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.
The pit is sometimes dug a foot wider than the frame is to be and not lined with boards, as previously described (156). Portable frames are placed on top of the manure, which is about even with the surface of the ground, and the frames are then banked with manure. This plan requires more manure, although the beds furnish heat for a longer period, and they settle with the manure, and so the plants are always the same distance from the glass. Another plan often used southward and on poorly drained land is to place the manure on top of the ground, and in this way dispense with the pit. This requires more manure than either of the plans described, because there is no protection at sides and ends. Liberal space must be provided for banking the frame when it is placed so high above ground. Manure greenhouses are sometimes used by placing hot manure under the benches; a central alley is provided, so it is possible to work in the house with the same convenience as in houses heated by flues, steam or hot water. The plan is not recommended, because steam or hot water heating is more economical and much more satisfactory.
Fig. 20. Flue-Heated Frame.
The manure from spent hotbeds has lost most of the nitrogen, but it is useful for composting and fertilizing crops which do not require large amounts of this element. It is also useful for placing in the bottom of flats before transplanting.
Many growers heat their frames by a system of flues leading from a furnace burning wood or coal. The plan is regarded as more economical than the hotbed method, and the results, with proper management, are equally satisfactory. The furnace or fire box at one end of the frame is cheaply constructed, and should be 3 or 4 feet long, and about 18 inches square, with provision for an ash pit under the grate bars. A main flue is built from the fire box, extending 8 or 10 feet under the soil of the frame, where it separates into two smaller flues that continue almost to the other end of the frame, which may be at least 60 feet long, before they join again and enter a chimney. Figure 20 shows this system of heating, but in a different form of construction. The flues may be made of brick, stone or tile, and may vary from 6 to 12 inches in size. Ten-inch tile are very satisfactory. Near the furnace the flue should be about 3 feet under the surface, rising gradually to 1 foot at the chimney. The air in the flue-heated frames has a tendency to become very dry. To prevent this, pans of water should be kept in the frames. Sheds may be built at the furnace end of the frames, to provide comfortable quarters for the work of seed sowing and transplanting.
A frame piped and heated by steam is shown in Figure 21. Hot water may be used in the same way. When heated by either method, mats may be dispensed with. The principles of steam or hot water heating must be observed to secure satisfactory results. These systems of heating are becoming more popular every year. Drain or sewer tile may also be laid in the soil of the frames, steam being conducted through them under some pressure.
Hotbeds are used for starting early vegetable plants. The seedlings are started in the hotbeds and transplanted into other hotbeds, cold frames or the field. Hotbeds are also used in forcing some crops to maturity. The most popular vegetables for hotbed forcing are radishes and lettuce. Spring and summer crops are frequently matured in hotbeds, being started over the fermenting manure, or over manure whose heating qualities have been (exhausted. Among the crops grown in this manner are cucumber, musk-melon, eggplant, squash, tomato, cauliflower, kale, spinach, radish and lettuce.