Practically all manure hotbeds are made with horse manure. Poultry and sheep manures are also desirable, although they are too valuable in fertilizing garden crops to be used for this purpose. Spent hops from breweries, and forest leaves, may also be worked into the hotbed manure, which should be fresh and not too compact nor too loose. If it is composed almost entirely of the solid excrements of the horse, with whatever urine it has absorbed, it may fail to heat, or the fermentation may be too violent and of too short duration. Two parts of excrement to one part of litter give satisfactory results. If too rich, forest leaves may be added. Manure with shavings as the litter is never satisfactory. The manure from horses well fed on grain is the best for hotbed use.
Preparation of the manure should begin 10 or 12 days before the time when the beds will be wanted for seed sowing or transplanting. A stable or a protected shed is necessary in preparing the manure for the best results in cold weather, because hard rains and severe weather may hinder or prevent fermentation.
Fresh manure from city livery stables, shipped on cars or barges, is often in proper condition for the pit when received, although piling and turning are usually necessary. The piles should be 4 or 5 feet wide, about 4 feet high and of any convenient length. When the manure is thrown from the wagon it should be tramped, but not too compactly. If the manure is rather dry, the addition of hot water will help to start fermentation. Ordinarily, the pile will begin to steam in a day or two. When fermentation is well under way, restack, placing the exterior of the pile on the interior of the new pile. After the manure is allowed to remain in this condition two or three days, or until the entire mass is hot, the pit should then be filled.
The manure should be thrown into the pit in successive layers of 5 or 6 inches and tramped firmly, especially along the sides and in the corners of the frames. The manure will settle several inches, and allowance should be made for this when filling. After the manure is in, from 4 to 6 inches of good soil is placed on the manure. Two inches of soil is sufficient covering to absorb steam and to keep the air pure if flats are used instead of sowing or planting directly in the soil. The frame is also banked with manure.