Most hotbeds are heated by the fermentation of manures in pits excavated for this purpose. The first essential of the pit is good drainage, natural or artificial. Artificial drainage may be provided by running tile from the bottom of the pit. In most soils, however, this precaution is unnecessary. The pit should be dug in the fall before the ground is frozen, and a few inches of leaves or coarse manure placed in the bottom during the winter. It should be of the same width as the frame (156) and of any desired length.
The proper depth of the pit depends upon several factors. In the North it is customary to use 15 to 30 inches of manure. The pit should, therefore, be a few inches deeper than the depth of the manure. For starting early vegetable plants in the North, 18 inches of good manure is ample, while 24 to 30 would not be too much in forwarding the eggplant, which requires a high temperature for seven or eight weeks. Tender plants, like tomato and pepper, also require more manure. Southward, the depths of manure vary from 6 to 12 inches. The kind of manure used and the length of time the hotbed will be needed also determine the proper depth of the pit.