This section is from the book "The Gardener's Monthly And Horticulturist V23", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
The Irish Gardener's Record says: "The first thing to do in the way of pre paring materials for growing mushrooms is to obtain a quantity of fresh horsedung that has not been heated. This latter condition is absolutely necessary, and where there is a difficulty in obtaining a quantity this way that has not been heated, it is advisable to make some terms with the stable-keepers, and remove the dung every day, spreading it out not more than a foot thick on the floor of some open shed, or it might be under a tree, or anywhere secure from rain; turn it every day or so until it be partly dry, and when a sufficient quantity is ready a bed may be made. Generally speaking, the end of September is as early as most people make mushroom beds, and from that time up to Christmas they may be made in succession as wanted. About six weeks after being made is as soon as mushrooms may be expected, sometimes they come sooner, but often much later. The situation for a bed is not material - any place not too damp, but secure from rain and frosts will do. One of the best mushroom beds we have ever had was under a tree covered up pretty thickly with litter to keep out the frost; and this bed answered admirably, producing abundance of mushrooms in long succession.
The bed was formed as a ridge with a base of about five feet and as steep as it could well be made, with the short loose dung trod firmly up to the ridge. Being spawned and covered with mould in the usual way, well beat over, and covered up with litter, it received no further attention than uncovering and gathering the crop, and a heavy watering towards the end of the season to revive it again, which it did for a time. But it is needless to say that for an out-door bed like this more dung was used than would make several shelf-beds in a well constructed mushroom house. A lean-to shed is also a pretty good place for a bed, and if it be a very cold place a greater proportion of dung must be used than if it were warmer; something like fifteen inches of solid dung well trod on will not be too much, taking care to have the dung tempered by exposure and frequent turning before making up, and in such a place a good crop may be expected. A cellar is also a very good place for a mushroom bed, and certainly better than any open air place; for mushrooms seem to like a still atmosphere - almost stagnant in so far as regards motion, so that a large airy apartment, however useful for most other things, seems unnecessary, if not absolutely hurtful, to the mushroom; so that amongst the many places we hear of their being produced, very exposed places are the most rare."