Explanation of microbial amendments
The term mycorrhizae means “fungus roots” in Greek and refers to a mutually beneficial association between mycorrhizal fungi and plant roots
Such symbioses have been found in nearly all families of crop plants (the primary exceptions being spinach and canola). Mycorrhizal fungi may colonize roots of the host plant internally (endomycorrhiza) or externally (ectomycorrhiza). As mycorrhizal fungi colonize plant roots, the volume of soil that plants can explore for nutrient uptake of phosphorus (especially) and some other mineral nutrients is expanded. Thus, mycorrhizae as members of the soil community help ameliorate plant stress associated with low soil levels of various mineral nutrients, and they can further improve plant growth and yield. Mycorrhizae also contribute to soil aggregate formation by providing aggregate formation nuclei and producing glomalin, a sugar-protein compound that works as a stabilizing coat on soil aggregates. And some strains have been found to be significant contributors to plant disease suppression, though the conditions under which such benefits are conferred are not entirely clear.
Rhizobia and Associative Nitrogen Fixers
Rhizobia are soil bacteria that form symbiotic relations with plants (mostly with legumes). They belong primarily to the genera Rhizobium, Azorhizobium, Bradyrhizobium, and Sinorhizobium. Rhizobia first invade plant roots and form nodules in which they convert atmospheric nitrogen into plant-accessible forms of nitrogen (Figure 3). There are also some Actinomycetes strains that form nodule-like structures on non-leguminous woody shrubs or trees such as Alnus (alder) and Malus (apple).
Responses to inoculation are greatest on land during the initial plantings of a particular legume species (different inoculants are required for different crops). However, enhanced yield responses have been seen with regular inoculation as well. In contrast, associative nitrogen fixers (Azospirillium, Azobacter, and Acetobacter) colonize close to the roots but do not form nodules. Also included are blue-green algae, which are also bacteria that fix nitrogen. Rhizobia and associative nitrogen fixers can provide substantial nitrogen to plants and soil without additional nitrogen fertilizer input. Nodule-forming symbiotic nitrogen fixers can provide 45–360 lbN/A/yr on legumes and 18–270 lbN/A/yr on non-legumes. Nitrogen input from associative nitrogen fixers can range from 9–180 lbN/A/yr depending on soil composition. The actual