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Clovers are among the most widely known and most important of cultivated legumes. They are a basic, crop in the agriculture of the greater part of the northeastern States and as far west as the Great Plains. The great value of clovers as pasture plants and as hay crops is familiar to nearly every one. Since they are able, with the aid of bacteria in their root tubercles, to make use of the free nitrogen of the air and to store it in their tissues, their use in crop rotations is very important in maintaining soil productivity.
Red clover (Trifolium pratense) is a biennial or perennial plant. It is one of the most important and most widely known of all cultivated legumes. The life period is a varietal character, the average, perhaps, being about 2 years. The top develops from a pronounced taproot which penetrates very deeply and possesses an extensive system of laterals.
Early Development.--The root system of a plant 2.5 months old, grown in a field near Lincoln, Nebr., in rich, moist, silt loam soil is shown in Fig. 103. The top, at this time, was scarcely 12 inches tall. The strong taproot, the widely spreading and muchbranched laterals, and the abundance of tubercles are all characteristic.
Fig. 103.--Root system of red clover about 2.5 months old.
By the middle of August of the first growing season, the plants had blossomed profusely, and some were ripening seed. They were about 13 inches tall and had from 3 to 13 stems from each root. Several taproots, 5 to 7 millimeters in diameter near the soil surface, were traced to depths of 4.5 feet. These tapered so rapidly that at a depth of 9 inches they were only a millimeter thick. Frequently, as many as 10 major branches arose from a single root in the surface 8 inches of soil. They ran horizontally or slightly obliquely 4 to 12 inches before turning downward, where, tapering rapidly, some of the longest reached depths of 3 to 4 feet. In addition to these laterals, the first 6 to 12 inches of taproot were clothed with very numerous smaller branches 1 to 7 inches long. Below the first foot, no large branches occurred, although, infrequently, the taproot divided into two more or less equal parts. Short branches, only a few millimeters apart, arose throughout the course of the root to a depth of 3 to 4 feet. The joints of the deeper soil, especially, were filled with networks of rootlets. The taproot was prominent throughout and, in general, pursued a vertically downward course.
On drier upland soil, the plants were smaller and slower in blossoming. There were, also, fewer stalks per plant. The largest roots were only 5 to 6 millimeters in diameter. None penetrated deeper than 4 feet. The root habit was very similar to that described, but the larger laterals were often fewer.
That red clover has a deep, well-branched taproot is further shown by studies in Minnesota. 79 Here, in a rich drift soil of clay, sand, and loam, plants at the ages of 1, 2, and 5 months, respectively, had roots which reached depths of 7, 22, and 68 inches. On all the plants, the large laterals were very abundant, and many were as large as the taproot and penetrated to as great a depth as the taproot.
In New York, where heavy clay soil was underlaid with tenacious, gravelly clay, the taproots reached a depth of 34 inches the first season. Branching occurred throughout their length, some of the branches extending 12 inches from the taproot. 14
Mature Root System.--Further development of the roots consists largely in their deeper penetration and the growth of some additional laterals. A mature root system is shown in Fig. 104. A comparison with the young plant in Fig. 103 readily reveals the fact that the territory later fully occupied by the roots was already blocked out early in their growth. The fully grown plants were excavated in deep, fertile silt loam which was underlaid to a depth of at least10 feet with a fairly compact, moist, loess subsoil. Strong taproots, nearly half an inch thick, were common but tapered so rapidly that at a depth of 1 foot, they were seldom more than half as large. The roots penetrated nearly straight downward to depths of 8 to 10 feet. A great mass of fine rootlets arose from the crown and first few inches of the taproot and, running laterally for distances of 6 to 8 inches, quite filled the surface soil. As illustrated in Fig. 104, a few larger branches, 2 to 3 millimeters in diameter, arose in the surface foot and extended out horizontally or obliquely for distances of 0.5 to 1.5 feet before turning downward. These usually extended to the third- or fourth-foot level. Below the surface 1.5 feet , lateral roots were more sparse. In the deeper soils, the branches were very fine and ran for several inches without branching.
Fig. 104.--Mature root system of red clover.
Why red clover makes its best growth on rich, deep, well-drained soil and why it does not thrive on low, poorly drained soil may be readily understood from a knowledge of its deeply penetrating root system. One reason why it is often advantageous to seed red clover in a mixture with other clovers and cultivated grasses is that the root systems of the different species vary widely. As a result, the soils of both the upper and lower layers are more fully occupied than they would be by a stand of a single crop.
White clover (Trifolium repens) is a perennial legume with a root habit which is very similar to that of red clover, although somewhat finer, at least, during the first year of its development. Plants grown at Lincoln had pronounced taproots. These reached depths of 2.5 feet by midsummer in both upland and lowland silt loam soil, although the tops were only 3 to 5 inches tall. The taproots were 2 to 2.5 millimeters thick, and the branching throughout was very similar to that of red clover.
At the New York Experiment Station, the taproots of plants growing in stiff clay soil were found to branch about 2 inches below the surface, many roots extending 9 inches from the taproot. While the majority of the roots were within the surface 15 inches of soil, a few reached depths of 2 feet during the first year of growth. 14
Mature plants possess long, deeply penetrating taproots from which originate many profusely rebranched laterals. Root tubercles occur throughout.
White sweet clover (Melilotus alba) is a biennial crop and roadside plant. Like many other legumes, it is characterized by a strong taproot. It makes a fair growth even in soils so depleted of nitrogen and humus that they will not produce other crops profitably. It even thrives on newly exposed heavy clay soils or upon steep embankments where little else will grow. But it makes its best development in fertile, moist soils where it may reach a height of 1 foot in 7 weeks.
Early Development.--Seven-weeks-old plants were found to possess strong taproots which ran vertically downward to depths of 2 to 2.6 feet, although the roots were only 1 to 3 millimeters in diameter. They were exceedingly well branched throughout their course, even to their tips. The branches were small and mostly 2 inches or less in length, although some had reached lengths of 5 to 6 inches. Tubercles were abundant (cf. Fig, 105).
Fig. 105.--Sweet clover plant 63 days old, grown in upland soil.
Four-months-old Plants.--When 16 weeks old, the stems were about half a centimeter in diameter and 21 inches tall. A typical root system is shown in Fig. 106. The taproots were of the same diameter as the stems. They tapered rapidly, some being a little more than 5 feet deep. Rarely more than two large branches occurred on a plant. They originated at various depths to 4 feet. In thin stands,. the roots are often much more branched. Many small laterals clothed the taproot and major branches throughout their course. These varied from a few millimeters to several inches in length. The younger, glistening white roots of the deeper soils branched not at all or only poorly, except where they occurred in the crevices of the jointed soil. Here, perhaps because of better aeration, they branched profusely. Nodules were of frequent occurrence to depths of 3 to 4 feet. In upland soil, plants of the same age but with smaller tops had roots which extended even deeper (6.5 feet).
Fig. 106.--Sweet clover root system about 4 months old.
A similar rapid development of the roots of this species was found at Geneva, N. Y. Here in stiff clay soil, the taproot had reached a depth of 33 inches by midsummer (July 27). Branches extended laterally 18 inches. The surface 1.5 feet of soil was quite filled with these and their sublaterals. 14
Twelve-months-old Plants.--The root system of a 12-months-old plaint is shown in Fig. 107. This grew in rather low level land adjoining a sandy ridge near Central City, Nebr. The old bluegrass sod had been covered to a depth of 1.2 feet by windblown sand from the adjacent hills, and the sweet clover had been planted to prevent further local soil shifting. Below the drifted sand, there was a layer of 1 foot of black sandy-loam, underlaid by 2.8 feet of fairly pure sand. Seepage water occurred above a layer of clay at the 5-foot level. This undoubtedly hindered further root penetration, for mature plants, in welldrained soil, may reach depths of 8 feet or more. The numerous large branches, which, on many other plants, originate also nearer the soil surface, were very characteristic. The first 1.5 feet of the taproot had numerous small laterals only a millimeter or two in diameter. These ran horizontally to distances of 0.5 to 1.5 feet, or even more, and filled the surface sand with a delicate network of absorbing roots. Few of the main laterals spread to a greater distance than 2 feet from the downward course of the taproot, but most of them reached depths of about 5 feet. All were abundantly supplied with extremely well-branched sublaterals so that the soil was well filled with roots to this depth. Thus, sweet clover is not only quite deeply rooted but is also fitted to absorb at all levels in the soil.
Fig. 107.--One-year-old sweet clover root system. Scale in feet.
The root system of the annual white variety, Hubam clover, may exceed 6 feet in length. 231 Comparative field tests in Ohio have shown that the biennial plants have a very much larger and deeper root system than Hubam. The weight of the roots to a depth of a foot was seven times greater, and the percentage of nitrogen in the roots four times as great as in the annual variety. 229
Other Root Relations.--The effects of roots of leguminous crops upon their death and decay in loosening and aerating the deeper soil are very great. In old clover and alfalfa fields, the soil is quite filled with root channels which greatly modify its structure and promote aeration. The amount of manure they add to the soil is likewise significant. This has been calculated from careful measurements made over small areas. In the case of a good stand of 2-year-old red clover, 6,580 pounds of vegetable matter--over 3 tons per acre--were left as roots in the soil. Chemical analyses showed that this included 180 pounds of nitrogen, 71 pounds of phosphoric acid, and 77 pounds of potash. 197 Of course, a long series of decomposition processes is necessary before the materials in the decaying roots again become available for plant use. Similar studies in Michigan have shown, that, during the first season, biennial sweet clover produced 1,825 pounds of air- dried, roots per acre, Hubam clover, 290 pounds, and alfalfa, 1,195 pounds. 137 In a sandy soil at Logan, Utah, 5,630 pounds of clover roots and short stubble were produced per acre in the surface 12 inches alone. 176 In general, there is about 1 pound of roots to 2 pounds of red clover plant aboveground.
It has been shown in the case of both red clover and alfalfa that these crops, through the work of the bacteria in their tubercles, may take more nitrogen from the air than is contained in the hay. 27 Therefore, the mere turning under of the roots and stubble may increase the nitrogen content of the soil. These legumes are, therefore, well suited to a place in a crop rotation which requires the removal of the parts aboveground and the use of the stubble and roots for keeping up the yield of other crops.
Red clover has a pronounced and deeply penetrating taproot. During its first season of growth, it frequently reaches depths of 4 to 6 feet, while mature plants in mellow well-drained soil penetrate to 8 feet or even deeper. Numerous long branches arise from the taproot, especially in the first foot of its course. They occur also, although in less abundance, on the second and third foot. They usually spread laterally 12 to 18 inches before turning directly downward, the longest frequently ending in the third or fourth foot of soil. Supplemented by smaller laterals which, like the larger ones, are fairly well branched, they form with the taproot an excellent absorbing system. White clover has a root habit quite similar to that of red clover, at least, during its first season of growth.
White sweet clover rapidly develops a deep fleshy taproot. This was found to reach a depth of 2.5 feet at the end of 7 weeks and about 6 feet after 4 months of growth. Mature plants have roots 1 to 1.5 inches in diameter and 5 to 8 feet deep. The degree of branching varies somewhat with the rate of planting, being greater in plants that are not too crowded. Many large and numerous smaller branches, all well supplied with laterals, spread rather widely and penetrate deeply, furnishing the plant with an excellent absorbing system. The annual variety has a less extensive root system.
Leguminous crops have a markedly beneficial effect upon soil structure; they add large amounts of nitrogen through their tubercle development, and upon the decay of the roots, the organic matter of the soil is greatly increased.