Affects of Soil Microorganisms
On Plant Health and Nutrition

Soil microorganisms, sometimes spelled as soil micro-organisms, are a very important element of healthy soil. Knowing what microbes in soil eat, the conditions they thrive in and the temperatures that they are most active in is important in organic gardening and organic lawn care. From a practical standpoint, it boils down to organic matter, but not just any organic matter. These facts below will help you plan your activities around the time they are most beneficial. Below is a partial list of important functions they perform.

Soil microorganisms are responsible for:

  • Transforming raw elements from one chemical form to another. Important nutrients in the soil are released by microbial activity are Nitrogen, Phosphorus, Sulfur, Iron and others.
  • Breaking down soil organic matter into a form useful to plants. This increases soil fertility by making nutrients available and raising CEC levels.
  • Degradation of pesticides and other chemicals found in the soil.
  • Suppression of pathogenic microorganisms that cause diseases. The pathogens themselves are part of this group, but are highly outnumbered by beneficial microbes.

Types of Microorganisms in Soil

There are several types of microorganisms in soil that benefit plants. Together they make up an immense population of living organisms. One teaspoon of soil may contain millions of various types. Below is a list of common soil microorganisms found throughout the world.

  • Bacteria – Small, single cell organisms that make up the single most abundant type of microbe. They have a very wide range of conditions that they live in from the artic wastelands to the steaming waters of volcanic hot springs. In soils, they multiply rapidly under the proper conditions. When conditions are wrong for one species, it is right for another. This is not always a good thing since a balance is what is required.
  • Fungi – The largest microbe group in terms of mass. Some fungi are beneficial, called mycorrhiza, that form a symbiotic relationship with plant roots, either externally or internally. Within the fungi group are pathogen fungi. These are disease causing fungi, some of which can be quite devastating to plants.
  • Protozoas – Small single cell microbes that feed on bacteria.
  • Actinomycetes – Necessary for the breakdown of certain components in organic matter.
  • Algae – Beneficial groups such as blue-green algae, yellow-green algae and diatoms. Some of these can produce their own energy through photosynthesis.

Soil microorganisms are living, breathing organisms and, therefore, need to eat. They compete with plants for nutrients including Nitrogen, Phosphorus, Potassium and micronutrients as well. They also consume amino acids, vitamins, and other soil compounds. Their nutrients are primarily derived from the organic matter they feed upon. The benefit is that they also give back or perform other functions that benefit higher plant life.

What is Organic Matter

It is a variety of natural substances including decomposed leaves, grass clipping, shed roots, wood chips, etc. Humus (well decomposed organic matter) is the richest source for plant growth. Organic matter comes in many different nutrient levels, especially Nitrogen. While soil microbes need carbon (C) to live, they also need the nitrogen contained in organic matter. Therefore, the Carbon to Nitrogen ratio (C:N) is very important.

Problems with Low Nitrogen Organic Matter

Organic matter low in Nitrogen will also have a slower breakdown rate than organic matter with higher nitrogen levels. The microbes will consume the Nitrogen element first and the grass will get what is left over. So organic matter high in carbon and low in Nitrogen will provide little nitrogen to the grass. However, if another N source is applied over the organic matter, it will speed up the decomposition.

Therefore, the rule of thumb is to choose an organic matter with higher levels of nitrogen. Anything lower than four percent (4%) Nitrogen with high Carbon content should be considered a soil amendment and not a fertilizer.

How Temperature Affects Soil Microbe Activity

Soil Microbe activity is dependent on soil temperatures. For simplicity, all essential soil microbes are classified into the three different temperature ranges they are most active in.

  • Psychrophiles – active in temperatures less than 68 degrees.
  • Mesophiles – Active in temperatures between 77 degrees and 95 degrees. This makes up the largest group of soil microbes and the range most activity charts are based on.
  • Thermopholes – Active in temperatures from 115degrees to 150 degrees. From a plant and landscape view, this group will rarely apply.

Since the primary group contains Mesophiles, this has a great influence on the degree of soil microbe activity. Areas where the temperatures are warm most of the year, organic matter can be consumed very quickly. Tropical rainforests are so lush in part because of consistently warm temperatures, which promote fast breakdown of organic matter and the release of nutrients into the soil.

Cooler areas, such as Canada and parts of Europe, that have extended periods of cool weather below 77 degrees will benefit far less from the additions of organic matter. This is because far fewer microbes are active in that temperature range. It is possible to build unhealthy levels of organic matter if you follow the example of those in warmer climates.

The scientific rule is this: With every 18 degree rise in temperature, from 32 degrees to 95 degrees, there is a 1.5 to 3.0 % increase in microbial activity. (Carrol/Waddington/Rieke)

Remember, the food availabity to microbes, the quality of organic matter, soil types, pH level, percent of Nitrogen, etc. will also have an effect on microbial activity level.

Soil pH Factor

Most soil microorganisms can tolerate a wide range of soil levels. However, bacteria favors a neutral to slightly alkaline soil up to 8.0. When pH drops below 6, fungi begin to dominate as bacteria finds it less favorable.

Soil Moisture

Just as temperature levels stimulate different soil microbes, so does soil moisture. Some are obvious. Persistent, damp conditions with heavy shade will promote the growth of algae while hindering microbes that thrive in sunny locations. Proper lawn watering requires deep watering so that the soil is wet 4 inches deep. Shallow watering means only the surface is wet. It drys out quickly and can greatly hinder soil microbes.

Oxygen levels necessary for healthy microbes

There is a balance to everything, including oxygen in soil. Compacted soil will have less oxygen and less water holding capacity. Clay soil consists of extremely tiny particles, even smaller than silt. Clay with proper structure can have sufficient oxygen, but it can also compact easily. Since soil microorganisms consume oxygen, but low oxygen soils will quickly deplete what oxygen it has and lower the soil microorganism levels. In lawns, deeply water to a level of 4 inches deep and allow it to dry before watering again. The deeper soil will remain moist at sufficient levels. Shallow watering means when the surface moisture is gone there is no moisture deeper to support a healthy microbe population.


Well decomposed organic matter is the oldest form of biostimulant and very effective. Modern advancements allow us to apply specific bacteria and other ingredients necessary for healthy plants. Other than organic matter, humic acid is a well performing biostimulant for lawns and gardens that has shown to stimulate soil microorganisms.

Humic acid is known to:

  • Enhance soil nutrient content.
  • Increase CEC.
  • Stimulate and increase microbial numbers.
  • Increases soil moisture holding capacity.
  • Improves soil structure.

Soil microorganisms are one of the most important elements of a healthy soil. A good lawn care program will take advantage of the many benefits of soil microbes and the biostimulants that encourage them.

References (Turfgrass Soil Fertility and Chemical Problems - Humates/Humic Acid - R.N. Carow/D.V. Waddington/P.E. Rieke P.277)

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