Fairy rings appear on the surface of the lawn as a circle whose perimeter is dark green foliage. The diameter of the circle varies from one place to another and from one lawn to another depending on the content of soil organic matter and the activity of soil fungi.
When the soil contains some organic matter from thatch, a dead tree trunk or dead tree roots, some soil fungi gather in circles to decompose that organic matter and release nitrogen that feeds lawn roots thus producing a circle of dark green foliage. If the content of soil organic matter is high, fairy rings persist for years.
Solutions: If you cut a tree, dig up the trunk and roots. If a lawn has a thick thatch, aerate frequently the soil to decompose its organic matter. Analyze the soil for nutrients. Neutralize soil pH. Do not over-fertilize with nitrogen.