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CHAPTER XXV

SWEET POTATO

  The sweet potato (Ipomoea batatas) is a prostrate, trailing, perennial herb, grown as an annual for its enlarged, fleshy roots. (The potato is discussed in "Root Development of Field Crops.") It is a member of the morning-glory family, of tropical origin, and one of the leading crops of southern United States. It is also grown to a considerable extent in the North. It rarely forms seed in the United States and is propagated vegetatively. Methods of propagation are pertinent here since they are concerned with root development.

  Methods of Propagation.--Tubers of moderate size are placed in a hotbed and shallowly covered with loose earth or sand. Sprouts soon develop from adventitious buds which are usually located near the base of former, fibrous, lateral roots. After these have grown up through the soil covering, more sand or soil is added to promote the development of a good root system. When the shoots are 3 to 5 inches high, they are removed and set in the field. In this manner two to five crops of slips may be taken from one root. 37

  When the plants are set into the field, they are watered like tomato or cabbage transplants if the soil is dry. Frequently, the roots are dipped into a thin paste made of mud and water. This not only, prevents them from drying in the process of planting but also causes the soil to adhere to them. Pressing the soil firmly about the plantlets to establish a good contact is essential. The plants are sometimes propagated by root cuttings placed directly in the field.

  Sweet potatoes are also propagated, especially in the South where the growing season is longer, by cuttings made from the ends of the vines. Slips are planted early in the season, and when the vines begin to run, cuttings are made. In making the cutting 8 to 12 inches of the end of the vine is used. All of the leaves are removed except the partially grown ones near the tip. The cutting is shallowly buried in the soil in the field in a more or less horizontal position and the soil firmly packed about it. Only a small portion of the tip is permitted to extend aboveground. Such vine cuttings have several advantages over slips. They produce roots more nearly uniform in size and shape and are free from some of the sweet-potato diseases that are carried from the seed bed to the field on the slips. They are also cheaper to produce and afford a yield which is quite as large or larger than from plants grown from Slips. 156

  Sweet-potato plants of the Yellow Jersey variety were transplanted into the garden on June 1. The plants were set 15 inches apart in rows 4 feet distant.

  Early Growth.--At the time of the first root excavation, 23 days later, plants of average size had 30 to 35 leaves with blades that varied from 1 to 2.5 inches in length and width. Many branches had grown from the original root cuttings. A typical case will be described.

  The transplanted root, which was 4 millimeters thick and 5 inches long, gave rise to 71 branches. These originated rather uniformly throughout, beginning just beneath the surface of the soil. Their general course was outward and downward, nearly all running obliquely at angles of 45 degrees or less from the old root (Fig. 68). Many were only about 1 inch long; others reached a maximum spread of 4 inches and a depth of 17 inches. This depth was not exceeded by. roots which penetrated rather vertically downward from their origin near the base of the cutting. None of these fine, rather tough, glistening white roots were more than 1 millimeter in diameter, even at their origin, and they were usually much less. When injured they exuded a white sap. Laterals were fairly abundant, about six per inch, thread-like but rather firm. They grew at right angles from the main roots. Usually they were unbranched, but a few of the longest had the beginnings of branchlets. The whole rapidly growing root system, because of its light color, stood out in striking contrast to the dark soil. The long, glistening white, unbranched root ends were very characteristic.

  Fig. 68.--Root system produced from a root cutting of the Yellow Jersey sweet potato 23 days after transplanting it into the field.

  Midsummer Growth.--One month later, July 25, root development was again investigated. Typical specimens now had 12 to 19 prostrate branches varying in length between 1 and 4.5 feet. Frequently, there were about 8 of the longer ones. In addition some new vines less than 1 foot long were found. The older, basal leaf blades were 2.5 by 3 inches in size; the younger ones near the ends of the branches were about half as large. Some of the vines, were rebranched. As many as 400 to 450 leaves were found on individual plants, thus presenting a rather large transpiring and food-making surface.

  A thorough study of the underground parts showed that they had developed in proportion to the tops. The roots ramified the soil 2 to 3 feet on all sides of the plant and penetrated to a working level of 2.6 feet. A maximum depth of 41 inches had been attained. A study of Fig. 69 gives a fair idea of root development and distribution.

  Fig. 69.--Development of sweet-potato root system on July 25. It is 1 month older than shown in Fig. 68.

  Each plant had 12 to 16 large roots 1.5 to 4 millimeters in diameter and many smaller ones, all of which originated from the transplanted cutting. Some of the larger roots had become quite fleshy (diameter 0.2 to 0.5 inch) throughout 2 to 4 inches of their course. In the surface inch of soil there arose numerous horizontal laterals only about 1 inch long and entirely unbranched. Others ran horizontally or nearly so 2 or more feet, branching freely but ending in the surface 6 to 8 inches of soil. Still others ran obliquely outward and downward in such a manner that when they reached a lateral spread of 1.5 to 2.5 feet they were still within the surface foot. These roots frequently gave rise to numerous, long, vertically penetrating branches which reached depths of 2 to 2.5 feet. For example, one main lateral, which finally ended at a depth of 21 inches but 40 inches horizontally from the base of the plant, gave rise to 11 roots of this type. Still other main roots turned more obliquely downward, some descending almost vertically and filled the soil beneath the hill. They branched widely and ended near the working level. Some of the large laterals from these more vertical roots spread obliquely in the deeper soil and ended, but more usually turned downward, 6 to 17 inches horizontally from their origin. The usual spreading of branches, whether vertical, horizontal, or intermediate, at wide angles (often at right angles) with the branch from which they originated is characteristic.

  Branching of the laterals was very similar throughout. An average rate of 3 to 8 branches per inch was determined. There were seldom fewer than 2 and sometimes as many as 12. Length of branches was more variable. Those that clothed the longer roots, whether of the first or second or sometimes the third order , usually ranged between 0.2 and 2 inches in length. Longer or shorter ones were much less frequent. These laterals were only rarely rebranched. As shown in the figure, however, the soil was well filled with roots. Both tops and roots were growing vigorously. This was clearly revealed underground by the glistening white root ends, the last 2 inches of which were usually unbranched.

  Mature Plants.--A final examination of the root habit was made on Oct. 3. The vines had made an extensive growth, frequently 12 occurring on a single plant. Many of them had a length of 14 feet and gave rise to 175 leaves. Since the leaf blades were approximately 3.5 inches in length and width, a very extensive transpiring surface was presented. The average spread of a single vine was about 20 inches and an individual plant covered nearly 200 square feet of soil surface.

  There were about 12 "potatoes" per hill, the largest being 2 inches thick and 5 inches long. Most of these fleshy roots extended downward rather obliquely and were usually found in the surface 6 to 9 inches of soil. More than 1 foot of the large roots extending beyond the swollen portion was also yellow in color. The thick root cutting to which these "potatoes" were attached was only 3 inches long. It gave rise to 8 to 15 absorbing roots that extended outward rather horizontally. Many of the roots that extended downward from the root cutting were dry in the hard, fissured soil layer which occurred at a depth of 21 inches. They penetrated far beyond this depth, however, usually to the 4-foot level. These roots were furnished with laterals at the rate of 3 to 5 per inch, some of which extended outward 2.5 feet.

  The fleshy portion of the roots gave rise to three to eight roots per inch. These were 8 to 12 inches long, pursued an almost horizontal course, and were rebranched at the rate of three to six short branches per inch. In addition about three large laterals 3 millimeters in diameter arose from each "potato." These usually extended horizontally 1 to 1.5 feet and then, turning downward, reached depths of 4 feet or more. The main roots, of which the "potatoes" were the enlarged parts, were 3 to 4 millimeters in diameter. They usually extended downward. But at the base of nearly all' of the "potatoes" one or two large horizontal laterals also arose. These ran rather horizontally 10 to 18 inches and then penetrated downward to 4 feet or more. Branching on these roots and on the main roots was very similar. Most of the branches were 1 to 6 inches long and furnished with short laterals. On the main roots, in addition to a few short rootlets, to a depth of 2.5 feet, there were one or two roots per inch that extended almost horizontally 6 to 8 inches and then ran downward to about 3 feet. These were rebranched at the rate of three to five short branches per inch.

  Between dept hs of 2 to 3 feet the branches on the roots were more sparse and usually partly dried. These occurred at the rate of two to four per inch, being quite unbranched and only 1 to 2 inches long. But below the 3-foot level, where the soil was more moist, the branches were more numerous, longer, and in good condition. The maximum depth of penetration was 69 inches; the working depth 51 inches.

  In addition to the roots described the stems gave rise at their nodes, and sometimes on the internodes, to an enormous number of roots. As many as 10 to 15, usually in 2 rows, arose from a single node, and there were 3 or 4 such clumps per foot of stem. Not infrequently a node would give rise to a single root. These single roots were profusely furnished with laterals, 1 to 1.5 feet long, which ran mostly horizontally, were rebranched to the third order, and formed a very intricate root network. These main roots were usually only 1 millimeter in diameter but many were swollen to 5 times this thickness for a distance of 3 to 6 inches. Most of them extended almost vertically downward but some ran obliquely outward several inches and then turned downward in their course. A working depth of 40 inches was ascertained although some were found 10 inches deeper. The roots which arose in clumps were very profusely branched in the surface 6 to 8 inches of soil. As many as 12 to 18 branches per inch were common. They extended horizontally 2 to 10 inches. Those that exceeded 1 inch in length were rebranched at a similar rate with rebranched rootlets 1 to 2 inches long. In fact, the soil was literally filled with the profuse network of these adventitious roots and their numerous branches. Undoubtedly they were carrying on much of the absorption for the plants. This was indicated by their fresh color and more turgid condition when compared with the main root system.

  The most striking character about the root system of the sweet potato is the intricate network of roots that underlies all of the stems and leaves. This undoubtedly promotes the rapid growth and wide spreading of the stems since the source of water and nutrient supply is not far removed from the leaves.

  Summary.--The sweet potato is propagated vegetatively by slips developed from root cuttings, by cuttings made from the ends of vines, and sometimes by root cuttings themselves. Plants grown by the last method rapidly develop an extensive fibrous root system. Three weeks after transplanting, roots are abundant from the soil surface to a depth of 17 inches. Nearly all run obliquely downward, the lateral spread not exceeding 4 inches. A month later the vines are 1 to 4.5 feet long. The roots extend from just beneath the soil surface to a working depth of 2.6 feet. Laterally, they run 2 to 3 feet. They are well branched throughout. Mature plants have vines 14 feet in length. In addition to about 12 fleshy roots per plant, the attenuated portions of which branched widely and penetrated deeply, there are usually an equal number of extensive roots of small diameter. These penetrate far outward and downward, mostly to a depth of 4 feet. Long sublaterals are common and shorter branches very profuse. A network of roots ramifies the soil from its surface to a depth of 51 inches. Abundant, profusely branched roots arising from the nodes of the vines penetrate deeply and add materially to the absorbing area. Thus the soil under the widely spreading vines is completely filled with a dense, absorbing network of roots.

  Root Habits of Certain Sweet-potato Relatives.--The root habits of certain relatives of the sweet potato are of interest for comparison, especially since some are noxious weeds. The hedge or great bindweed (Convolvulus sepium) is a perennial plant that propagates freely from a rootstock, the numerous roots penetrating deeply. In the moist subsoils of eastern Nebraska a depth of 5 to 7 feet is usually attained. The small or field bindweed (Convolvulus arvensis) propagates from creeping roots. Under a surface area covered with the vines of this species, the soil is thoroughly occupied with roots. In eastern Nebraska depths of penetration of 15 to 17 feet are not uncommon. It is of interest that the rhizomes and root offshoots of these plants vary in depth with the compactness of the soil. In very hard soil they may occur almost entirely in the surface 2- to 10-inch soil layer. In mellow, cultivated soils they are much deeper and frequently most abundant at the 6- to 18-inch soil level. 79 The widely spreading and deeply penetrating root habits of the morning-glory have been confirmed by studies in Colorado. 119

  A greater root extent, however, is attained by a plant more nearly related to the sweet potato, the bush morning-glory (Ipomoea leptophylla). This species is common in sandy soil of the semiarid, central portion of the United States. It is a perennial with an enormously enlarged taproot, which is often 1 foot or more in diameter and tapers to 1 inch or less only at a depth of 4 to 6 feet. This enlarged portion of the taproot not only furnishes an enormous reservoir for food but also a storehouse of water upon which the plant may draw during a period of drought. The taproots extend to great depths, at least 11 feet and probably to twice this depth. Throughout their course they are profusely branched. Many of the branches penetrate obliquely outward or downward. Others pursue a nearly horizontal course. Many have an enormous spread, the roots running outward to distances of 15 to 25 feet or more. The surface foot of soil, as well as the 10 feet below it, is literally filled with the glistening white, brittle branches of this remarkable root system. 167

  The root habits of the wild ancestors of cultivated plants, so far as the plants are known, would make an interesting and valuable study. The object sought in cultivating plants is usually to produce a growth of some of their parts, whether flower, seed, fruit, leaves, stem, or root, that is unnatural to the species in its native habitat. Undoubtedly in so doing other parts of the plant are also greatly modified. The modern varieties of vegetable crops have become accustomed to growing in soil especially prepared and liberally supplied with everything the roots need. In fact, the plants are unable to fully develop in soils of moderate fertility. Surely this has brought about great, changes in the root habit.

  Root Habit in Relation to Cultural Practice.--A number of interesting and important root relations are concerned with the growing of sweet potatoes as a result of the cultural condition imposed upon this plant of wild ancestry.

  Soils and Fertilizers.--Since the sweet potato is a tender plant of tropical origin, it grows best in an easily warmed soil. For proper growth of the fleshy roots soil should not be plowed too deeply. In such an instance, there is a tendency for the roots to grow slender and too long. On very rich soil the crops produce too much vine growth and the "potatoes" are likely to be too large and rough. In heavy clay soils they are also likely to be rough and irregular in shape but in lighter, easily moved soil they are usually more uniform in size and smoother. Hence a light, moderately rich soil, such as a sandy loam with a clay subsoil, is perhaps best, although under irrigation light sandy loam and coarse sandy soil are most suitable. 122

  Fresh manure causes a rank growth of vines and the development of large, rough roots.

  Well decomposed stable manure applied in the row is beneficial in producing maximum yields but when manure is used in the drill, the surface of the sweet potato is more likely to be disfigured with black marks known as scurf and spoken of as "soil stain" "mottling" "rust," etc. This spoils the appearance of the sweet potatoes and reduces their keeping qualities. 31

  It has also been shown that nitrogen in excessive quantities tends to produce long "potatoes." 132

  Water Relations.--It has been observed in New Mexico that if sweet potatoes are kept well irrigated and the surface soil moist, a larger crop is produced and the "tubers" are nearer the surface of the ground than where the soil is allowed to become too dry. In the latter case, the roots tend to grow deeper in the soil. There is a development of the root system, seemingly at the expense of the crop and the "tubers" are found quite deep in the soil. 37 This, of course, increases the difficulty and expense of securing them and they are much more apt to be bruised in the process, thus increasing subsequent losses from decay.

  The plant roots so deeply and extensively that, once the roots are well established throughout the soil, it endures drought much better than most vegetable crops. In fact, it will produce a fair crop in semiarid regions where most vegetable crops will not thrive at all. Under irrigation in California good growth and yields have been observed where by midsummer there was no available soil moisture nearer than 20 to 24 inches from the soil surface. 122 The roots, however, are intolerant of poor aeration resulting from a wet soil. Hence the common practice of setting the plants on ridges often 8 to 15 inches high. 122, 143, 178 The ridges should be as low and flat as the drainage conditions will permit for the higher and narrower the ridges the more rapidly water is lost. The effectiveness of this method, of course, depends upon the season, but experimental evidence shows that the yields are often increased.

  Cultivation, Rooting of Vines.--The practice of thoroughly preparing the soil by repeated cultivation before the plants are set is an excellent one since it avoids a later disturbance of the roots in cultivation. It is the common practice also to maintain a surface mulch and to keep the weeds out by shallow cultivation at least until the vines cover the ground. Usually at each cultivation the soil is gradually moved toward the rows. But the vines must not be covered with soil; otherwise they will develop roots early in the season and hinder the best development of the main roots. The rooting of the vines between the rows later in the season does not reduce the yield. 31 Many gardeners continue cultivation later by turning the vines first to one side of the row and then to the other during the process. This prevents the development of adventitious roots, as already described, and it often prevents the full development of the crop. For example, in Georgia the yield of the Pumpkin Yam variety was decreased 107 bushels per acre. 143 Similar results, with a decrease varying from 5 to 126 bushels per acre, have been obtained in Louisiana and Arkansas. 111 The extensive development of adventitious roots greatly reduces the distance water and nutrients must travel to supply the vines with these materials essential to food manufacture. On the other hand a certain amount of the energy of the plant is expended in growing these roots. During dry years, especially, preventing the vines from rooting would seem to be especially injurious to the plants. 178,143

  Pruning and Spacing.--Although the effect of pruning the vines has not been studied in connection with root development, it would seem that pruning would retard it. Indeed, this has been found to be the case in regard to the fleshy portion of the roots, although some growers believe that reducing the growth of foliage stimulates root development. For example, in New Mexico, hills pruned back to 12 inches in diameter yielded 6,012 pounds per acre, those pruned back to 24 inches produced 8,690 pounds per acre, and the yield where no pruning occurred, was 16,520 pounds. 37 Similar results were obtained in Georgia. Unpruned plants produced 201.3 bushels per acre but plants pruned weekly throughout the season to a length of 2 feet produced only 104.9 bushels. 143

  Spacing of the plants seems to be determined in practice largely by the spreading of the aboveground parts and the case of cultivation. To afford room for the widely spreading vines, the plants are widely spaced. A planting distance of 12 to 18 inches apart in rows 3 to 6 feet distant is usual. Under these conditions root interlacing occurs to a marked degree and the soil is thoroughly occupied.

  Harvesting.--It is of the utmost importance that the roots be removed from the soil soon after the leaves have been killed by frost since water accumulates in the roots. This results from the cessation of transpiration in consequence of the destruction of the leaves. The "potatoes" may safely be kept in the ground until mature or until the leaves have been injured by frost. During the latter part of the period of growth and up to the time of the death of the vines, the root is characterized by a high starch and a low sugar content. The choice of the time of harvesting is not a matter of the maturity of the roots but is governed by other factors. With the termination of the flow of materials from the vines, the carbohydrate transformations characteristic of sweet potatoes in storage are inaugurated. These are principally the changes of starch to sugars. 53

  Summary.--The root habits of sweet potatoes are closely related to cultural practice in many ways. Chief among these are the selection of the proper kind of soil and its thorough preparation before the plants are set; the effects of the application of fertilizers and water of irrigation upon the root habit; the relation of the roots to drought and soil aeration; manner and time of cultivation in relation to the root system and the rooting of the vines; the effect of pruning the stems on root development; spacing of the plants in the field; and, finally, physical and chemical activities of the root in relation to time of harvesting.


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