Trichoderma are fungi that colonize the root system. Their presence stops harmful fungi from colonizing the same root area. They effectively stimulate root development and increase a plant's ability to handle environmental stress i.e. High temperatures.
Mycorrhiza or "Fungus roots" which are symbiotic with the plant roots on which they occur. This mycorrhiza relationship facilitates the sharing of some of the plant's storehouse of organic compounds (which are essential to fungi, as they are to all living organisms). In addition, water is exchanged along with the organic compounds for assistance from the fungus in the absorption of nutrients like phosphorus and some other minerals. There are endo- and ectomyccorhizae, the endo- aiding in material uptake, the ecto- forming a rhizosphere protecting the roots from potential pathogens or diseases.
Bacteria fall into four functional groups. Most are decomposers that are especially important in immobilizing, or retaining, nutrients in their cells, thus preventing the loss of nutrients, such as nitrogen, from the root zone. They are also responsible for the biochemical breakdown of organic matter into organic compounds and nutrients, and ultimately into its original components. These "good" bacteria are called rhizobacteria, because they occur in the rhizosphere. These bacteria produce a variety of chemicals that stimulate plant growth. While common in natural settings, their populations are often very low or absent in nursery potting soils, urban environments and disturbed man-made landscapes.