We all have it—a range of temperature and humidity that we feel most comfortable and perform most efficiently within. Plants have it, animals have it and so do bacteria—every living thing has an optimal comfort zone.

For most living things their comfort zone can be defined as a particular combination of temperature, humidity and air—all obviously necessities for life here on Earth, but different organisms require different permutations of this formula.

Bacteria are no different—they need the right combination of conditions to thrive like all living things. Despite their tiny size they are crucial to successful plant growth and under the proper conditions can accelerate and increase plant growth rate and yield.

When it comes to growing plants, in fact, bacteria play a role that is out of all proportion to their size. The successful gardener will make a point of taking care of these smallest of creatures and many larger issues will take care of themselves.

What are bacteria and why do our roots need them?

Bacteria are tiny one-celled organisms—generally about 4/100,000s of an inch wide (one µm) and somewhat longer in length. What bacteria lack in size they make up in numbers. A teaspoon of productive soil generally contains between 100 million and one billion bacteria.

Every grower should know the basics about bacteria and how critical the right organisms can be to successful root growth and development. As growers we are in control of our bacteria populations and the types of bacteria we make available to our crops are governed to a large extent by the conditions we keep our plants’ roots in.

Bacterial populations vary based on three primary soil conditions—moisture, temperature and aeration. We have direct control over these conditions and we need to manage them properly—a consistent root zone environment that is ideal for bacterial growth will result in a proliferation of beneficial bacteria and lead to healthy roots and plants.

Soil moisture and bacteria

The moistness of the soil is one of the three most important factors influencing the microbial population of your garden. Water (as soil moisture) is essential to healthy bacteria in two ways: it supplies hydrogen and oxygen and serves as a solvent and carrier of food nutrients.

Beneficial microbial populations proliferate best in a moisture range of 20 to 60 per cent. Under waterlogged conditions anaerobic microflora become active due to lack of soil aeration and the ‘good’ aerobes get suppressed—some beneficial microbes will die out due to tissue dehydration and some will change their forms into ‘resting stage’ spores or cysts in order to survive adverse conditions.

That’s why you shouldn’t overwater your plants—the soil in your garden should remain consistently somewhere in this optimum range of between 20 to 60 per cent moisture in order to promote the activity and increase of beneficial bacteria.

Soil temperature and bacteria

Next to moisture, temperature is the most important environmental factor influencing the biological, physical and chemical processes that govern microbial activity and populations in soil. Though some microorganisms can tolerate extreme temperature conditions, the optimum temperature range at which beneficial soil microorganisms can grow and function actively is actually rather narrow.

There are three soil temperature ranges within which microorganisms can grow and function, which divides microbes into three groups: psychrophiles, which grow at temperatures below 50°F, mesophiles, which thrive between 50 to 113°F and thermopiles, with an optimum temperature range between 113 and 140°F.

Most soil microorganisms are mesophilic and their optimum temperature is around 98.6°F. True psychrophiles are almost absent in soil and thermopiles—though present in soil—tend to behave like mesophiles. True thermopiles are found more often in decaying manure and compost heaps, where high temperatures prevail.

Seasonal changes in soil temperature affect microbial population and their activity, especially in temperate regions. In winter, when temperature is low, the number and activity of microorganisms decline and as the soil warms up in spring they increase in number as well as activity.

In general, the population and activity of soil microorganisms are highest in spring and lowest in winter, but growing in a greenhouse or under controlled conditions will provide consistent soil temperatures in a managed soil environment—eliminating these extremes.

Soil temperature greatly influences the rates of biological, physical and chemical processes that take place in the soil. Within a limited range, the rates of chemical reactions and biological processes double for every 10 degree increase in temperature. Different pathogen species have different thermal limits for survival, germination and infection, so temperature can also control soil-borne diseases.

Soil aeration and microorganisms

For the optimal growth of microorganisms, good aeration in the soil is essential. Microbes consume oxygen from the air found in soil and exhale carbon dioxide. The activity of soil microbes is often measured in terms of the amount of oxygen absorbed or the amount of CO2 exhaled by these organisms in the soil. Under waterlogged conditions gaseous exchanges are hindered and an accumulation of CO4—which is toxic to microbes—occurs in the soil air.

Depending upon their oxygen requirements, soil microorganisms are grouped into three categories: aerobic (requiring oxygen), anaerobic (not requiring oxygen) and microaerophilic (requiring low levels of oxygen). The ones your plants need to thrive—the ‘good’ bacteria—are the aerobic microbes.

Using devices or management practices that provide optimal moisture, temperature and aeration conditions in your garden soil will help to foster a healthy population of beneficial bacteria and microbes—you will see healthier plants and use less water and fertilizer while achieving improved results and production.

Bacteria are the most critical elements of a healthy soil and root system. Healthy soil equals healthy plants; it's that simple. What happens below the soil surface is just as important as what happens above—and it’s all based on the health and vigor of the lowly bacteria.