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The sugar beet (Beta vulgaris) is a biennial. During the first year of growth, it stores a large amount of food in the fleshy taproot and crown or fleshy stem. Most of this surplus is used, if the beet is grown the second year, in the production of aerial shoots. The "beet" itself is largely the very fleshy upper portion of an extensive taproot. The upper part or crown is a very much shortened fleshy stem, upon the apex of which the leaves occur. The root proper may be distinguished from the stern or crown by the two opposite, longitudinal rows of secondary roots (Fig. 91).
The root habits of sugar beets have been studied at Greeley, Colo., at various periods during their development. 104 The fine sandy loam soils are very dry unless irrigated, since the precipitation averages only 13 inches per year. The beets were grown in plots of very similar soil; one lot without irrigation, a second under light irrigation, and the third plot fully irrigated. Barnyard manure had been uniformly spread over both irrigated plots at the rate of 5 tons per acre before the seed bed was prepared. Throughout the growing season, the aerial environment as regards temperature, humidity, wind, and evaporation was, nearly the same in the several plots. Moreover, physical and chemical composition of the soil was much the same (except for manuring), and temperatures at all corresponding depths were almost identical. Hence, the marked differences in root habit were due mainly to other soil differences, the controlling factor being water content with its attendant effects upon aeration and ease of root penetration. Plants in fully irrigated soils, of course, showed the more typical root habit.
A widely grown variety of sugar beet known as Kleinwanzlebener was used. The seeds were drilled about 1 inch deep, in rows 18 inches apart, and in well-prepared seed beds. Later, the crop was thinned so that the beets were 12 inches apart in the row. The plots were subjected to shallow cultivation from time to time.
The Young Root System.--When 2 months old (June 8), the plants had 8 to 10 leaves each and were 4 to 5 inches tall. The root development in the dry land and irrigated plots is shown in Fig. 90. All of the plants were characterized by a strong taproot.
Fig. 90.--Sugar beets about 2 months old: A, dry land (practically no water available in the second foot); B, irrigated soil.
In the dry land, practically no available water occurred in the second foot during April and very little during May, and the widely spreading, much-branched laterals compensated in part for the lack of penetration. In moist soil, the roots already averaged ¼ inch in thickness but tapered rapidly and in descending 3 to 4 inches lost half of their width. Beginning just below the soil surface and for a distance of 2 to 4 inches, laterals occurred in two rows, on opposite sides of the root. They were grouped in clusters of 2 to 4 and seldom exceeded 1.5 inches in length. This root zone was more extensive on the larger roots in the moist soil. Larger branches, originating from all sides of the root, occurred in the deeper soils. Some arose much nearer to the end of the taproot in the less rapidly growing dry-land plants. In dry land, they were 1 to 7 inches in length, the longest ones being profusely rebranched, though in the irrigated soil, they were not only shorter but much more poorly furnished with laterals. Moreov er, these primary branches were less numerous, 6 to 8 per inch, than in dry land, where 8 to 12 occurred regularly on an inch of taproot. Differences in lightly and fully irrigated soil consisted of lateral branches being somewhat more numerous and their sublaterals longer in the drier soil. Thus, the white, succulent, rather tender beet roots are considerably modified by environment.
The Half -grown Root System.--Excavations were again made and the roots examined a month later. Since practically no efficient rainfall occurred during this period, the dry land became almost depleted of available water. Only 2 to 4 per cent remained at a depth of 3 feet. The number of leaves varied from 15 to 18 and was about the same On plants in all plots, but those in irrigated soil were considerably larger., They were 14 inches high in the latter cage, where the tops had a spread of 18 inches, but only 5 inches high in dry land, where the spread of tops was 11 inches.
The roots had made a remarkable growth during the 30-day interval, branching profusely and extending into the third or fourth foot of soil (Fig. 91). The taproot had doubled in length and the absorbing area had increased many fold. Differences in development were even more pronounced than at the earlier examination. The outstanding features of the root system in the fully watered plot were the marked growth of the shallow absorbing system and the development of long, deeply penetrating branches in the subsoil. The dry-land plants had neither of these habits but were characterized by large numbers of horizontally spreading major laterals in the surface 12 to 18 inches, where soil moisture had been constantly more abundant. Among the dry-land plants, the taproots were slightly less than 2 inches in diameter, but in the fully watered plot, they were 2.5 inches thick. All tapered so rapidly that even the larger ones were scarcely half an inch in diameter 6 inches below the surface. In the hard soil of the dry land, they zigzagged downward to depths of 30 to 46 inches, but in moist soil, they pursued a more even vertical course, reaching a depth of 50 to 54 inches.
Fig. 91.--Sugar beets about 3 months old: A, dry land with low water content of subsoil; B, fully irrigated soil.
Branches in the surface soil were very limited in dry land; only tiny rootlets less than an inch in length occurred. Their growth had apparently been stimulated by very recent showers, and they were not yet clothed with root hairs. In striking contrast, the beets which had been irrigated about 9 days earlier had developed 60 to 75 roots per linear inch in zones 4 to 5 inches long on two sides of the taproot. They spread horizontally for distances of 3 to 5 inches and were profusely branched with secondary laterals at the rate of 8 to 12 per inch. This portion of the root system, which had failed to develop in the drier soil, added materially to the absorbing area of the rapidly developing plants. The smaller dry-land plants had many strong, widely spreading laterals in the surface foot, a response, no doubt, to the very low water content of the deeper soil. These became progressively younger and shorter downward, but the number throughout the course of the taproot was usually 4 to 7 branches per inch. The sublaterals were also long and rebranched. In the moist soil, however, long branches occurred regularly in the first, second, and third foot, some being over 2 feet long. The direction of these roots away from the horizontal was marked. Like the taproots, they were profusely branched and rebranched with both long and short laterals.
The roots of the lightly watered beets were in many ways intermediate to those described. The course of the taproot was somewhat irregular but not to so great an extent as in the dry land. The surface roots were, in extreme cases, an inch long though, as a whole, poorly developed since water in the surface soil was scarce, but the larger branches ran more horizontally than those of the more normally developed fully watered plants.
Mature Root System.--A final study of the beets was made in mid-September. Heavy showers had somewhat replenished the soil moisture, although in the unirrigated plot, very little water was available at any time. Each plant possessed 20 to 30 leaves. Those in the irrigated plots were twice as long (about 18 inches) as those in dry land and the photosynthetic area was three to four times as great. Of course, those in the fully watered plot were largest. Effects of drought were very evident. Some of the plants in the dry land had died, and on the rest, many of the outer leaves were dead and the others frequently wilted so badly that they did not recover even at night. The spread of the tops of these drooping plants was 19 inches, As was also that of the lightly watered plants, but in the other plot, it was 23 inches. In the fully irrigated plot, some of the older outer leaves were dead or drying, but this condition was more pronounced in the lightly watered area.
Root development was correlated with that of tops. The fleshy taproots were 3 to 6 inches in diameter in the irrigated soil but only about 2.7 inches in the dry land. Moreover, the dryland plants had increased their depth from the third-foot level to about 3.5 feet (maximum 4.5 feet) since the last examination. Those in the irrigated plots, which were about 4 feet deep, now occupied the fifth and a part of the sixth foot of soil (maximum depth 6 feet, Fig. 92). The root plan of the irrigated plants, with their widely spreading, horizontal surface root system and rather vertically descending and deeply penetrating major branches, was fairly well blocked out at the preceding examination on July 7. Marked changes had occurred in the root habit of the dry-land plants. Here, the two zones of surface laterals, which had then just appeared, were now well developed. At a depth of 1 to 7 inches, in response to the increased surface moisture due to rains, these roots arose in great profusion from two sides of the taproot and ran horizontally for distances of 3 to 7 inches. Near the lower edge of these zones , they were supplemented by larger and longer roots. These spread 10 to 34 inches and were rebranched to the fifth order at the rate of 10 to 14 rootlets per inch, the whole forming a close network in the dry soil. On the larger roots of the irrigated plants, these lateral root-producing zones were longer, as were also some of the smaller roots, but the larger ones, originating from the base, did not extend beyond 2 feet and were less profusely branched.
Fig. 92.--Sugar beets on Sept. 12: A, dry land; B, fully irrigated soil.
A root habit found frequently in the harder soils of dry land and also occasionally in the lightly watered plot was that of the taproot breaking up into two, three, or sometimes more branches of similar diameter. This frequently occurred at depths of 6 to 8 inches and sometimes deeper.
Many of the deeper and formerly short branches in the dry land had now reached 2 to 3 feet in length. Their course was almost invariably rather obliquely downward, practically none spreading laterally through a distance greater than 7 inches. These, with their profuse branches, occupied, more or less completely, the third and sometimes a part of the fourth foot of soil. In the irrigated plots, the roots usually spread more widely, in some instances more than 2 feet.
Seepage water occurred late in the season at a depth of about 5 feet in the fully irrigated plot, and the root penetration was not so great as in the lightly watered one, probably because of deficient aeration. Branching was similar in the two plots, except for a greater tendency for a wider lateral spread of roots in the shallower portion of the soil of the lightly irrigated plot. The sublaterals were longer, sometimes reaching 25 inches, while none were found to exceed 8 inches in the more moist soil. The surface root system of the fully watered plants contained a greater number of roots in the furrow slice, but they were hardly as long as in the drier soil of the lightly irrigated plots.
The beets in the dry land, harvested late in September, yielded at the rate of only 2.5 tons per acre, those in the lightly watered plots, 21 tons, and those in the fully watered plot, 22.5 tons per acre.
Root Development under Increased Rainfall.--During the following, much more humid season the young beets in all the plots were found to have root systems practically the same and were thus in accord with the uniformity of soil moisture. Later, because of a constantly good water supply, the surface laterals spread much more widely in all plots than during the preceding year, and correlated with this was a lesser depth of penetration. As the surface soil in the unirrigated plot became drier, a pronounced tendency was found for the branches to turn downward. As a whole, the root habit more nearly resembled that in the irrigated plots than that of the last year's dry-land plants. The lightly watered soil was sufficiently wet so that little difference was found between the root habit here and in the fully watered plot. In both cases it resembled that of the fully watered plants of the preceding season. Observations at Fargo, N. D., also show that the sugar beet is deeply rooted. 202
Relation of Root Habit to Tillage Practice.--A consideration of the very fleshy portion of the taproot explains why a deep, mellow, easily moved soil is essential for a proper development of the beet. The actual depth of cultivation varies, of course, with the nature of the soil and the previous depth of cultivation. Early tillage prevents the formation of a crust on the soil surface and subsequent difficulty in the emergence of the shoot, as well as in aeration. The time and manner of thinning is closely connected with root injury. Thinning should always be done before the seedlings are too far advanced, usually in the fourth-leaf stage. Later thinning greatly disturbs the roots of even the more vigorous plants, left to mature. The widely spreading, deeply penetrating root systems are undoubtedly In important factor in competition and resultant reduced yields where the crop is grown too thickly. The extensive root habit explains why a deep soil is best for the growth of this crop and why a soil 2 to 3 or more feet deep and free from hardpan and standing water is absolutely essential. Since beets can use generous amounts of water to advantage and since an ample moisture supply is essential for the production of large yields, enough water should be added at each irrigation to promote good root growth. The water should moisten the soil as deeply as the roots penetrate. Cultivation following every irrigation prevents the surface soil from baking, forming a crust, and cracking with resultant loss of moisture and "pinching" of the beets. Cultivation should be shallow so as not to disturb the roots in the surface soil.
Root Growth of "Mother Beets" Used for Seed Production.-- Since the sugar beet is a biennial it does not form seed during the first year of growth. In temperate climates it is necessary to protect the food-stored "beets" from freezing by removing them from the soil and storing them during the winter. Root development of the mother beet during the second year of growth awaits more detailed study but the following facts are of interest. Beets planted in April and May showed some enlargement of the old root and especially a marked development of new side roots. On beets planted in March the side roots were numerous and large while on those planted later in the season they were few and very much finer. The difference in root development was correlated with soil and air temperatures, the lower temperatures of early spring being more conducive to a better growth. This was shown further by the fact that " beets" planted in September when soil and air temperatures were again lower showed extensive root development. New roots are necessary to supply the demands for water made by the new shoots. They also undoubtedly absorb food materials from the soil which supplement the supply accumulated the preceding year and thus promote a good growth of tops and abundant seed production. 150a
The sugar beet has a strong, deep, very fleshy taproot, which in moist soil grows rapidly and almost vertically downward, reaching depths of 5 to 6 feet. Beginning just below the soil surface and extending to a depth of 6 to 10 inches, laterals occur in two opposite rows on the sides of the roots. These begin to appear when the plants are 6 to 8 weeks old and finally occur in great numbers, 60 to 70 per linear inch. Running horizontally 6 to 18 inches or more on all sides of the plant and branching profusely, they form an excellent absorbing system in the surface foot of soil. Numerous larger and longer branches arise usually at depths of 8 inches to 4 feet. They spread from 6 inches to 2 feet laterally and penetrate deeply into the subsoil. Like the taproot, they are profusely branched throughout, at maturityto their tips, with both long and short branches. Thus, the plant is well provided for absorbing water and nutrients at all levels to near the maximum depth of root penetration. This root habit, however, is greatly modified by variations in soil conditions. In dry soil, the taproot is smaller, pursues a more tortuous course, does not penetrate so deeply, and is branched more nearly to the tip. The larger, deeper-seated branches turn downward rather abruptly, reaching depths of 3 to 4 feet. Branching is more profuse throughout. Development of the surface absorbing system may be greatly delayed, although it branches more profusely and may extend even more widely when the soil becomes moist.