Mushroom compost is not made from ground up mushrooms as you might think. It's the soil used to grow mushrooms. Since mushrooms are such a delicate crop, the soil is used only once. Then we compost and bag it for home gardeners. This rich, soil-building medium is perfect for vegetable and flower gardens, hanging baskets, shrubs and trees. Mix it with your planting soil or native soil or as a top-dressing on potted plants of all types.