Habitat: grasslands and pastures
Conservation Status: Not of concern
Edibility: Edible
Edible
Spores are chocolate-brown; growth up to 5 inches; cap diameter up to 6 inches. Agaricus macrocarpus grows slightly larger and in woods. Agaricus urinascens grows even larger and develops an ammonia smell with age. Agaricus xanthodermus has a bulbous base and flesh that turns bright yellow when cunt while releasing a chemical, ink-like smell.
Sources: Roberts, Peter and Evans, Shelley. The Book of Fungi. Chicago, The University of Chicago Press, 2011.
PNW Herbaria: Specimen records of Agaricus arvensis in the Consortium of Pacific Northwest Herbaria database
CalPhotos: Agaricus arvensis photos