Casing is a top-dressing applied to the spawn-run compost and is necessary for mushrooms to grow from the mycelia, which have grown throughout the compost. Sugar beet lime mixed with peat moss and limestone are used as a casing material. It is very critical that nematodes carried by dust not get into the casing material as that can cause major crop losses. For this reason, dust control on our farm is very important. Casing is not used to supply nutrients; rather, it acts as a water reservoir and provides a place where rhizomorphs form. Rhizomorphs look like thick strings and form when the very fine mycelia grow together. Casing must be able to hold moisture since moisture is essential for the development of a firm mushroom (a mushroom is 97% water). During the period following casing, some water in a very fine spray must be applied intermittently to maintain the moisture level at its maximum water holding capacity before the mushroom pins form.