A fungus that is to a greater or lesser extent always available in most all soil. Many quality potting mixes have it added now a days. I intentionally add it on a regular basis to transplants. I use the product Mycos.
From this web site (lots of good info here):
http://mycorrhizae.com/
“Mycor” – “rhiza” literally means “fungus” – “root” and defines the mutually beneficial relationship between the plant and root fungus. These specialized fungi colonize plant roots and extend far into the soil. Mycorrhizal fungal filaments in the soil are truly extensions of root systems and are more effective in nutrient and water absorption than the roots themselves. More than 90 percent of plant species in natural areas form a symbiotic relationship with the beneficial mycorrhizal fungi.
Example with (left) and without added mycorrhizae.
Non-Commercial info here on benefits:
http://aggie-horticulture.tamu...
