Soil microbes

Many types of microscopic life live in the soil, such as the fungi pictured here, and provide multiple benefits to gardens.

If you look at soil close enough, through a microscope of course, you will find some of the greatest of gardening assistants hidden among the many forms of microbial life that make their home in your garden.

Of the many microscopic life forms found in soil, a large portion of them are highly beneficial to plants. From the mycorrhizal fungi to the nitrogen fixing bacteria, these tiny garden assistants perform valuable service that makes it much easier for us to grow healthy plants. Successful gardening often requires that we take good care of these “friendlies” in the soil.

Not all of the soil dwellers, of course, are beneficial. Included within the soil microbial world are the plant pathogens, those that attack plants and cause plant diseases. Some of them are quite deadly and we must strive as gardeners to minimize or eliminate their interference. By properly managing fertilizers, water, and other inputs we can often avoid their nasty effects.

Nevertheless, these disease agents manage occasionally to get a toehold and cause problems for their targeted plants. Sometimes these diseases will simply slow down the plant. Sometimes they kill them. Among our local soil-dwelling pathogens are the many once-celled fungi that cause root rot. Others types, once inside the plant, clog up water conducting tubes or cause some other kind of health problem. These are not friends, for sure. Fortunately, many of the beneficial soil microbes, our garden assistants, help plants ward off these diseases by offsetting their activities or even by attacking the pathogens themselves.

Helpful microbes perform many other tasks. Some help the plants take up water and nutrients more efficiently. That is what the mycorrhizal fungi do best. I am sure you will recall the Rhizobium bacteria that exist in the roots of the many members of the pea family. They help by converting free nitrogen into a nutrient that the plants can easily use. Stated more simply, they provide free food to their plant hosts.

Other microbes enable plants to cope with drought and extreme temperatures. In the desert, we need those types around, don’t we? Still others break down harmful toxins. Some stimulate plant growth while others turn phosphorus and other minerals into forms that plants can use. Perhaps the most important task of the microbial world is to decompose even the toughest of plant residues and break them down into humus and other materials that improve the health of soils. All of these, and many more important tasks, are done quietly and efficiently by these soil-dwelling assistants.

It makes good sense then, as we manage the soil microbes in our gardens, to provide the conditions that encourage their activity and reduce or eliminate the conditions that favor the non-beneficial soil microbial life. We need to better understand these invisible life forms and what we can do to favor their growth.

First of all, we need to know that these different forms of soil life are plentiful. There are a lot of them. Some estimates place the number of soil bacteria alone in one gram of soil at approximately ten billion individual cells. One gram of soil, we realize, does not take up a lot of room. It is hard to imagine that there would be enough space for that many of them to do their thing. Yet, there are more. In addition to the bacteria, there are literally hundreds of different types of fungi, algae, and protozoa in that same space. Obviously, the traffic can get pretty congested down there.

If we are going to take care of all this livestock, it helps to know where in the soil they are found. By far, most of them live within the upper level of soil where the roots of plants grow. They are most prevalent there because it is in the root zones of plants that sufficient water, air, and food supplies necessary for their growth are most commonly found. How does this come about?

Roots growing through soil release chemicals to encourage microorganism growth. Cells scraped off of the roots during growth become a source of food for the microbes. Rainfall and irrigation water help keep the soil moist. Compost is added to the surface of soils and leaches down into the root zone. Roots open up channels in the soil through which air and water can penetrate. When the roots die, they provide still more food material for the microbes to break down and digest. All of these conditions together help create an environment that sustains the growth and development of these beneficial microbes.

What can you and I do to help encourage these unseen friends? First of all, we can manage our garden environments correctly. Every time we add compost or leaf litter to the soil; every time we correctly irrigate our plants; every time that we till the soil to aerate the root zone; we are helping these garden assistants to grow, develop, and perform their life functions. When that happens, we benefit, and so do our plants.

Unfortunately, it does not help if we fail to do these tasks consistently and regularly. A skipped irrigation, a postponed tillage, a forgotten application of compost and soil microbial activity can slow way down. Once it grinds to a stop, it is hard to start these factories again. A good garden calendar to help remind us of what needs to be done can be a critical tool. It helps us ensure that these unseen friendlies in the soil, while they might be out of sight, are not out of mind.

Still, it is important to remember that even when we do everything right, not everything may go according to plan. The soil is still a biological system. There are many factors that can interrupt the workings of the microbes down in the soil. Pathogens can overwhelm the defense. Cold weather can slow down activity. Internal forces, such as pH, soil moisture, and availability of food sources can shift microbial populations from one type of microbe to another. There is just no telling what can or will happen. Nevertheless, by managing the soil conditions to benefit the soil microbes, we can generally provide the right environment for them to thrive. In so doing, we reap the benefits of their help.

By understanding the microscopic life that lives in the soil, and learning to work with them, we can create healthy conditions in our soils and enlist the aid of these unseen assistants to help improve the health of our garden and landscape plants.


If you have questions, you can reach one of the Master Gardeners at the Cooperative Extension office, 820 E. Cottonwood Lane, Building C, in Casa Grande. The telephone is (520) 836-5221, 204.