This section is from the book "Studies of American Fungi: Mushrooms, Edible, Poisonous, Etc.", by George Francis Atkinson. Also available from Amazon: Studies of American Fungi: Mushrooms, Edible, Poisonous, Etc..
One pound of mushrooms from every two square feet of surface is considered a very good crop. Sometimes it exceeds this, the beds bearing one pound for every square foot, though such a heavy yield is rare. Oftener the yield is less than half a pound for a square foot of surface.
The beginner should study very carefully the conditions under which he grows his crops, and if failure results, he should attempt to analyze the results in the light of the directions given for the curing of the manure, its moisture content, "sweetness," character of the spawn, temperature, ventilation, etc. While there should be good ventilation, there should not be drafts of air. A beginner may succeed the first time, the second or third, and then may fail, and not know the cause of the failure. But given a good spawn, the right moisture content of the material at time of planting and running of the spawn, the sweet condition, or proper condition of the curing of the manure, proper sanitary conditions, there should be no failure. These are the most important conditions in mushroom culture. After the spawn has run and the crop has begun to come, the beds have been known to freeze up during the winter, and in the spring begin and continue to bear a good crop. After the spawn has run well, beds have accidentally been flooded with water so that manure water would run out below, and yet come on and bear as good a crop as adjoining beds.
Volunteer mushrooms sometimes appear in greenhouses in considerable quantity. These start from natural spawn in the manure used, or sometimes from the spawn remaining in "spent" mushroom beds which is mixed with the soil in making lettuce beds, etc., under glass. One of the market gardeners at Ithaca used old spawn in this way, and had volunteer mushrooms among lettuce for several years. In making the lettuce beds in the autumn, a layer of fresh horse manure six inches deep is placed in the bottom, and on this is placed the soil mixed with the old, spent mushroom beds. The following year the soil and the manure at the bottom, which is now rotten, is mixed up, and a fresh layer of manure is placed below. In this way the lettuce bed is self-spawned from year to year. About every six years the soil in the bed is entirely changed. This gardener, during the winter of 1900-1, sold $30.00 to $40.00 worth of volunteer mushrooms. Another gardener, in a previous year, sold over $50.00 worth.
In some cases gardeners follow the practice of inserting a forkful of manure here and there in the soil where other vegetables are grown under glass, and planting in it a bit of spawn.
Some combine a mushroom house and house for vegetables in one, there being a deep pit where several tiers of beds for mushrooms can be built up, and above this the glass house where lettuce, etc., is grown, all at a temperature of about 6o° F.