Microbes in the soil are important for the nourishment of plants, and many of them facilitate the chemical conversions and physical transport needed to make nutrients available. Proponents of organic agriculture and others claim that soil microbes should supply all the nutrients needed by plants, and that applying soluble forms of plant nutrients harms the biology in the soil and reduces its capacity to make the native soil nutrients available. It is worth examining these claims more closely.

The microbes that supply nitrogen are from two categories: symbiotic and free-living. The symbiotic types are mainly rhizobial bacteria that infect the roots of legumes, such as ...