"Most gardeners think of plants as only taking up nutrients through roots systems and feeding the leaves. Few realize that a great deal of the energy that results from photosynthesis in the leaves is actually used by plants to produce chemicals they secrete through their roots. These secretions are known as exudates. A good analogy is perspiration, a humans exudate.
Roots exudate in the form of carbohydrates (including sugars) and proteins. Amazingly, their presence wakes up, attracts, and grows specific beneficial bacteria and fungi living in the soil that subsist on these exudates and the cellular material sloughed off as the plant's root tips grow. All this secretion of exudates and sloughing off of cells takes place in the rhizosphere, a zone immediately around the roots, extending out about a tenth of an inch, or a couple of millimeters. The rhizosphere, which can look like a jelly or jam under the electron microscope, contains a constantly changing mix of soil organisms. All of this "life" competes for the exudates in the rhizosphere, or its water or mineral content.
At the bottom of the soil food web are bacteria and fungi, which are attracted to and consume plant root exudates. In turn, they attract and are eaten by bigger microbes, specifically nematodes and protozoa, who eat bacteria and fungi (primarily for carbon) to fuel their metabolic functions. Anything they don't need is excreted as wastes, which plant roots are readily able to absorb as nutrients. How convenient that this production of plant nutrients takes place right in the rhizosphere, the site of root-nutrient absorption.
At the center of any viable soil food web are plants. Plants control the food web for their own benefit, an amazing fact this is too little understood and surely not appreciated by gardeners who are constantly interfering with Nature's system. Studies indicate that individual plants can control the numbers and the different kinds of fungi and bacteria attracted to the rhizosphere by the exudates they produce. During different times of the growing season, populations of rhizosphere bacteria and fungi wax and wane, depending on the nutrient needs of the plant and exudates it produces.
Soil and bacteria are like small bags of fertilizer, retaining in their bodies nitrogen and other nutrients they gain from root exudates and other organic matter (such as those sloughed off root tip cells). Carrying on the analogy, soil protozoa and nematodes act as "fertilizer spreaders" by releasing the nutrients locked up in the bacteria and fungi "fertilizer bags". The nematodes and protozoa in the soil come along and eat the bacteria and fungi in the rhizosphere. They digest what they need to survive and excrete excess carbon and other nutrients as waste.
Left to their own devices, then, plants produce exudates that attract fungi and bacteria (and, ultimately, nematodes and protozoa); their survival depends on the interplay between these microbes. It is a completely natural system, the very same one that has fueled plants since they evolved. Soil life provides the nutrients needed for plant life, and plants initiate and fuel the cycle by producing exudates."
There's the tip of the ice-burg for you. That is an excerpt from the very first chapter of the book "Teaming with Microbes" by Jeff Lowenfels & Wayne Lewis.
