It is preferable, however, to have a pit beneath the frame in which the manure is placed. If the bed is to be started in midwinter or very early in the spring, it is advisable to make this pit in the fall and to fill it with straw or other litter to prevent the earth from freezing deep. When it is time to make the bed, the litter is thrown out, and the ground is warm and ready to receive the fermenting manure. The pit should be a foot wider on either side than the width of the frame. Fig. 205 is a cross-section of such a hotbed pit. Upon the ground a layer of an inch or two of any coarse material is placed to keep the manure off the cold earth. Upon this, from twelve to thirty inches of manure is placed. Above the manure is a thin layer of leafmold or some porous material, that will serve as a distributor of the heat, and above this is four or five inches of soft garden loam, in which the plants are to be grown.
It is advisable to place the manure in the pit in layers, each stratum to be thoroughly trodden down before another one is put in. These layers should be four to eight inches in thickness. By this means the mass is easily made uniform in consistency. Manure that has too much straw for the best results, and which will therefore soon part with its heat, will spring up quickly when the pressure of the feet is removed. Manure that has too little straw, and which therefore will not heat well or will spend its heat quickly, will pack down into a soggy mass underneath the feet. When the manure has sufficient litter, it will give a springy feeling to the feet as a person walks over it, but will not fluff up when the pressure is removed. The quantity of manure to be used will depend on its quality, and also on the season in which the hotbed is made. The earlier the bed is made, the larger should be the quantity of manure. Hotbeds that are intended to hold for two months should have about two feet of manure, as a rule.
The manure will ordinarily heat very vigorously for a few days after it is placed in the bed. A soil thermometer should be thrust through the earth down to the manure, and the frame kept tightly closed. When the temperature is passing below 90°, seeds of the warm plants, like tomatoes, may be sown, and when it passes below 80° or 70°, the seeds of cooler plants may be sown.
If hotbeds are to be used every year, permanent pits should be provided for them. Pits are made from two to three feet deep, preferably the former depth, and are walled up with stone or brick. It is important that they be given good drainage from below. In the summer-time, after the sash are stripped, the old beds may be used for the growing of various delicate crops, as melons or half-hardy flowers. In this position, the plants can be protected in the fall. As already suggested, the pits should be cleaned out in the fall and filled with litter to facilitate the work of making the new bed in the winter or spring.
Various modifications of the common type of hotbed will suggest themselves to the operator. The frames should ordinarily run in parallel rows, so that a man walking between them can attend to the ventilation of two rows of sash at once. Fig. 206 shows a different arrangement. There are two parallel runs, with walks on the outside, and between them are racks to receive the sash from the adjacent frames. The sash from the left-hand bed are run to the right, and those from the right-hand bed are run to the left. Running on racks, the operator does not need to handle them, and the breakage of glass is therefore less; but this system is little used because of the difficulty of reaching the farther side of the bed from the single walk.
If the hotbed were high enough and broad enough to allow a man to work inside, we should have a forcing-house. Such a structure is shown in Fig. 207, upon one side of which the manure and soil are already in place. These manure-heated houses are often very efficient, and are a good make-shift until such time as the gardener can afford to put in flue or pipe heat.
Hotbeds may be heated by means of steam or hot water. They can be piped from the heater in a dwelling-house or greenhouse. Fig. 208 shows a hotbed with two pipes, in the positions 7, 7 beneath the bed. The earth is shown at 4, and the plants (which, in this case, are vines) are growing upon a rack, at 6. There are doors in the end of the house, shown in 2, 2, which may be used for ventilation or for admitting air underneath the beds. The pipes should not be surrounded by earth, but should run through a free air space.