This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
In most cases, perhaps, farmyard manure is stacked or thrown loosely in heaps and left exposed to the weather. Unless frequently turned over and kept moistened with water or urine the manure heap will gradually allow the best part of its fertilizing ingredients, namely, the ammonia gas, to vanish into the air, a misfortune readily recognized by the smell given off. Or the heavy rains wash out all the soluble salts into the drains, where they are lost. The result often is that the "life" of the manure has departed, and nothing is left but the dead carcass. To avoid these calamities it is therefore best to have the manure under cover if possible, and placed on concrete bottoms, so that any liquid oozing out may be afterwards collected and thrown over the heap. If a piece of moistened red litmus paper be placed near a steaming manure heap it will turn blue; and a glass rod dipped in spirits of salts (hydrochloric acid) will be covered with a white crust of sal ammoniac, produced by the union of the acid with the escaping ammonia.
Manure should be well packed or trodden down, as it loses ammonia more readily if left in a loose condition, Wonderful chemical changes take place rapidly in the heap, and micro-organisms are at work reducing the organic material into a finer, less littery, and more fertilizing compost. In this way the manure heap loses considerably in bulk, and the farmer and gardener must take care not to let it remain too long before working it into his soil. It has been computed that 100 loads of fresh dung left exposed to the action of the weather loses nearly 27 loads in 81 days, 35 1/2 loads in 254 days, 37 1/2 loads in 384 days, and about 53 loads (over one-half) in 493 days.
Growers of flowers, ferns, palms, etc, use stable or farmyard manure in fairly large quantities, but not of course so largely as market gardeners and farmers; and many of them preserve all the ingredients of the manure by stacking it in layers with soil. Thus a bed is marked out, and perhaps a layer of soil 1 ft. thick is spread over it. On top of this a layer of manure 3 or 4 ft. thick is placed. Then another layer of soil, followed with a layer of manure, until the material is used up - the top layer always being soil. Arranged in this sandwich-like way, the layers of manure decay evenly, and at the same time fertilize the layers of soil. In due course the compost is chopped down with the spade, and is used in various proportions with other soil for any special crops. While it may not be always possible or convenient for market gardeners to store manure in this way, those who cultivate plants of any kind in pots will find it an excellent method of producing a rich and agreeable compost.