of samples arriving to AGVISE that request a biomass measurement is on the increase. Often we are asked the question “What is an average number for the biomass of a soil?” or “What is a reasonable number for a biomass reading?” We have prepared the following tables that list the average biomass readings on soil received into our laboratory. We have attempted to sort out the biomass readings by both texture and organic matter. As you will note in the tables, both the texture of the soil and the organic matter appear to influence the level of biomass found in samples received at AGVISE. The relationship between biomass and organic matter appears to be a little stronger than the biomass/texture relationship.

A biomass reading is a snapshot picture of the microbial population on the day the sample was tested in the laboratory. It is not a stable number like the CEC or the texture of a soil. The biomass number will change with the environment in which the soil undergoes. After the soil is removed from a field setting, the biomass can and will change with storage conditions of the soil. Recommended storage conditions for biomass samples is 4oC. Freezing of soil samples is not recommended due to the adverse biocidal effects on the soil microbial biomass. If samples must be frozen, they should be preincubated for 7 to 10 days prior to testing. The drying of soil samples for biomass readings must be strictly avoided. Also, after the sampling procedure takes place, samples should not be exposed to direct sunlight. Our experience at AGVISE also indicates the long-term storage under refrigerated conditions in sealed containers can and will affect the biomass results too. Samples reanalyzed after a month or more of storage can have a significant reduction in the biomass reading. I strongly suspect the microbes may have used up the oxygen supply in sealed containers after prolonged storage.