Green Arcs in the Lawn: Fairy Ring or Fertilizer?

Two Likely Causes

1. Fairy ring fungus (Marasmius oreades) – Forms arcs/circles of dark green grass, later browning. Mushrooms appear at edges. Dig out infected soil or apply labeled fungicide.
2. Subsurface nitrate release – Decaying roots or bird droppings create localized fertility spikes.

Management

• Harvest mushrooms before they spore.
• Acidify soil with iron sulfate.
• Improve drainage, sun, and air flow.
• Dry conditions may naturally suppress the fungus.

Without a sample, exact diagnosis is hard. But fairy ring is common in sandy soils with organic history.