Mushrooms and toadstools are merely the above-ground growths of fungal organisms living in soil. Some fungi live on buried organic matter such as construction lumber, or dead plant roots. Others live in harmony with tree roots, assisting in the uptake of water and nutrients. A few cause plant disease. The sudden appearance of mushrooms after a heavy rain does not mean the lawn needs lime, fertilizer of anything else. If you object to mushrooms sprouting in lawns or gardens, rake them up and dispose of them. There is no chemical control for mushrooms.