The real deal is that without soil microbes, we would all die. The work they do in our soil is incredibly complex but it all boils down to this: microbes eat, thus we eat. Plants are unable to take from the soil the nutrition they need without microbes working in the soil. Microbes are alive, and must have nutrition to survive, and that nutrition comes from organic matter. As they consume the nutrients they need, microbes create foods like nitrogen, carbon, oxygen, hydrogen, phosphorus, potassium and trace minerals for our plants. It is the microbes that convert the NPK and minerals in the soil into a form our plants can use to grow and produce food and flowers for us.
Microbes are everywhere. They are in the air, in the rivers and oceans, in our drinking water, in the soil, and on our skin. Of course we know some microbes are bad, like e. coli and salmonella, but more are considered beneficial and out-compete pathogens for survival in the soil. There are all kinds of microbes, like algae, protozoa, bacteria and fungi, with many others waiting to be discovered. Their populations in soil are numerous: as many as one billion of up to 13,000 species can reside in a single gram of soil. (1 gram = 0.0022 pounds, so maybe a teaspoon?)
Muck from horse stalls
Most microbes need organic carbon to live; they get this from eating wood chips, leaves, manures and other organic materials added to the soil. As microbes digest organic matter, they create humus which increases soil structure, good for root penetration and development. (Compaction can nullify much of this action.) Microbes also get some carbon from the rhizosphere (the area immediately around plant roots) because roots give off substances the microbes can use, like sugars and amino acids... and then the microbes convert some of it back in forms the plants can use, as minerals, vitamins, nitrogen and amino acids. (Amino acids are the building blocks of protein. Humans need 20 amino acids to make muscle, hair, skin, and connective tissue, and human bodies only make 10 of them. The others must be supplied by food from plants and animals.)
Some microbes (like some bacteria and blue-green algae) are able to “fix” nitrogen from the air and make it available to plants. Some plants and trees cannot grow if deprived of specific microbes (mycorrhizal fungi) around their roots. That’s why some plants need a good shovelful of additional soil from around their roots for company when transplanting. There are microbes that break down contaminants and toxins, like oil spills and toluene from gasoline leaks. The word for that action is bioremediation and research is ongoing to select microbes that digest other toxins in our soils.
There are many garden products available containing beneficial microbes for the soil. Some are in the fast-acting form of foliar sprays, and some are home-brewed compost teas used to spray or drench. In all cases, foliar sprays are not enough; there must also be organic matter continually available in the soil for the microbes to eat. The leaves, compost and/or manure you added last summer need continual replenishment. Microbes multiply, and if your microbe population is low due to lack of organic matter, it can be easily rectified by amending the soil with organic matter and allowing time for microbial growth. Jump-starting the reproduction of microbes by adding beneficial microbes along with organic matter is an option depending on how soon you want to see results, and the cost.
We can readily say that microbes are the workhorses of our gardens. Microbes make nutrients in the soil available to plants in a form the plants can use. Microbes create some of those nutrients, and we (with Nature’s help) add the rest. Because the plants are healthier, they resist disease better, and tolerate environmental stress better. Microbes improve soil structure by the humus they create while digesting organic matter. Microbes help in nitrogen fixing.
 Microbes in soil and sand-based root zones, by David A. Zuberer. Soil and Crop Sciences, Texas A&M University
On the Horse muck heap, iStockPhoto.com # 5392702, Used by Permission
Garden Waste, iStockPhoto.com #1260031, © Loretta Hostettler, Used by Permission
Home composting, iStockPhoto.com # 6781437, © Sebastien Cote, Used by Permission
Bacteria flowing (green), iStockPhoto.com # 6906975, © David Marchal, Used by Permission
3D bacteria, iStockPhoto.com # 4556554, © Chris Dascher, Used by Permission
Fungi (Macro), iStockPhoto.com # 2190943, Used by Permission