From the name it is seen that the sweet potato does not belong to the same botanical family as the potato itself. By reason, however, of its similar condition of growth and, to a certain extent, its chemical composition and uses, the term potato has, in this country at least, become to be universally applied to both, although the prefix “sweet” is quite commonly used with the sweet potato, whereas if any prefix is used with the potato, properly so-called, it is the word “white” or “Irish.” The sweet potato is grown extensively in the United States and in other respects, agriculturally, may be regarded as complemental to the potato.
While the potato grows best in the northern parts of the country and in mild climates, the sweet potato flourishes in the greatest abundance in the southern and warmer portions. In respect to the character of the soil the two vegetables are quite similar, both doing best in a sandy or loose soil, provided it is sufficiently supplied with plant food for the use of the growing plant. The sweet potato is a thickened root, and is propagated almost exclusively by means of shoots called “slips.”
Planting and Cultivation.
—There is a very distinct difference between the planting of the sweet potato and that of the potato. The former are rarely planted in the field where the crop is to mature. It is quite a universal custom to plant the sweet potato in beds where the young growth can be forced both by means of artificial heat and by a generous mulch of highly nutritious soil. The plants can then be set very early in the spring and by the time they are ready to be transplanted to the field have acquired a considerable size. When ready for transplanting the seed bed is prepared with the same care as that required for the potato. The ridging of the rows, which in the case of potatoes takes place during cultivation, is accomplished in the case of sweet potatoes before planting. If the soil is moist and the temperature not too high the young plants are removed from the seed bed and set on top of the apexes in the formed rows. The cultivation of the field during the growth of the crop is sufficient to keep the surface in good tilth and prevent the growth of weeds, grass, etc. Care must be exercised in the cultivation not to draw the earth away from the ridges which have been formed, but to increase their size by drawing the earth more and more toward the apex of the ridge. The cultivation is continued until the growing vines practically cover the surface of the soil and thus form a natural mulch, which not only conserves the moisture and tilth of the soil but also prevents the growth of weeds and grass. The sweet potato, in respect of its flavor, is particularly sensitive to the influence of frost, also the leaves are more sensitive to frost than those of the potato. If a heavy frost is experienced before the tubers are harvested it is apt to impart an unpleasant taste to the potato and injure its edible qualities. For this reason, if it is not possible to harvest the potato before the advent of frost, it is advisable to cut the vines at the point where they emerge from the soil. When this has been done the injurious effects of the frost, above mentioned, are not experienced. In the southern portion of the country the sweet potato is often allowed to remain in the soil during the greater part of the winter, and, if the vines are removed, it keeps in excellent condition.
Yield and Composition of the Sweet Potato.
—As has already been mentioned, there is a general resemblance, in so far as chemical and nutritive properties are concerned, between the sweet potato and the potato. The sweet potato is usually colored a yellowish tint, due to the distribution of more or less xanthophyll throughout its substance. The sweet potato also contains notable quantities of cane sugar, to which its name is due. It, however, contains large quantities of starch and fiber and small quantities of protein, resembling in this general manner the potato itself. The sweet potato has not been used in the United States for the making of alcohol. In the Azores great quantities of sweet potatoes are grown for this purpose, and make an alcohol of fine quality, which is used to a large extent in fortifying port wines. There are large areas in the United States, especially in the Southern States, where the sweet potato can be grown in great abundance. The experiments at the South Carolina station show that as high as 11,000 pounds of sweet potatoes can be grown per acre. The percentage of starch is markedly greater than in the white or Irish potato. In all cases over 20 percent of starch was obtained in the South Carolina sweet potatoes, and in one instance over 24 percent. As high as 2,600 pounds of starch were produced per acre.
In addition to starch, the sweet potato contains notable quantities of sugar, sometimes as high as six percent being present, so that the total fermentable matter in the sweet potato may be reckoned at the minimum at 25 percent. A bushel of sweet potatoes weighs 55 pounds, and one-quarter of this is fermentable matter, or nearly 14 pounds. This would yield, approximately, 7 pounds, or a little over one gallon of 95 percent alcohol. It may be fairly stated, therefore, in a general way, that a bushel of sweet potatoes will yield one gallon of industrial alcohol. The average yield of sweet potatoes, of course, is very much less than that given in the South Carolina reports, where heavy fertilization was practised. On plots to which no fertilizer was added the yield was about 8,000 pounds of sweet potatoes per acre, yielding in round numbers 1,900 pounds of starch. The quantity of sugar in the 8,000 pounds is about 350 pounds, which, added to the starch, makes 2,250 pounds of fermentable matter per acre. This will yield 1,125 pounds of industrial alcohol of 95 percent strength, or approximately 160 gallons per acre.
The yield of sweet potatoes in the above computation must be regarded as exceptionally high. A safer calculation will be based upon the yield of 100 bushels of sweet potatoes per acre, a little above the average of the yield of the potato, or a total of 5,500 pounds per acre. One-quarter of this amount is fermentable matter—about 1,400 pounds—which would yield, approximately, 700 pounds of 95 percent alcohol, or 100 gallons of 95 percent alcohol per acre. In addition to the sugar in the form of sucrose, or common sugar, which the sweet potato contains, there is also an appreciable amount of non-crystallizable sugars. The total sugars in the sweet potato have not been overstated in the above estimate. In fact, the contrary, rather, is true, since the two sugars together probably average about six percent of the weight of the potato. If the average quantity of starch in the sweet potato is 20 percent, which is rather a low estimate, the total fermentable matter in the sweet potato is 26 percent instead of 25 percent, as estimated above.
Changes in Composition of the Sweet Potato of Different Varieties on Storing.[33]
| First Lot (November 28). | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Name of Variety. | Original. | Air-dry. | Water-free. | ||||||||
| Water. | Starch. | Invert sugar. | Sucrose. | Water. | Starch. | Invert sugar. | Sucrose. | Starch. | Invert sugar. | Sucrose. | |
| Per- cent. | Per- cent. | Per- cent. | Per- cent. | Per- cent. | Per- cent. | Per- cent. | Per- cent. | Per- cent. | Per- cent. | Per- cent. | |
| Georgia Buck | 75.35 | 13.13 | 0.77 | 4.31 | 6.79 | 49.65 | 2.93 | 16.31 | 53.27 | 3.14 | 17.50 |
| Bunch Yam | 72.37 | 15.12 | 1.09 | 4.45 | 6.67 | 51.06 | 3.67 | 15.04 | 54.71 | 3.93 | 16.11 |
| Do. | 67.99 | 19.58 | .56 | 4.49 | 7.24 | 56.70 | 1.61 | 13.02 | 61.18 | 1.74 | 14.04 |
| Horton Yam | 70.29 | 15.06 | 1.05 | 6.23 | 6.24 | 47.52 | 3.31 | 19.67 | 50.68 | 3.53 | 20.98 |
| Georgia Buck | 71.56 | 14.35 | .73 | 6.61 | 6.88 | 46.98 | 2.40 | 21.63 | 50.45 | 2.58 | 13.23 |
| Vineless Yam | 70.03 | 16.85 | .54 | 5.01 | 7.90 | 51.78 | 1.67 | 15.40 | 56.22 | 1.81 | 16.72 |
| Hanover Yam | 76.16 | 13.61 | 1.10 | 4.22 | 7.37 | 52.89 | 4.29 | 16.40 | 57.10 | 4.63 | 17.70 |
| Georgia Yam | 70.01 | 18.87 | 1.00 | 4.08 | 7.57 | 58.17 | 3.07 | 12.59 | 62.93 | 3.32 | 13.62 |
| Average | 71.72 | 15.82 | .86 | 4.93 | 7.08 | 51.84 | 2.87 | 16.26 | 55.82 | 3.09 | 16.16 |
| Second Lot (January 7). | |||||||||||
| Name of Variety. | Original. | Air-dry. | Water-free. | ||||||||
| Water. | Starch. | Invert sugar. | Sucrose. | Water. | Starch. | Invert sugar. | Sucrose. | Starch. | Invert sugar. | Sucrose. | |
| Per- cent. | Per- cent. | Per- cent. | Per- cent. | Per- cent. | Per- cent. | Per- cent. | Per- cent. | Per- cent. | Per- cent. | Per- cent. | |
| Georgia Buck | 69.74 | 12.72 | 1.75 | 9.25 | 8.80 | 38.34 | 5.27 | 27.87 | 42.04 | 5.78 | 30.56 |
| Bunch Yam | 67.31 | 13.66 | 2.02 | 9.90 | 9.49 | 37.83 | 5.60 | 27.40 | 41.80 | 6.19 | 30.27 |
| Do. | 67.29 | 13.83 | 2.40 | 9.43 | 10.00 | 38.04 | 6.61 | 25.94 | 42.27 | 7.34 | 28.82 |
| Horton Yam | 71.39 | 9.57 | 2.57 | 9.69 | 7.18 | 31.05 | 8.35 | 31.43 | 33.45 | 9.00 | 33.86 |
| Georgia Buck | 67.63 | 14.43 | 2.12 | 7.85 | 8.46 | 40.80 | 6.00 | 22.21 | 44.57 | 6.55 | 24.26 |
| Vineless Yam | 67.33 | 12.03 | 2.90 | 10.09 | 7.90 | 33.90 | 8.19 | 28.44 | 36.81 | 8.89 | 30.88 |
| Hanover Yam | 70.13 | 14.13 | 1.66 | 6.58 | 9.29 | 42.90 | 5.05 | 19.99 | 47.29 | 5.57 | 22.04 |
| Georgia Yam | 71.78 | 11.21 | 2.26 | 8.10 | 8.62 | 36.30 | 7.31 | 26.24 | 39.72 | 8.00 | 28.72 |
| Average | 69.08 | 12.70 | 2.21 | 8.86 | 8.72 | 37.40 | 6.55 | 26.19 | 40.99 | 7.17 | 28.68 |