Many synthetic fertilizer (chemical) regimens push top growth at the expense of root development, making plants vulnerable to stressful environments.  Frequent, high level of synthetic fertilizer produce an unbalanced and often unsustainable shoot-to-root ratio.  Mycorrhizae on the hand, feed your plants and simulate root growth.  Unlike Mycorrhizae, fertilizers cannot help prevent root disease, improve soil structure or promote other beneficial microbes.  Synthetic fertilizers can lead to other side effects, such as deterioration of water quality, soil structure and excess soil salinity.  The Mycorrhizal relationship improves feeder-root production, and a mycorrhizal plant can better utilize added fertilizer (preferably organic).  Many Organic fertilizers, when not overused can actually promote more microbial activity.