Soil Microbes
Keeping their Populations Healthy

In organic lawn care and organic gardening, beneficial soil microbes are the driving force that makes everything happen. They are of absolute importance. Without these microbes, there can be no mineralization of elements necessary for plant health.

It is almost beyond imagination to consider that only one ounce of healthy, fertile soil can contain billions of tiny beneficial bacteria. Of all the types of soil microorganisms, beneficial bacteria are by far the most abundant, and in some ways, the most important.

There is much talk about purchasing microbes to increase the populations of soil microorganisms. For those who wish to consider that, companies like PHC (Plant Health Care) is a reputable distributor of many types of beneficial bacteria and fungi. Simply adding water to the measured ingredients forms a solution that can be sprayed or poured.

This is important to consider: Many organic sites state that simply adding microbes will make a remarkable difference in plant health. This is not always true. In some cases the additions of certain microbes will help greatly, but in other situations it will only be temporary at best.

Below are some important questions to consider that can also help test your knowledge of soil microorganisms.

  • Do microbes always work as they are expected regardless of the conditions or temperature?
  • If your soil's microorganism levels were actually low, (which is difficult to tell without a specific test) will simply adding more microbes be the solution?
  • Are all microbes the same in importance or function?
  • What situations may indicate that soil microorganisms are not what is needed?
  • What is the most important fungi that is often added when transplanting trees and plants.
  • Do microbes favor a healthy, vigorously growing turf over a poor quality turf? Does it matter?

You will find the answers to these questions within the text below.

Keeping Soil Microbe Populations Healthy

When talking about increasing microbial activity, it is also important to remember the other factors that must be considered. Beneficial soil microbes are living organisms that are subject to their environment.

Like most all living organisms, they must have air, water, acceptable soil temperatures, proper soil chemistry for growth, and an abundance of food to survive and grow. If one of the necessary items are missing, the soil microorganisms will decline.

Remember, the work associated with these beneficial soil microbes are the only means of converting organic elements into forms the roots can take up and use. If they decline, so will your grass and other plants.

As far as turf quality goes, healthy turf is capable of sustaining tremendously larger amounts of soil microorganisms than poor quality turf. It is exactly the same for healthy forests, grasslands, marshes, etc.

Soil Microbes Need Sufficient Oxygen

Ensuring that the soil has plenty of oxygen is extremely important. This is especially true for compacted soils. Core aeration opens the soil allowing more air and water to reach the roots and is a great benefit to aerobic bacteria. (aerobic means the bacteria are of the type that need oxygen to survive.)

In compacted or waterlogged soils, standing water can be a problem. You have probably seen a black film left behind after standing water has evaporated. This is caused by anaerobic bacteria that thrive is oxygen depleted soil.

There are also liquid spray on products that are supposed to provide aeration, but not all work. Some that do are short lived. You will have to experiment with the better products if liquid aeration is the method you choose.

Soil pH and Bacteria Decline

Since most beneficial bacteria thrive best in neutral to near neutral pH (6.5 - 7.5), when it starts to get far away from that the populations can become stressed.

Important: In low pH soils fungi begin to dominate as bacteria decline. This is one reason why people can apply organics, but not see much happening. It doesn’t matter how many microbes you add to the soil, if the pH is too far off, they will not thrive. Once the pH problems are fixed the bacteria quickly rebound.

Soil Moisture

Without moisture, soil microbes cannot survive. In the absence of rain, watering becomes necessary. A common problem is shallow watering. With shallow irritation the water is not absorbed deeply into the soil. As the sun heats up the soil, the moisture quickly evaporates. Deeply watering provides moisture deeper into the soil and keeps the microbe populations happy.

Soil Temperature

Soil microbes thrive most in 70 to 90 degree temperatures. The hottest part of the summer is the busiest time for microbes, provided the other elements mentioned here are in order. Colder locations or places with long winters, such as in Canada or in parts of the Northern U.S., can actually see over accumulations of organic matter due to the shorter period of microbe activity. In contrast, year around tropical climates consume carbon so quickly that it can be difficult to provide enough.

The Most Important Element - Carbon

However, with all things equal, the most limiting factor in microbe populations is available carbon. Compost, which is decomposed organic material, does not have quite as much available carbon as you might think. The work of soil microorganisms during composting returns about one third of the carbon back into the air as carbon monoxide (CO₂). While compost still provides a good amount, it is extremely valuable in amending soil composition.

The majority of available carbon consumed by soil microbes comes from shed roots, decomposing organisms, clippings, fallen leaves, etc. Aged manure may have more available carbon compared to compost, but it is not as fun to work with.

Generally speaking, as far as soil microbes are concerned, additions of compost will increase the populations of microorganisms. This will continue until soil or environmental conditions change that do not favor them or until all the substrates in the compost are consumed.

Material left behind after the microbes consumed the available carbon becomes part of the humus fraction. (That is a good thing.) As the food source is steadily consumed, along with the depletion of food will come the depletion of soil microbes. Eventually the microbes themselves will be consumed by other soil microbes until there is a microbial balance based on the available food supply.

Mycorrhiza Fungi

Most of the above material is about bacteria. However, mycorrhiza is a naturally occurring beneficial fungus that is just as important as bacteria, but functions differently.

Mycorrhiza works by attaching directly to the roots. It is often lacking, especially on new lawn where the soil is not good. Mycorrhiza forms long chain-like structures on the roots of plants that actually helps funnel nutrients back to the roots.

This beneficial fungi can be added to your soil or applied in other ways. It comes in gel form as a bare root dip, made into a liquid for soil injection, or can be applied as a soil drench.

Short Recap of Important Material

  • Adding beneficial soil microbes to the soil without providing addition food needed to sustain the microbe population may only be a short term benefit to your soil or plants .
  • Adding soil microbes to the soil without correcting imbalances that may be present, such as low pH, low soil moisture, compacted soils, etc. will not greatly benefit your soil or plants no matter how many microbes you add.
  • Soil microorganisms are living, breathing, organisms that need a suitable environment to thrive. As long as the right elements come together with an ample food supply, beneficial microbes will usually increase on their own.
  • Adding soil microbes will benefit your plants when the other environmental conditions are good. It can greatly speed the build-up of beneficial microorganisms when conditions are right.
  • Providing compost, manure, or other sources of carbon (food for microbes) will provide a natural increase in soil microbes. Adding additional microbes may speed the increase.

Do not forget about aged manure as a topdressing as well. It often has more available carbon than compost, but it is not as nice to work with. However, many farms may offer it for free.

Understanding Soil Microorganisms

Soil microorganisms are one of the most important elements of a healthy soil. A good lawn care program will use the many benefits of soil microbes.

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