30 June 2019

“Fairy Rings” or “Hexinringe” and the Role of Fungi in Weathering and Soil Fertility

Posted by John Freeland

As our three-year old chocolate-golden Labrador mix looks happiest when he’s running, we try to get him out to a park once or twice a day. While he’s chasing noisy killdeer and attacking discarded plastic water bottles, I have a chance to look at the land and sky and assess the progress of the new growing season. In springtime, at one of the parks near our home in Lenawee County, Michigan, strange dark green circles emerge in the turf grass, persisting into the summer. The circles consist of darker, greener, more robust grass surrounded by shorter, lighter green grass. Some of the circles are entirely closed rings while others are incomplete and form crescent shapes. Some double up as overlapping hoops. They vary in diameter with some upwards of about twenty-five feet (8m), or so. Most are a bit smaller.

Figure 1. Soil fungus growing outward from a central origin produces “fairy rings” in turfgrass.

The dark circles have bright white mushrooms in them. Our dog, who ordinarily seems willing to eat most anything, leaves the mushrooms alone. While some of these ring-forming mushrooms are apparently safe for eating, some are deadly and the ones that destroy your liver might look a lot like the ones that don’t.

Figure 2. Mushrooms emerge in the darker, thicker grass, which benefits from nutrients released by fungal mycelia in the soil

These structures have a rich folklore in Europe, for example, the German term for them is “Hexinringe, or “witches’ rings” where witches gather to dance. Some traditions regarded them as entrances to the supernatural realm, dangerous places where, if entered, the unfortunate pedestrian became cursed with a shortened life expectancy.

As a simple work-around, supposedly, if the visitor wears their hat backwards, then the witches and fairies will be confused long enough for the “intruder” to make a clean get-away, without the curse.

From a more rational perspective, these rings are a reminder of the life that exists below the surface, in the dark, damp recesses of soil interstices where air, water, minerals, detritus and living organisms are involved in complex processes, releasing essential nutrients made available to plants for photosynthesis. Vital plant nutrients such as phosphorus, potassium, calcium, magnesium, iron, and sulfur are derived, ultimately from dissolution of rock minerals, including the following common examples listed with their chemical formulas:

Plagioclase feldspar (Ca, Na)(Al, Si)AlSi2O8.
Orthoclase feldspar KAlSi3O8
Hornblende Ca2Na(Mg, Fe)4 (AlFe) [(Si, Al)4O11]2 [OH]2
Augite Ca (Mg, Fe, Al) (Al, Si)2O6:
Biotite K (Mg, Fe)3, (AlSi3O10) (OH)2
Apatite Ca₅(PO₄)₃(F,Cl,OH)
Pyrite FeS₂

Mycorrhizae are fibrous fungal structures having an intimate relationship with plant roots. In mutualistic symbiosis, the fungi break down organic matter, mobilizing plant nutrients, and produce enzymes and organic acids that help dissolve soil minerals, making more nutrients available. More completely, as Finlay describes:

“(m)ycorrhizal fungi connect their plant hosts to the heterogeneously distributed nutrients required for their growth, enabling the flow of energy-rich compounds required for nutrient mobilization whilst simultaneously providing conduits for the translocation of mobilized products back to their hosts. In addition to increasing the nutrient absorptive surface area of their host plant root systems, the extraradical mycelium of mycorrhizal fungi provides a direct pathway for translocation of photosynthetically derived carbon to microsites in the soil and a large surface area for interaction with other micro-organisms.”

A basic understanding of soil fungi has serious implications as we consider how we are connected to the complex biosphere and, ultimately, the rock minerals that support it.