Button mushrooms are the young, emergent stage of a number of mushroom varieties, but the term is most often applied to the white button mushroom (Agaricus bisporus), which is the most common variety of supermarket culinary mushroom. Small quantities of button mushrooms can be grown from ready-made kits, but these are expensive for the quantity of mushrooms they produce. You can grow your own button mushrooms from mushroom spawn and compost for a fraction of the price of store-bought mushrooms.
Mix equal parts of straw and horse manure together using a pitchfork. Add about 1 pound of gypsum for every 5 gallons (the volume to fill a standard utility bucket) of straw-manure mixture and mix it in thoroughly with the pitchfork. Wet the pile with water until it is completely moist but not saturated.
Turn the pile once a week with the pitchfork, adding water as necessary to keep the compost pile moist. When the the pile has fully decomposed, cover it with a tarp until you are ready to plant your mushroom.
Obtain dry flake mushroom spawn for white button mushrooms from a reputable mushroom supply vendor. When the mushroom spawn arrives, fill the wooden citrus or vegetable crates, or scrap-lumber trays, heaped slightly over the top of the crates or trays, with compost.
Add one large handful of mushroom flake spawn for every square foot of surface area to each crate or tray. Mix the spawn into the compost thoroughly by hand. Mist the trays with a plant mister or a fine-spray attachment to a hose. Place the crates or trays in a warm, dark location.
Mist or spray the compost frequently enough to keep it moist, but not saturated. When fine white webs of mycelium appear on the surface of the compost, soak peat moss in a bucket. Spread one to two inches of soaked peat moss over the surface of each crate or tray.
Watch the crates or trays for white pin-head sized mushrooms to emerge. Harvest the mushrooms when they reach the button stage; that is, are full round mushroom heads about 1 inch to 2 inches in diameter.
Mist or spray the trays daily to keep the growing medium moist; additional crops, or flushes, of mushrooms will emerge every 10 days to 2 weeks until the nutrients in the compost have been exhausted.