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The common onion (Allium cepa) is a biennial with large bulbs that are usually single. It is the most important bulb crop and is exceeded in value by only four other vegetable crops grown in the United States, viz., potatoes, sweet potatoes, tomatoes, and cabbages. Like all the other bulb crops it is hardy and, in the North, is planted very early in the spring. In the South it is grown as a winter crop. The bulk of the crop is grown from seed sown in the open, but the plants are also grown from transplanted seedlings that have been started under glass or in a seed bed and by planting sets. 93
For root examination the Southport White Globe onion was planted Apr. 10. Especial care was exercised to have the soil fine and loose and with a smooth surface, since the seeds are small and do not germinate quickly. Thorough preparation of the soil is an essential feature in the successful growth of nearly all crops. It is especially important in the production of vegetables. The seed was sown in drill rows in the field where the crop was to mature. The rows were 14 inches apart and the seed was covered 1/2 inch deep. From time to time the soil was shallowly hoed and all weeds were removed from the plots.
Early Development.--The onion develops a primary root which, under very favorable conditions, may reach a length of 3 to 4 inches 10 days after the seed is planted. In the meantime the cotyledon comes from the ground in the form of a closed loop. By the time the first foliage leaf emerges from the base of the cotyledon, several new roots make their appearance near the base of the stem.
The first field examination was made June 4. The plants were about a foot tall and had an average of four leaves each. These varied from 4 to 12 inches in length and 2 to 5 millimeters in diameter. The total area of the cylindrical leaves per plant averaged 8.5 square inches. Each plant was furnished with 10 to 12 delicate, shining white, rather poorly branched roots (Fig. 7). The longest usually pursued a rather vertically downward course to a maximum depth of 12 inches. The lateral spread from the base of the bulbous stern did not exceed 4 inches. The roots were rather poorly branched and frequently somewhat curled or pursued a zigzag course.
Fig. 7.--An onion 8 weeks after the seed was planted.
Effect of Soil Structure on Root Development.--To determine the effect of loose and compact soil on root growth, onions were grown in rectangular containers with a capacity of 2 cubic feet and a cross-sectional area of 1 square foot. A fertile, sandy loam soil of optimum water content was screened and thoroughly mixed and thus well aerated. One container was filled with very little compacting of the soil. It held 173 pounds. Into the second container 232 pounds of the soil were firmly compacted. Surface evaporation was reduced by means of a thin, sand mulch. Onion seeds were planted in each container early in April, and a month later the sides were cut away from the containers and the root systems examined. Owing to clear weather and favorable greenhouse temperatures the plants had grown rapidly and were in the third-leaf stage.
Fig. 8--Onion seedlings of the same age. The one on the right was grown in compact soil and that on the left in loose soil. Both drawings are made to the same scale.
The effects on root growth are shown in Fig. 8. In the hard soil the plants nearly always possessed six roots but only five were found on those in the loose soil. Under the former conditions the vertically descending roots reached depths not exceeding 5 inches; the others spread laterally 1 to 1.5 inches and then, turning downward, penetrated to a total depth of 2.5 to 3 inches. In the loose soil one root from each plant grew downward to a depth of 12 to 15 inches; the others spread laterally 1 to 2 inches and then turned downward and reached a maximum depth of 7 inches. Thus the root system in the loose soil was not only deeper but more widely spread. The roots in the hard soil, moreover, were much more kinky. In the loose soil they made long, gradual curves; in the compact, hard soil, short abrupt ones. Under the latter soil condition the root ends were often thickened. The roots running laterally from the plant were more horizontal in the hard soil during the first 1 to 2 inches of their course. Thus this portion of the root system was shallower than in loose soil. Branches were shorter throughout. Similar results were obtained with lettuce seedlings (see Chapter XXXIV).
Half-grown Plants.--By July 25 the bulbs had reached a diameter of 0.5 to 1.8 inches. From 28 to 33 roots arose from the base of each of the bulbs, i.e., at a depth of 1.5 to 2 inches. Nearly all of these smooth, shining white roots were a millimeter in diameter although a few were only ½ millimeter thick. The longest frequently maintained their initial diameter for distances of 5 to 10 inches; others quickly tapered to a thickness of only ½ millimeter. In fact the roots showed considerable variability in this character, sometimes tapering only to enlarge again. The deepest roots penetrated vertically downward or ran obliquely outward for only a few inches and then turned downward. A working level of 20 inches and a maximum depth of 27 inches were attained.
Some of the main roots ran outward, almost parallel to the soil surface, to distances of 6 to 8 inches before turning downward at an angle of about 45 degrees. These had a maximum lateral spread of about 12 inches on all sides of the plant. Between these horizontal roots and the vertically descending ones, the soil volume thus delimited was filled with numerous roots Which extended outward to various distances and then turned downward, or pursued an outward and downward course throughout their entire extent (Fig. 9).
Fig. 9.--Fibrous root system of onion, 3.5 months old.
As a whole the main roots were poorly branched. In the surface 8 to 10 inches of soil branching occurred at the rate of 2 to 6 laterals per inch of main root, although sometimes as many as 12 were found. Many of the laterals were only 0.5 to 1 inch long, but some reached lengths of 4 to 5 inches. Below 8 inches depth, branches were often fewer and usually shorter, seldom exceeding 1 inch in length. The last few inches of the rapidly growing roots were unbranched. All of the laterals were slender, white, and entirely destitute of branches. Usually they were much kinked and curved and as often extended outward or upward as downward. The absence of roots in the surface 1 or 2 inches of soil is an important character in relation to cultivation. In this respect the onion is quite different from many garden crops.
Mature Plants.--By Aug. 21 the plants had reached a height of 19 inches. Each had four to six leaves which varied in diameter from 0.5 to 0.75 inch. Owing to dry weather the plants were not flourishing and the leaf tips were dry. The bulbs averaged about 2 inches in diameter.
From 20 to 25 roots arose from the base of the bulb. A few ran vertically downward but most of them ran outward at various angles, even to near the horizontal, and then gradually turned downward. The volume of soil delimited at the previous excavation (which had an area on the surface of about 4 square feet) had not been increased except in depth. The former working level of 20 inches had been extended to 32 inches. A maximum depth of 39 inches was found (Fig. 10).
Fig. 10.--A maturing onion excavated August 21. Root growth is not yet completed. Some of the roots shown in Fig. 9 had died.
The uncompleted root growth was shown by the 2 to 5 inches of unbranched or poorly branched, glistening white, turgid root ends. The main roots were very uniform in appearance and about 2 millimeters in diameter throughout their course. They were somewhat kinked and curved, perhaps owing in part to the difficulty in penetrating the rather compact soil. The branching was somewhat uniform throughout and at the rate of three to five, rarely more, branches per inch. These were usually only 0.5 to 1 inch in length although sometimes they reached lengths of 3 to 5 inches. No secondary branches were found. The laterals, as before, were much kinked and curved. Many of them ran in a generally horizontal direction; others extended upward throughout their entire length; still others turned downward or outward and then downward. The surface 1 or 2 inches of soil were entirely free from roots. Compared with most garden crops the onion has a rather meager root system not only in regard to lateral spread and depth but also in degree of branching.
Death of the Older Roots.--The decrease in the number of roots from 28 to 33 on July 25 to 20 to 25 on Aug. 21 is of interest for it is one of the few cases found among vegetable crops where the roots die before the plant approaches or reaches maturity. That death of the older roots originating from the center of the bulb actually occurs was further confirmed by greenhouse studies.
Plants were grown from seed in appropriate containers which held 2 cubic feet of soil. When the plants were nearly 3 months old (June 23) and some roots had attained a depth of 22 inches, the sides of the containers were cut away and the roots examined. From one to three roots per plant arising from the center of the base of the bulb were dead. These were surrounded by six to eight living roots. The dead roots, especially one of them, held a more vertically downward course than the live ones.
Similar observations have been made in both water and soil cultures. "It was observed that the roots formed at the time of germination died at about the time of the formation of the bulb, and that new ones then developed and carried the plants through their complete life cycle." Almost all of the original roots of plants which germinated May 12 were dead 2 months later (2.5 months where grown in soil), at which time the new roots were 5 inches long. It is pointed out that the death of the original roots, which come from the older tissue at the base of the bulb, may be due to a number of causes:
. . . (a) to the senility of the stem tissues, (b) to the senility of the roots proper, and (c) to the convolutions which are formed in the tissues of the stem as the result of irregular growth between old and new tissues and which bring about, quite often, disconnections in the water- and food-conducting tissues. 138
Onions of the Yellow Bermuda variety were studied at Norman, Okla. The seeds were sown Mar. 16, ½ inch deep, and rather thickly to insure a good stand. When the plants were well established they were thinned to 4 inches distant in the row.
Early Development.--Although growth had been slow yet by May 9, when the first root excavations were made, the plants were 8 inches tall and had four vigorous leaves. The root system consisted of 16 to 22 delicate, white, fibrous roots about 0.5 millimeter in diameter. They varied in direction of growth from horizontal to vertical. The roots were rather lax and their course in the soil wavy. The maximum lateral spread was 10 inches and the maximum depth of penetration 12 inches. All except the shortest roots were clothed with unbranched laterals which reached a maximum length of 1 inch, becoming shorter near the root ends. These were rather evenly distributed at the rate of 2 to 4 per inch. The last 2 to 5 inches of rapidly growing main roots were free from branches.
Half-grown Plants.--A second examination was made June 14 when the plants were 16 inches tall. The bulbs were 1.5 inches in diameter and the plants examined had five large leaves. The number of roots had increased to 32, the lateral spread to 16 inches, and the maximum depth to 22 inches. The roots were also much larger than before, with an average diameter of 1 millimeter which they maintained throughout their course. Many of the formerly horizontal or obliquely descending roots had now turned downward at various distances from the bulbs. Branching was at about the same rate as before but much more irregular. The longest branches reached 2 inches. None of the laterals examined were rebranched. On some plants the laterals occurred to within 0.5 inch of the bulb; on others an unbranched area of 2 to 3 inches was found. The cause for this difference was not ascertained.
Growth during the Winter and Second Spring and Summer.--Yellow Bermuda onions, left in the field during the dry late summer and fall of 1925, suffered the death of both their tops and root system.
By the middle of December, the plants had developed new tops about 3 inches tall. These consisted usually of three shoots bearing three leaves each. No relation was found between the size of the bulbs and the number of roots to which they had given rise. Bulbs 2.5 inches in diameter bore from 28 to 97 roots. On a plant with 53 roots their lengths were as follows: 9 were 1 inch or less long, 14 ranged between 1 and 10 inches, and the remainder between 11 and 21 inches in length. They were 1 to 1.5 millimeters in diameter. Their distribution from horizontal to nearly vertical was identical with the root systems already described. The distribution of lateral roots was very irregular. They occurred only on the longer roots and within 1 foot of the bulb. The longest were 2 inches. All of the roots were white and rapidly growing.
Further examination was made on Mar. 9, when the leaves were 6 to 9 inches tall. The number of roots varied from 64 to 207. The soil was well filled with roots within a radius of 18 inches from the plants and to a depth of 18 inches. A maximum lateral spread of 22 inches and a maximum depth of 33 inches were found. Many of these fibrous roots pursued an outward course within 3 inches of the soil surface. At distances of 16 to 20 inches from their origin the larger, horizontal roots usually, but not always, turned downward. Lateral roots, for the most part, were more or less at right angles to the main ones, but considerable variation occurred. They formed a rather complete network of glistening white rootlets to within ½ inch of the soil surface. Even at a distance of 16 inches from the bulb many were 2 to 4 inches in length but quite unbranched. Nearer the base of the plant laterals 8 to 10 inches long were not infrequent. All of the primary laterals, except a few of the longest, were devoid of branches.
By June 20 the plant had an average height of 40 inches, although some exceeded this by 1 foot. Since each bud on the original bulb had developed into an individual plant, a hill now consisted of two to five plants with bulbs 2 inches in diameter but still loosely united by the crowns. In part the heads consisted of many flowers in full bloom but mostly of nearly mature seed pods.
Each stalk was furnished with 32 to 48 roots, the strongest and most profusely branched of any yet observed. Many were 3 millimeters in diameter, often retaining a thickness of 2 millimeters throughout their course. The roots, however, had extended their area of occupation but little to a maximum lateral distance of 22 inches and to a depth of 34 inches. This lack of linear growth had been met in two ways. New roots had replaced some of the older ones which had died and the branches throughout were longer and more profuse than heretofore. In fact the whole root system from near the bulb to within a few inches of the root tips was well supplied with branches. Many of these were I millimeter in diameter and frequently 10 to 12 inches in length. Occasionally, they bore sublaterals. These were never abundant. On a few roots they were found to occur at the rate of two per inch throughout a distance of 3 or 4 inches. They were about 0.5 millimeter in diameter and averaged about ½ inch in length.
Summary.--The onion has a fibrous root system consisting of 20 to 200 shining white, rather thick roots. Some of these spread horizontally just beneath the surface soil 12 to 18 inches on all sides of the plant before turning downward. The soil area to be occupied is blocked out by the plant rather early in its development, and later growth consists chiefly in root elongation. In compact soil the roots are shorter, do not spread so widely, and have shorter branches than in loose soil. The rate of root growth of the Southport White Globe onion is shown in Table 10. Depths of 18 to 32 inches are usual. The roots are poorly furnished with rather short laterals which are usually unbranched As, the older, centrally placed roots die, they are replaced by peripheral new ones. Compared with most garden crops, the onion has a rather meager root system not only in regard to lateral spread and depth but also in degree of branching.
TABLE 10.--RATE OF ROOT GROWTH OF ONION Age, Lateral spread, Working depth, Maximum depth, days inches inches inches 55 4 10 12 106 12 20 27 133 12 32 39
Other Investigations on Onions.--At Geneva, N. Y., onions of the Blood Red variety were examined in the middle of September. The plants were grown in a fertile, clay loam soil overlying a tenacious, gravelly clay subsoil. It was found that
. . . the root system is by no means extensive but it is very much concentrated. The roots seem to take complete possession of the soil beneath a, circle about 8 inches in diameter to a depth of about 10 inches.
Very few roots penetrated beyond these limits. A few spread horizontally 10 inches from the bulb. The laterals are never rebranched. 43
The roots of the Large Red onion were washed from the soil at the same station on June 21. The leaves were but 8 inches long and the bulb only a few millimeters thick, but the roots were found to extend to a depth of 16 inches. Further examination, in September, showed that most of the roots extended no deeper than at the June examination. In a few cases roots were found at a depth of 18 inches but no horizontal roots could be traced farther than 1 foot. 44
The roots radiated from the base of the bulb in all directions below horizontal, some lying no more than 1 inch beneath the surface. They were of equal size throughout their length, except that they were slightly swollen close to the terminus. New roots appeared to be growing, as some were decidedly shorter than the others . . . The branches were all short and never subdivided.
In order to note whether the root growth in this plant the second year differs from that of the first, the roots of a plant of which the full-grown bulb was planted out in the spring were washed out June 10. The system was the same, in general, as that of the first year. An estimate made it probable that the plant had formed at least 400 feet of roots and root branches, though it had been set out but about 40 days. 44
Very few roots of onions grown on gravelly sandy loam soil at Ithaca, N. Y., reached a depth of more than 10 inches. A few were found as deep as 20 inches. The greatest lateral spread was 12 inches but few reached out more than 6 inches. The main root zone was found within a radius of 6 inches. 152
The growth of seedling Yellow Globe onions at the same station was as follows: "At the time when the onion seedlings were set out, each plant had about ten to twelve roots averaging about 3 inches long. Twenty-five days after the plants were set in the field, the roots had reached a depth of only about 2 inches and had a lateral spread of 3 inches. At this time there was an outer whorl of about ten to fourteen coarse roots without branches. Inside of this whorl were about twelve to fourteen finer roots with a few short branches. These smaller roots grew downward. Twenty days later, when the plants were 12 inches tall and a little larger at the base than a lead pencil, the roots had reached a depth of 4 inches. At this time there were about ten to twelve roots growing vertically downward and about twenty-two to twenty-four larger lateral roots extending from 3 to 4 inches from the plant. Ten days later a few roots had reached a depth of 10 inches and a lateral spread of 6 inches, although most of the growth was within from 3 to 4 inches of the plant. At this time there were about twenty-eight to thirty coarse roots with a very few short branches, and twelve finer roots, each having several branches, growing downward from the base of the plant. When the bulbs were 2 inches in diameter the greatest depth reached was 18 inches and few had gone lower than 12 inches. The greatest lateral spread was 9 inches, with very few roots extending to more than 6 inches from the plant. At this time there were thirty-six coarse roots at the base of the outer scales and twelve finer-branched ones growing out of the center of the bulb at the base. When the bulb was full-grown a few roots had reached a depth of 20 inches, but most of them were in the surface 6 to 8 inches of soil and within a radius of 6 inches: Many roots were found very near the surface, especially close to the plant, but the greatest number were found at a depth of from 2 to 3 inches. The tendency of growth of the outer whorl of roots is outward and downward for a few inches, and then nearly vertically downward. The main root zone was within a radius of 6 inches . . . which leaves a space of 6 inches between the rows with few or no roots." 159a
Relation of Root System to Cultural Practice.--The relatively meager root system and correspondingly limited extent of above-ground parts explain why onions can endure crowding better than most vegetable crops. Plants thrive when grown only 3 to 4 inches apart in rows 12 to 15 inches distant. In fact close planting increases the yield. 182 The concentration of the roots into a relatively small area helps to make clear the marked response to fertilizers worked into the surface soil and also why a very fertile soil is necessary for abundant yields.
Few crops require such thorough seed-bed preparation as does the onion and this is undoubtedly due in part to the poor rooting habit of the seedlings. The roots have difficulty in penetrating stiff clay soils and this is one reason why such soils are unsatisfactory unless there is sufficient humus present to lighten them. Neither are the plants adapted to a light, open, gravelly soil, as the roots must grow in a continually moist soil, a good supply of water being especially necessary during early stages of growth. A soil retentive enough to keep a constant supply of moisture about the roots but friable enough to permit expansion of the bulbs and easy root penetration is ideal.
The present-day tendency to sow the seeds more thinly and dispense with thinning is certainly a logical practice as regards the lack of disturbance of a poorly branched root system. Pulling out plants in the process of thinning undoubtedly disturbs the root system and breaks many of the roots of those left in the rows. Thinning when the soil is moist causes less harm. In fact one of the most harmful effects of weeds may be not so much that of shading or absorbing nutrients and water which should be used by the crop as that of root disturbance of the cultivated crop occasioned by their removal. If the root system is thus much disturbed, the onions are likely to ripen prematurely before reaching normal size.
Weeds should be destroyed as soon as they appear above the soil surface, for then their roots are poorly established. The lack of the dense network of laterals, associated with the roots of most vegetable crops, probably accounts for the need of an especially well-prepared, deep seed bed and the inability of onions to compete successfully with weeds. Actual practice has shown that to produce a good yield of onions the weeds must be kept under control and a surface mulch be maintained to conserve moisture.
For many densely rooted crops, experiments in New York have shown that cultivation for the purpose of moisture conservation is not profitable. 152, 158,159a Corn, for example, has such an extensively developed network of roots in the surface as well as in the deeper soil (Figs. 4 and 6) that the roots absorb the moisture before it reaches the surface. 25,135 Likewise, carrots, beets, cabbage, and beans, all deeply and densely rooted. plants, show very little or no advantage in cultivation, as regards moisture content of the soil, over those in plats where the weeds were removed by scraping the surface soil. Although a difference of only about 1 per cent more moisture was found in the plats cultivated once a week, the yield of onions was greater than in plats where the soil was scraped to keep down the weeds. In these experiments, on a gravelly sandy loam, the onions were grown in rows 18 inches apart and 4 inches distant in the row. Since, in this experiment, the main root zone was within 6 inches of the plant, a space 6 to 12 inches wide between the rows contained very few roots. Moreover, since the tops shade the soil but little, it may be clearly seen how a soil mulch resulting from cultivation would conserve the soil moisture. The roots are not so superficially placed as to be injured by shallow cultivation.
There is apparently a correlation between the extent and distribution of the root system and the response of the crops to cultivation for purposes other than the destruction of weeds, moisture being conserved by cultivation for plants like the onion with meager root development. 152,158 Thus it would seem that the usual distance between the rows, about 14 inches for hand cultivation and 18 to 24 inches for cultivation with a horse-drawn cultivator, is quite ample for full root development with little or no competition between plants in adjacent rows. In fact the bulb crops are about the only vegetable crops for which this statement is true.
When onions are transplanted in the "new onion culture," it is usual to trim both tops and roots to a considerable extent. Trimming-the tops reduces the transpiring area and makes reestablishment more certain. The roots are trimmed to facilitate planting and to avoid having any long roots curl upward.
In growing onion sets, the older practice was to sow the seed late in the season on poor soil. Lack of sufficient water and nutrients, resulting in part from thick planting, combined with hot weather, resulted in the desired small bulbs. Now many commercial growers depend upon competition alone. Seeds are sown very thickly on rich soil, often 200 per foot of drill. The rows are spaced only about 1 foot apart. Under these conditions, it is impossible for the competing roots to make a normal growth and secure enough water and nutrients to supply the top which is dwarfed as a result. Consequently, the bulbs never become very large. 93
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