The rhizosphere

When we think of roots, we should also think of the rhizosphere which is a very important, tiny zone, about one millimeter in width, full of activity and surrounding the white feeder roots, especially root hairs as well as the outer hyphal cells of mycorrhizae that may also be present, as can be seen in figure 2. The rhizosphere is a very dynamic environment inhabited by a constantly changing mixture of microorganisms such as bacteria , fungi, protozoa, slime molds, algae, nematodes, soil viruses and microscopic insects and animals which are in constant competition with each other for water, food, and space. Another major characteristic of the rhizosphere’s environment is the presence of exudates from the stored organic compound released by the roots. These exudates, with their carbonic acid ions, increase the availability of nutrients, and also provide a food source for rapid multiplication of microorganisms in quantities much greater than what is available for those thriving in the other parts of the soil. Furthermore, the rhizosphere is the environment where root hairs and mycorrhizae absorb nutrient elements into their cells. The concentration of nutrients in the rhizosphere being higher than in the surrounding soil, makes it possible for water to move by diffusion towards the roots and rhizosphere.

Maintaining a normal equilibrium between roots, nutrients and the many types of microorganisms present makes the rhizosphere quite fragile, and it can completely change every time a treatment is carried out on the tree or in the soil. When a tree’s health declines, there usually is a problem somewhere in the rhizosphere. For example, if something goes wrong at the foliage level of the tree, like disease or too much loss of leaves, less carbohydrates are manufactured, depriving the roots of enough energy for growth and defense. The decline is more obvious if pathogens invade the weakened roots. Furthermore, any kind of root injuries, causing easy entrance for pathogens, or any soil treatments that kill beneficial microorganisms in the rhizosphere, may have ill effects on the health of the tree. Therefore, a good knowledge of the rhizosphere gives us a better chance to know if our soil treatments are beneficial rather than harmful to the tree.