How do mycorrhizal fungi help plants absorb nutrients?

The answer

Mycorrhizal fungi form symbiotic relationships with plant roots in which the fungi extend ultra-fine hyphal threads, as thin as one-sixtieth the diameter of a root hair, into soil pores that roots cannot access. These hyphal networks can extend the effective absorptive surface area of a root system by 100 to 700 times. The fungi are especially adept at acquiring phosphorus, nitrogen, and water, which they exchange with the plant for carbon in the form of sugars, typically receiving 10 to 20 percent of the plant's photosynthetic output. This partnership is so ancient (dating back over 400 million years) and so fundamental that most plants cannot thrive without it, and some scientists argue that mycorrhizal fungi were essential to plants' original colonization of land from water.