Mycorrhizal fungi form a mutually beneficial relationship with a host plant's roots. Increasing the plants ability to uptake nutrients and water, plus helping to increase the plants resistance to diseases, pests, and drought. Fungal hyphae extend into the soil serving as an extension of the roots system, allowing it to essentially mine for nutrients and water in the soil at greater distances than would be possible without this connection. 
Roots of a kale transplant grown in soil inoculated with mycorrhizal fungi
In old growth forests these fungal systems and networks are a way for the trees to communicate and share nutrients as well. There are more plants that share this connection, than don't. Over 90% of all plant species form symbiosis with soil fungi.  Found naturally in the environment, Glomus intraradices (also known as Rhizophagus irregularis) is a prolific endomycorrhizal fungi / arbuscular mycorrhizal fungi that can greatly benefit many different plants. It is added into soil mixes and soil amendments, helping plants to uptake phosphorus, zinc, potassium, sulfur, nitrogen and copper. There are many different mycorrhizal fungi that will create this beneficial relationship with your plant's roots. Glomus mosseae is another important one. The fungal hyphae extend far and wide making this exchange of nutrients and micronutrients possible, in return the plant provides carbohydrates for the mycorrhizae. Because the mycorrhizae can extend further than the plants roots, it increases the plants surface area within the soil for nutrient and water absorption.
Transplant hole sprinkled with a mixture of feather meal, bone meal, fish meal, fish bone meal, alfalfa meal, sulfate of potash magnesia, blood meal, bat guano, kelp meal, and gypsum. Also includes 14 different mycorrhizae, and 5 different beneficial bacterial. When inoculating your transplant hole, make sure your plants roots come into direct contact with the mycorrhizae.
In order to thrive and create this symbiotic relationship the environment has specific needs that have to be met, such as no tillage. Because tilling breaks up the soil food web and the fungal hyphae it immediately affects the health of your plants and soil, and takes time, potentially years, to rebuild. Lowenfels and Lewis wrote, "If the salt based chemical fertilizers don't kill portions of the soil food web, rototilling will. This gardening rite of spring breaks up fungal hyphae, decimates worms and rips and crushes arthropods. It destroys soil structure and eventually saps soil of necessary air." (Lowenfels and Lewis, p. 27).  You can build your soil naturally instead of tilling or using chemical fertilizers. It takes patience but can be done very easily, through using compost, mulch, and natural soil amendments. Your plants will be much healthier, and you will be taking the necessary steps towards creating a living soil environment. ------------------------------------------------------------------------------------------------- References:
1 Buechel, Troy and Bloodnick, Ed (2016, April) Mycorrhizae: Description of Types, Benefits and Uses. Retrieved from https://gpnmag.com/article/mycorrhizae-description-of-types-benefits-and-uses/
2 Behie, Scott W. and Bidochka, Michael J. (2014) Nutrient transfer in plant–fungal symbioses. Trends in Plant Science, Volume 19, page 1. Retrieved from https://aglifesciences.tamu.edu/rootbiome/wp-content/uploads/sites/38/2015/06/2014-Behie-Trends-PLS-plant-fungal-symbioses-1-s2.0-S136013851400168X-main.pdf