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.