Back when I was picking puffball mushrooms I found a long, vivid green swath of grass about two feet wide. I didn’t think much of it, but a few weeks later when I returned to that field I realized that the green grass formed a long arc about fifty feet in diameter. It puzzled me for a while, but going back over the arc I observed that all the puffballs in that field were along the path of the arc. I found puffballs in three fields this year, but only this one had any discernable difference in the grass.
Looking around for an explanation led me to a few interesting mushroom websites. These patterns are called fairy rings and they are indeed caused by fungal activity. The underground mycelial growth makes more nitrogen available to the grasses above (at least with the Calvatia (puffball) family — other fungi can create a dead-zone ring).
Fungi are fascinating mysteries to me. I wonder if we had more of these in our fields would it increase grass productivity?
Clearly the cattle can taste the diference. I moved the cattle in to glean the last scraps of grass out of this hayfield. Look where this cow headed. Note the green arc (sorry, the colors don’t differentiate well with mobile-phone photography). If you zoom in you might be able to pick out the spory old puffballs.