Soil Bacteria

For every acre of garden, there is the equivalent of two mature cows, by weight, of soil bacteria living there. Ponder that a moment.

Biofilm of ‘Desulfovibrio Desulfuricans’, a soil bacteria believed to reduce toxic metals in soil and groundwater. Courtesy of Pacific Northwest National Laboratory.

Your average cow weighs about one ton. Two cows weigh about the same as a car. That’s a lot of soil bacteria! For a different view, you could fit 15 trillion bacteria into a single tablespoon, if nothing else was there.

What are all those one-celled creatures doing in your soil?

Bacterial behavior

Truth be told, much of your garden soil is made up of dead bacteria. Affectionately known as ‘bio bags of fertilizer’, soil bacteria are important players in nutrient cycling and decomposition. While still alive, their excretions improve soil structure by binding particles together into aggregates. This improved soil structure results in better water infiltration rates and it increases your soil’s water holding capacity. As bacteria breath, they release carbon dioxide into the soil. Plants love carbon dioxide.

Soil bacteria are most commonly found in the film of water that coats soil particles. Bacteria can’t move very far on their own. They generally move with water, though they sometimes hitchhike on passing worms, spiders, and insects. This is called phoresy.

Under ideal conditions, a single bacteria can produce 16 million copies of itself every 24 hours, doubling its population every 15-30 minutes. Conditions are rarely ideal, so bacteria reproduce as much as they can, whenever they can.

There are four basic groups of soil bacteria: decomposers, mutualists, lithotrophs, and pathogens. Most soil bacteria are beneficial. Pathogens are the troublemakers.

Beneficial decomposers

The majority of soil bacteria are decomposers that break down plant and animal debris into simple compounds which plants and other living things then use as food. This makes soil bacteria an important part the soil food web. Some decomposers can break down pesticides and pollutants. Decomposers also store a lot of nutrients in their bodies. When they die, those nutrients become available to your tomato plants. [Soil bacteria are 10-30% nitrogen.]

SEM image of bacterial biofilm on the surface of a sand particle. (Anthony D’Onofrio, et al) Public Domain

Mighty mutualists
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Mutualists have working arrangements with plants that benefit both sides of the equation. The most commonly known mutualists are the rhizobia bacteria which convert atmospheric nitrogen into a form useable by plants. Very often, these mutualists live on or in the roots of legumes, such as peas and beans. Other mutualistic soil bacteria are able to convert atmospheric nitrogen without the help of plants, but the plants still benefit.

Lithotrophs

You don’t hear much about lithotrophs, but this group is unique in that they don’t eat carbon compounds, the way other bacteria do. Instead, they manufacture their own carbohydrates, without photosynthesis, and feed on chemicals, such as hydrogen, iron, nitrogen, and sulfur. This group is also known as chemoautotrophs. These soil bacteria help break down pollutants and are an important part of nutrient cycling.

Apple twig infected with fireblight (Kate Russell)

Pesky pathogens

These are the disease-causing bacteria. They can cause fireblight, bacterial wilts, cankers, galls, and soft rot. The beneficial soil bacteria are always at war with these germs, competing for food, space, air, and moisture. 

Killing bacteria is difficult. Most often, if conditions become difficult, a bacteria will simply enter a dormant stage. This is why many Quick Fix treatments don’t work. They don’t kill the bacteria, they just send it on a temporary hiatus. There are some soil bacteria (Streptomycetes) that actively protect plants from bad bacteria.

Pseudomonas aeruginosa, which causes soft rot in plants (Janice Haney Carr, CDC) Public Domain

Why do soil bacteria matter to gardeners?

Most soil bacteria are valuable members of your team. They provide a huge benefit to your soil and plants. And you need to know what the bad bacteria look like when they start to set up housekeeping. The earlier you break those disease triangles, the faster your can return to harvesting your delicious crops.

Most bacteria are aerobic, which means they need oxygen. This is why turning your compost pile makes everything decompose faster. You are providing the decomposer bacteria with the air they need. If you don’t, the anaerobic, non-air breathing bacteria take over. Those are the ones associated with rot and purification. [Ew!]

Did you know that soil bacteria will consume more water than they can hold, causing their bodies to burst? Yet another argument against over-watering...