Plant roots perform double functions of anchoring the plants firmly to the soil and as organs for nutrients and water absorption. A nutrient element that is to be absorbed into the plant must be in a soluble form and must be located at the root surface, or within the area of the soil immediately surrounding the root hair.
This portion of the soil which is usually about 2mm away from the root surface is termed the rhizosphere.
Due to the activities of the root the rhizospheric soil has both biological and chemical properties different from the bulk soil. The soil in the rhizosphere differs from the rest of the soil in the following ways:
Soil acidity may be higher or lower in the rhizosphere than the bulk soil when plants take up Cationic elements such as K+, Ca+, Mg+, or HN+4 from the soil solution, hydrogen ions, H+, are released from the roots to balance the charge in the soil solution thus increasing the acidity.
On the other hand, uptake of anions (N03, N02, S0 =) makes the roots to exude bicarbonates (CH03–) in replacement thus lowering the acidity (increased PH level).
Due to uptake of dissolved nutrients by roots from this soil zone, nutrient concentration becomes low causing a form of the concentration gradient between the rhizosphere and the other soil volume. Nutrients then move from bulk soil to the rhizosphere by diffusion.
There is a higher concentration of energy-rich materials in the rhizosphere. These organic compounds include root exudates such as organic acids, sugars, amino acids, and phenolic compounds which are useful to the growth of micro-organisms.
The high concentration of root exudates and other high-molecular mucilages secreted by root cap cells give rise to a higher concentration of micro-organisms in the rhizosphere than the bulk soil.
The respiration of roots and microbes results in high concentration of carbon dioxide, (C02).
The PH of the area surrounding the root is also lower owing to the formation of carbonic acid.
Fungi usually have a beneficial association with plant roots to form mycorrhizae. These mycorrhizae accelerate the growth of certain trees spp such as pinus resimosa and Pinus carribea plant roots.
The fungal hyphae grow out into the soil to about 15cm from the root surface to scavenge for nutrients thus increasing the effective feeding zone of the root.
Fungi are also important because they excrete enzymes which dissolve nutrients in mineral soil. The xylem vessels conduct nutrients from the root to the shoot.