Understanding Soil Microbes

When discussing soils, we typically omit one of the most important pieces of the puzzle – the soil microbes.  Soil microbes are rarely discussed, and, if they are a topic of discussion, they don’t receive adequate attention.  Although invisible to the naked eye, the soil is teeming with billions of powerful microbes responsible for a plethora of soil and plant activities, making them the driving force of any ecosystem.

In this segment, I will discuss a few aspects of microbial life:

  • Why are soil microbes so important?
  • What is the Carbon to Nitrogen (C:N) ratio and why is it important to nutrient release?
  • Why add microbes to soils already teeming with native populations?

Why are soil microbes so important?

This can be answered somewhat concisely.  Microbial activity is the catalyst for transformation within the soil system.  Microbes excrete enzymes and other organic compounds that are responsible for much of the plant-beneficial activity within the soil profile.

Amongst an almost endless list of tasks they perform, these excretions:

  • Convert macromolecules into smaller molecules for easier and less energy-intensive uptake by the plant
  • Convert starch to sugar (the energy source for plants)
  • Break down chitin, a component of many pests and plant diseases
  • Make micronutrients usable by the plant
  • Act as a food source for different microbial species
  • Cycle nutrients in soil profile
  • Enhance organic matter decomposition
  • Form micro-aggregates in the soil which help to improve soil structure
  • Help to stabilize soil organic matter (SOM)

What is the Carbon to Nitrogen (C:N) ratio and why is it  important to nutrient release?

The C:N ratio is paramount to biological harmony in the soil.  By definition the C:N ratio is the ratio of the mass of carbon to the mass of nitrogen in a particular substance.  This is extremely important to remember, as we are asking microbes with a cellular C:N ratio of roughly 8:1, to consume residues and other materials with C:N ratios ranging from 11:1 to 82:1 and higher.


Table 1: the C:N ratio of crop residues and organic materials;


USDA Natural Resource Conservation Service

https://www. nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_021586.pdf

Why are C:N ratios so important to understand? 

The answer to the question above is quite simple –  microbes always eat first.  In soils with a very high C:N ratio, much of the N will be utilized by the microbes to keep themselves alive.   This can cause immobilization of N in the soil profile.  In some cases, it can tie up some of the N that you were anticipating for this cropping season.  In order to tap into this nitrogen reserve, there will be a certain period of time required for these microbes to satisfy their own N requirements before releasing N to the plant (mineralization).

Why add microbes to soil already teeming with native populations?

There is a war being waged in the soil profile between the beneficial bacterial and fungi (good guys) and the pathogenic bacteria and fungi (bad guys).  The addition of high concentrations of beneficial bacteria and fungi to the soil profile can help in the following ways:

  • Increase the odds of the good guys out-competing the bad guys for food and space. By utilizing the food sources present in the soil, these additional microbes (good guys) will help to increase the microbial community present, thus taking over the real estate around the plant’s root system, leaving the (bad guys) nowhere to set up camp.
  • Supplementing microbes along with a good biological food source can also increase native populations of bacterial and fungal microbes.
  • Increasing microbial populations will also increase the nutrient release around the new seed or seedlings. These microbes produce the enzymes needed for solubilizing N and P, breaking larger macromolecules into smaller more usable molecules, as well as helping the cycling of nutrients, etc.

Additionally, certain herbicides have been shown to reduce the diversity of beneficial soil microbial populations while increasing the populations of pathogens, such as fusarium, disturbing the natural order of the ecosystem.


As you can see, there is quite a lot happening in our soils.  Managing nutrient levels in the soil and understanding the levels of Organic Matter and TEC (Total Exchange Capacity), as we discussed in my last document, are very important. But it is only half of the story.  If we want to get the most out of our soils and crops, we need to recognize that there is a free work force present in the soil and they need to be managed for maximum efficiency.

Author: Jake Straub, Regional Sales Manager-Agricultural Products, Performance Nutrition