Pipe-heated beds are popular in some sections. Beat-tie (U. S. D. A. Farmers' Bul. 324, p. 12) gives the following description of this method: "Where a steam or hot water boiler is in use for greenhouse or residence heating, a very satisfactory plant bed can be constructed by burying four or five lines of pipes in the soil beneath the bed and supplying the heat from the boiler. This has been found to be an ideal method, as the lines of pipes can be controlled by valves and the temperature of the soil in the bed kept at the desired point. If hot water heat is used the pipes should be laid with the soil in direct contact with them, but for steam the pipes should be surrounded with 3-inch or 4-inch tiles to prevent too rapid passage of the heat and the consequent burning of the soil. The pipes should be placed 10 or 12 inches below the bottom of the special soil in the bed, and 14 to 16 inches below the line of the seed potatoes. If the pipes are laid 18 inches apart, an even distribution of heat will be secured. For hot water heating, 2-inch pipes should be employed, but 1¬-inch pipes will be large enough for steam, provided the lines are not more than 60 feet in length. The lines of pipe should be carefully graded and have a uniform fall toward the return end; in fact, the rules governing greenhouse heating apply to the piping of beds of this kind. A general idea of the construction of a pipe-heated bed may be gained from the cross section shown as Figure 102.

Fig. 102. Cross-Section Of Pipe-Heated Frame.

In the far South it is not necessary to provide any kind of protection to the seed bed. In the North, protection with glass or canvas will serve the purpose without artificial heating.

Any fine, light, sandy soil may be used for bedding the potatoes. First, there should be several inches of soil in the bed and the upper inch free from all coarse materials. Then the tubers are placed, by hand, on the surface of the bed so that they do not touch each other. Very large potatoes may be split lengthwise, the cut surfaces being placed next to the sand. After placing tubers, cover them with about 3 inches of fine soil and then water thoroughly.

At the time of bedding the temperature of the bed should range from 80 to 85 degrees and be gradually dropped during a period of six weeks, perhaps, to about 60 degrees, when the plants may be set in the field if weather conditions are suitable. The air temperature in the frame should never fall below 60, and higher temperatures are preferred for some time after the tubers have been bedded. Watering and ventilation must be attended as changing weather conditions demand. Flue or steam-heated frames require more frequent and abundant watering than manure-heated and nonheated beds. Strong, healthy plants are essential to large yields, but good plants cannot be grown without close, daily attention.

In the extreme South the tubers are sometimes cut into pieces and planted like white potatoes, and again the small tubers are occasionally planted whole and cuttings made to extend the plantation. Vine cuttings are commonly used in the most favorable regions. They are usually 10 or 12 inches long, and are made after the plants begin to vine. When planted, they are placed horizontally or obliquely with only 1 or 2 inches of the tip projecting above ground.

631. Soil Preparation

Weeds must be guarded against by clean cultivation, in making preparations for a crop of sweet potatoes. Rotation is important. A thoroughly cultivated crop, followed by crimson clover, puts the soil in excellent condition for planting sweet potatoes. In sections where climatic conditions permit, it is customary to grow an early crop of peas, beans, cabbage or other vegetables before planting sweet potatoes. While a comparatively dry soil is essential to this crop, the supply of moisture must be ample at transplanting, a fact which must always be kept in mind in all preparatory tillage operations.

632. Fertilizing

A yield of 200 bushels an acre will require, for the tubers alone, 30 pounds of nitrogen, 10 pounds of phosphoric acid and 45 pounds of potash. While these figures indicate the need of rather large amounts of nitrogen, it is universally conceded that excessive applications of this clement produce too much vine growth at the expense of tuber development. It is also agreed that free applications, unless too liberal, will increase the yield, but the quality will be inferior. Nitrogen, however, must be used under most conditions, and it is probable that not less than 3 per cent will produce the best results, unless manure has been used or a leguminous cover crop has been plowed down. In the far South probably all of the nitrogen should be derived from organic sources, while in the North a portion should be in soluble mineral forms.

The mineral elements are much more important than nitrogen. They are essential to large yields of high quality, but potash is more important than phosphoric acid. The growers of the famous sweet potatoes at Vineland, New Jersey, use fertilizers carrying about 3 per cent of nitrogen, 7 per cent of phosphoric acid and 12 per cent of potash. Voorhees ("Fertilizers." p. 223) recommends, to the acre, 20 pounds of nitrogen, 50 pounds of phosphoric acid and 80 pounds of potash. At the Georgia Station (Ga. Sta. Bul. 25, p. 128) 320 pounds of acid phosphate, 360 pounds of cottonseed meal and 640 pounds of kainit an acre have given the best results. Beattie (U. S. D. A., Farmers' Bul. 324, p. 7) recommends a mixture composed of 200 pounds of high-grade sulphate of ammonia, 200 of dried blood or 300 of fish scrap, 1,200 of acid phosphate and 400 of muriate of potash. Applications of fertilizer vary from 200 to 1,000 or more pounds an acre, depending upon the condition of the land. The best results are obtained from dressings made from 10 days to three weeks before planting. When very large amounts of the mineral elements are employed, it is especially important to apply them well in advance of planting, because the tender plants are very susceptible to injury from burning.