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.
Mushroom growing is reputed to be rather a gamble, owing to the doubtful results, and it is certain that success is not obtained unless the grower devotes a great deal of time and attention to every detail. It is only by taking every care and precaution that regular and paying crops can be secured.
If the grower succeeds in obtaining a good crop it is one of the most profitable that can be grown, but if only a poor crop is obtained, the returns are swamped by the outlay. Those situated near large cities have the best chance of success in the growing of mushrooms, as not only is the manure cheaper, but it is also fresher, and retains a far greater percentage of ammonia. Contracts are made to clear a number of stables regularly all the year round, and in this way the cautious grower makes sure that the manure is free from disinfectants. If he is unable to do this it must be bought by the yard - Is. 3d. to 1s. 9d. per yard, delivered, being the usual price. For those growers who are not near enough to a city for either of the above methods, manure must be obtained from a contractor, who will put it on rail at so much per ton - 2s. a ton is a fair price, with carriage in addition. But in whatever way the manure is obtained, the important points are to get it free from disinfectants and peat litter; with either in, manure is not worth the getting. The best plan is to have it put on hard ground under a roof of galvanized iron. At any rate, provision should be made to shoot off rain.
The first step is to turn over the heap and damp it out. It is possible sometimes in winter to prepare it without using a drop of water, but this is an infrequent occurrence. The objects to be arrived at are: to thoroughly mix the manure, to sweeten it, to fix the ammonia, and to add just sufficient moisture to carry the crop through. Not more of it should be placed on the heap than the workmen can manage in a day. With forks specially adapted for the purpose, the men then proceed to shake it out, throwing all rubbish away, and taking out merely the longest of the straw. In this and all subsequent turnings the manure should be shaken in little heaps at the workers' feet, and then thrown with a scattering motion to the front, starting the fresh heap about 9 ft. in front of the old one. As this is being done another man, stationed with a hose, must damp it down, a process that requires a good deal of discretion, and one that it is very difficult to describe in print. A rough-and-ready test is this: after a little has been done, draw a handful from the centre and wring it between the two hands. If it leaves them just wet, it will probably be sufficiently damp; if the straw itself is very dry, more water will be necessary. The ground must be kept clear as the work proceeds, using a shovel to throw the droppings to the top of the heap. Care should be taken not to pack the sides too firmly, or they are likely to dry out and will become covered with a white mould. The heap should be made about 5 or 6 ft. high on this occasion, reducing it a little at each subsequent turning. A brisk heat is required at first to fix the ammonia, but the same temperature is not necessary afterwards. The first turning-over always takes half as long again as the later ones, because the heap is consolidated and the rubbish and litter have to be rejected. After an interval of two or three days from damping, the heap must be turned again, starting from the same end as before, provided there is room; if not, it must simply be turned back again. The outside 6 in. at the end should be pulled off and thrown to the top of the heap, then pull down the heap and shake out as at first, throwing the sides into the middle and vice versa. It should be the workman's aim to thoroughly separate and break every portion of the heap. The manure must be tested for moisture as the turning goes on, any spots that show white being damped. If the heap is very dry it is better to soak the top 6 in. with the hose and then turn.
Turnings should afterwards take place on alternate days, arranging so that the last one takes place about the middle of the week, to allow time for the bed to be laid down in the same week. After the manure is ready, no time must be wasted.
Fresh-drawn-in manure will require five or six turns after the damping-out, but manure that has been stacked for any length of time may do with one or two turns less. To be ready for laying down it should have the straw broken into lengths of 3 or 4 in., be free from any offensive smell and give off an odour similar to that of a Mushroom. A further satisfactory test is that when rubbed between the fingers it has a greasy feeling, but is not wet. No moisture should ooze out if a handful of manure is twisted in the hands. A minimum amount of water should be used; only practice will determine exactly how much is necessary, but it is fatal to get the idea that the same amount of water will do for all seasons of the year. The test given above is for the winter and spring crops. For early autumn the manure must be wetter, to compensate for quicker evaporation, and in order that the temperature of the bed may be kept down after it is laid. It will be found, as a rule, that manure that is on the dry side gives a higher temperature in the bed, and moister manure keeps a lower and more even temperature. If it were as easy to grow Mushrooms in the autumn as it is in the winter and spring, prices would certainly not be as high as they are in October and November.