This section is from the book "Commercial Gardening Vol4", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
For the casing or landing, good virgin loam should be used if obtainable. If access can be had to a field, pare off the turf in slabs 1 ft. square and 2 in. deep, take out 4 in. of soil, and replace the turf. Weightier Mushrooms will be obtained from this than from old soil. But old soil has generally to be utilized; old Tomato or potting soil often does well, or even soil from an arable field. If it is at all inclined to be sour, lime should be mixed with it, 1 part to 100 being an average dressing. All soil should be put through a screen of 1/2-in or 3/4-in. mesh, and should be moist enough to bind together if squeezed in the hand. Before starting to case, the surface of the bed should be trodden flat with the aid of boards, and then the soil wheeled on the bed on boards to prevent crushing the spawn. The soil is then spread on as evenly as possible with the hands and beaten down firmly with the back of a flat spade or shovel. A depth of 1 1/2 to 2 in. should be aimed at. Less than 1 1/2 in. will not hold the "roots" firmly when picking is in progress, and a loss of crop through broken threads will ensue. The casing must be firm.
After the latter process is finished, an even temperature of 70° to 75° F. should be looked for. If lower than this, litter must be placed on the bed. From 3 to 6 in. is often necessary at first, and this should be turned after two or three weeks to allow the air to gain access to the bed, as well as to get rid of any very short stuff, or weeds that may have come up in the soil. As the spawn begins to show, the litter should be reduced gradually, and if necessary fire heat turned on, so that a house temperature of 55° by night and 60° by day is maintained, air being given whenever possible. A little litter is always an advantage on the bed; it prevents quick drying and so much damping down is not necessary. The Mushrooms also are weightier than when it is absent. All the time it is on, a careful watch must be kept to see that the surface of the bed does not become dry. If at any time this occurs, it should be remembered that two or three light dampings at intervals are better than one heavy one. The heavy damping will probably run through to the manure, and once the surface of that becomes too wet, hopes of a crop are nil, for the mycelium will be unable to get to the surface. When the Mushrooms are peeping through, daily dampings will be required. This should be done with a very fine rose and very lightly each time. Once a day is sufficient in cool weather when tire heat is not used, but two or three times a day may be necessary.