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Alfalfa (Medicago sativa) is a perennial. Its length of life is dependent both on the variety and on the environmental conditions. Usually, it live s only 5 to 7 years, but in semiarid regions, fields over 20 years old are found. Common alfalfa has no rhizomes, although they occur on some of the hardy, yellow-flowered varieties of Medicago falcata. They also sometimes appear on some variegated types, i.e., hybrids of ordinary and yellow-flowered alfalfa.
Like the wild legumes of the grassland, alfalfa is deeply rooted. The seedling usually gives rise to a single taproot which takes a vertically downward course. Considerable variation occurs in the number of side roots which arise from the taproot.
Early Root Habit of Common Alfalfa.--A 63-day-old alfalfa plant is shown in Fig. 93. The crop was grown on upland silt loam soil near Lincoln. The great extent of root in relation to top is very striking. When the plants were only a month old (May 1) and the tops were half an inch high, the taproots were already 5 to 6 inches long. The unfolding of the first pair of leaves was accompanied by the appearance of the first lateral roots. In the 2-months-old plants, the absence of laterals in the surface soil was as characteristic as was their abundance deeper, where 10 to 12 rootlets regularly occurred on an inch of the taproot. Some short secondary branches were present and nodules were abundant.
Fig. 93.--Alfalfa plant 63 days old.
Later Development.--By August 10, the roots had reached the stage of development shown in Fig. 94. Owing to a favorable season, the plants had made a fair growth and were 15 inches tall. Roots were fairly abundant to 5 feet, and some had penetrated to a maximum depth of 5.5 feet. The taproot was the prominent feature, laterals not being abundant below the surface 18 inches of soil. The longest branches did not exceed 6 to 10 inches in length. The general course of the roots was vertically downward. Nodules occurred to depths of over 3.5 feet. A crop of similar age on lowland soil had a better development of tops, which averaged 21 inches tall, and the roots were a little more extensive but otherwise similar. Root development in both upland and lowland during the preceding year was very similar to that just described.
Fig. 94.--Alfalfa root 4.5 months old.
Root Habit of Two-year-old Plants.--The lower plot was adjacent to a 2-year-old field of alfalfa. A long, deep trench was dug in this field, and the roots were thoroughly examined during the first week in June. No other plants were present, and the soil was occupied entirely by the roots of alfalfa. The taproots, near the soil surface, were 5 to 10 millimeters in diameter. Just below the crown and to a depth of 1.5 feet, the roots were well supplied with a great abundance of small laterals, usually less than a millimeter in diameter. These often ran parallel with the soil surface for a distance of 1 to 12 inches or even more. Other branches ran more obliquely downward, usually making wide angles with the taproot. They were well supplied with secondary laterals, mostly less than 3 inches in length. All of the rootlets, but especially those in the surface 2 feet of soil, were abundantly furnished with nodules 2 to 3 millimeters long and 1 to 1.5 millimeters in diameter. In fact, these occurred to the maximum depth of root penetration, about 12 feet. The taproots tapered rather rapidly, so that at a depth of 2 feet, the diameter was seldom greater than 1 to 3 millimeters, and below 9 feet none of, the roots were more than a millimeter thick and usually much less. Many ended at depths of 7 to 10 feet, and others extended to the water level at 12 feet, where they terminated with little branching. Not infrequently, the main root ran for distances of 2 to 5 inches in the deeper so il, giving off few or no branches. Branches more than an inch in length were rare in the deeper soil, and usually they were very much shorter. Here, the roots showed a marked tendency to branch only in the soil crevices. As a whole, the taproot predominated throughout, and typically it branched but little, many plants penetrating deeply without giving off any large laterals (Fig. 95).
Fig. 95.--Two-year-old alfalfa root grown in rich lowland soil. Water table at depth of 12 feet.
The actual number of roots in a vertical layer of soil extending only 4 inches into the wall of the trench, together with the lack of large branches, is shown in Fig. 96. The presence of earthworm burrows in the deeper, stiff, clayey subsoil is significant, These, with countless small holes left at all depths, even to 12 feet, upon the death and decay of older alfalfa roots, are very important in aiding soil aeration. It would seem that the excellent development of other crops upon old fields of alfalfa, sweet clover, and red clover, the roots of which penetrate deeply, is due not only to an increased nitrate supply but also to better aeration. The fertilizing effect of the deeper portions of these decayed root systems is below the reach of most crops.
Fig. 96.--Upper portion of 2-year-old alfalfa roots in their natural position in the soil.
Effect of Environment on Root Habit.--One part of this field of 2-year-old alfalfa extended up a rather steep hillside to the crest of the hill. Here, the plants had a good even stand, which was almost as dense as that on the low ground, and the tops were equally well developed. It was discovered that subsoil conditions were far from typical for the region. The surface 1.2 feet of dark clay loam was rather rich in humus, but below this was a subsoil of stiff yellow to slate-colored clay about 2 feet thick. It was somewhat intermixed with streaks of decomposed Dakota sandstone which modified its tenacity. Below 3.2 feet, the clay became very hard and much jointed, roots being largely confined to these joints. The clay was intermixed with pockets and streaks of chalk. The soil was so hard, especially the deeper soil, that it was removed with considerable difficulty. Its glacial origin was shown by pebbles, often 2 to 3 inches in diameter, which occurred throughout, although not abundantly.
The taproots were as large in diameter as those from the lowland, but a marked difference in branching-habit was apparent. Branches were not only very much more numerous but much larger (Fig. 97). In fact, the first 8 inches of taproot gave rise to two or three times as many branches. Some of the larger ones ran obliquely for 1.5 feet from the base of the plant in the surface 18 inches, and then, turning downward, reached depths of 5 to 6 feet. Frequently, they divided into a large, profusely branched, absorbing network, especially prominent in the first 2 feet of soil. Very rarely did any taproots extend beyond 7.5 feet in depth, and all but the deepest were well branched, especially throughout the last 18 inches of their course.
Fig. 97.--Portions of the root systems of alfalfa from lowland (left) and upland (right). Scale 1 foot.
These differences in root habit, namely, a deep taproot with no major branches and relatively few smaller ones in lowland and a shallower taproot with numerous large, widely spreading branches in the upland, must be attributed to soil conditions with their resultant effect upon water content, soil solutes, and perhaps aeration. Which of these or what combination of these is the controlling one can be definitely answered only by a series of experiments carried on under conditions where the effect of each factor can be evaluated. Experiments with several varieties and strains of alfalfa grown at Ithaca, New York, show that in compact soil all varieties and strains develop branch roots, but in open soil the taproots predominate. 39a
Year-old plants grown in fairly moist loess soil in north central Kansas, from the same lot of seed used at Lincoln, had a root habit like that described for plants of a similar age at Lincoln. But those grown on the short-grass plains of eastern Colorado were very different. Although the tops were 11 inches tall and had blossomed, root penetration was limited by a hardpan layer of very dry soil which occurred at a depth of 2 feet. The root habit was very similar to that of the 2-year-old plant shown in Fig. 98. Low water content of soil and very dry air caused the crop to pass many days in a semiwilted or wilted condition, growth being resumed when showers occurred. The taproots were only 2 to 4 millimeters thick but profusely branched with both large and small laterals. Not infrequently, some of these were equal in size to the taproot. A horizontal spread of 1 to 1.5 feet was characteristic, some roots running 2 feet in the dry soil. Small branches were so numerous that the soil was remarkably well occupied by a network of roots, a condition quite unusual in fields of young alfalfa of more humid regions. Indeed, the modification of the root habit was so great that one would scarcely recognize the roots as those of alfalfa.
Fig. 98.--Alfalfa excavated in the short-grass plains on June 28 during the second year of its growth.
Chemical analyses showed that the soils were rich in all the necessary nutrients; nodules occurred on the roots at all levels; aeration could not have been a limiting factor to growth in this dry soil; and, undoubtedly, water played the dominant rôle. Further investigations at Greeley, Colo., substantiated this conclusion.
Root Behavior under Irrigation and in Dry Land.--A study of crops growing with and without irrigation was made at Greeley, Colo. 104 The plots adjoined those already described for corn. Turkestan alfalfa was drilled 1 inch deep in rows 20 inches apart on the dry land but sowed broadcast in the irrigated plot where it was worked into the soil with a hand rake. The dry-land plot was hoed at frequent intervals throughout the season to conserve the moisture, but the irrigated plot was not tilled. Sowing was done on April 11.
Early Development.--Owing to favorable temperatures, the crop grew well, and when 2 months old, that in the irrigated and fertilized soil was 6 inches tall. Each plant had 6 to 8 leaves. In the dry land, owing largely to a lack of moisture, the plants were only half as tall, although they had the same number of leaves. Differences in root habit were already pronounced (Fig. 99). Plants in the dry land, where water was very scarce in the first foot, had penetrated deeper. They had slightly fewer but longer laterals and more secondary branches. Tubercles were also more abundant.
Fig. 99.--Roots of alfalfa plants 2 months old, those from dry land with greater depth and longer branches.
Midsummer Growth.--Root development was again examined on July 8. Practically no efficient rainfall had occurred up to this time. In the dry land, there was almost no water available in the surface foot and only 2 to 3 per cent remained in the deeper soil layers. In the irrigated field, the 3-months-old plants were 18 inches high, and many were in blossom. In the dry soil, they were only half as tall and most of them had not bloomed. A glance at Fig. 100 shows the marked differences in root habit. The prominence of the taproot, its greater depth of penetration, and the relative scarcity of large branches characterized the Plants in the moist soil. This contrasted sharply with the shallower, more profusely branched taproot found in the dry land, where several of the major branches frequently reached depths nearly or quite as great as the main root.
Fig. 100.--Roots of alfalfa plants 3 months old, the one from the dry land having the greater number of branches.
Most of the dry-land plants had from three to six large branches in the first foot; about half of the plants in the moist soil had none, and many had only one, although some, especially isolated individuals, frequently had two or more. There was a greater tendency for the branches to turn downward with less lateral spreading in dry land than in the irrigated soil. The number of small laterals varied from 6 to 10 per inch and was about the same in both cases, although they extended much closer to the root tips in the dry soil. This was due, undoubtedly, to the slower rate of elongation of the main roots. In the irrigated plot, the taproots had grown at the average rate of about half an inch a day, some reaching depths of over 3 feet. Nodules 1 to 2 Millimeters in diameter occurred abundantly over the entire root system of the irrigated plants, but in the dry land, they were smaller, not abundant, and fairly well distributed only in the first 18 inches of soil. Thus, it seems clear that an unfavorable environment not only affects crop growth directly but also indirectly through its influence upon nodule-forming as well perhaps as on other types of nitrogen-fixing bacteria. The activities of other soil organisms are, undoubtedly, also greatly modified.
Root Habit at the End of the First Year.--A final examination was made near the end of the growing season on Sept. 12. Rainfall since July 8 had been very light, and little water had been available at any level in the dry land. This was especially the case in the shallower soil. The second growth in the irrigated plots, the first crop having been cut on July 26, was 26 inches tall. The plants were in full bloom. In the dry land, the crop was only 8 inches tall. A root depth of 6.1 feet had been attained by the more vigorously growing crop, at which level seepage water occurred. Plants in dry land reached a maximum depth of 5.5 feet. The roots were much more kinked and curved, probably owing to the greater difficulty in penetrating the hard, dry soil (Fig. 101). Working depths were 4.6 and 3.2 feet, respectively. As before, the dry-land plants were characterized by a greater number of strong branches in the surface foot and by a marked tendency to spread but little before turning downward and penetrating deeply. The spread of branches in the better watered soil, where they frequently originated at a depth greater than 1 foot, was much more pronounced, sometimes reaching 2 feet . No nodules were found on dry-land plants, but they were abundant to 4 feet in the irrigated soil.
Fig. 101.--Alfalfa roots near the end of the first season's growth: A, dry land; B, irrigated soil.
Root Extent during the Second Year.--By July 10 of the second year, which was relatively very wet, the dry-land alfalfa had extended its depth from 5.5 feet, where dry soil had occurred, to 9 feet. Below 6 feet, the soil was sandy and gravelly. In the watered plot, growth had ceased the preceding season at the 6-foot level, owing to seepage water saturating the soil, but the roots now extended to nearly 10 feet. A comparison of Figs. 101 and 102 shows clearly the differences between the root systems in the two fields.
Fig. 102.--Root systems of alfalfa on July 2 of the second year of growth: A, dry land; B, irrigated soil.
Other Investigations of Alfalfa.--The root habits have also been studied in several other places in Colorado in soil varying from sandy loam to heavy clay. 84 In all cases, a marked taproot with relatively few large or widely spreading branches was characteristic. Six-year-old plants on stiff clay soil near Fort Collins were found to penetrate to a depth of about 12 feet, although 7 feet was the more usual depth of penetration found in the other Colorado soils. Little correlation was found between depth of roots and age of plants. Year-old plants in fine loam with a clayey subsoil were about 4 feet deep but 9-months-old plants had root lengths of nearly 9.5 feet. Roots of 6-year-old plants were found which were larger than 9-year-old ones. The causes for these differences were not determined. They may have been due in part to thickness of planting and the effects of frequency of cutting. In the sandy soils of Wisconsin, where the roots reach depths of 7 to 10 feet and sometimes more, a study of the effects of different treatments on the growth of the crop has been made. In these light soils, too frequent and especially too early cutting greatly retards root development. 145
At Stillwater, Okla., where a porous open subsoil is overlaid by a plastic clay hardpan, root penetration was limited to the soil above the hardpan. Where lime was applied, the roots entered the hardpan but did not pass through it, though on soil which had been treated with barnyard manure, the roots extended through the hardpan. The greatest depth of root penetration and the greatest root development were attained, however, where both lime and manure were applied. The taproots, under these conditions, extended below the hardpan into the more porous lower subsoil. Increased root length of 10 to 23 inches was thus brought about. Whether the results were due to the stimulating effect of the lime and manure on the plant or to their action on the hardpan was not ascertained. 15 Undoubtedly, in soils with a very deep, moist subsoil, alfalfa roots reach great depths. However, roots seldom reach a diameter greatly exceeding 1 inch and are usually about ½ inch thick.
On upland loam soil at Manhattan, Kan., roots have been traced to depths of 8.5 feet 204 and, in stiff clay soils, to a depth of 10 feet without finding an end. Alfalfa roots are said to extend 15 to 30 feet in depth in fairly good soil. It may be recalled that depths of 21 to 25 feet are attained by the wild rose and certain other prairie species. The need for an extensive absorbing system can be appreciated when it is recalled that the crown of alfalfa may produce from 100 to over 300 leafy stems.
Varietal Differences in Root Habit.--In regard to the root habits of different varieties, studies made in South Dakota are of interest.
There are outstanding differences between the root systems of southern-grown common and yellow-flowered alfalfas in the prominence of the taproots, the development of branch roots, the number and development of rhizomes, and in the number and place of most profuse production of fibrous roots. Between many plants of common alfalfa, especially of the less upright forms, and many plants of the Turkestan and Grimm alfalfas, however differences are not great, and it is often impossible to determine by their root systems the groups to which these plants belong. In brief, the root systems of the least hardy forms of purple-flowered alfalfa may be distinguished from the most hardy hybrid and yellow-flowered alfalfas with accuracy, but the intermediate forms are not sufficiently distinct to be distinguishable from one another or invariably from some forms of the non-hardy or yellow-flowered alfalfas. 65
Relation of Root Habit to Crop Production.--An examination of seedling alfalfa roots helps to make clear why a well-prepared seed bed is essential to a good stand, the delicate roots growing best when in close contact with well-pulverized soil. It also explains why a crop sowed early in the fall does not winterkill as badly as the less developed plants of a late-sowed one. Although alfalfa is not exacting as regards soil texture and will grow well even in heavy clay soils, a knowledge of its deeply rooting habit explains why it thrives best in deep, permeable soils, as loam, silt loam, or sandy loam. The roots are very sensitive to poor aeration resulting from standing water, complete submergence for only a day frequently being fatal, and the crop grows poorly or not at all if water stands within 2 to 3 feet of the. surface. Hence, the soil must be well drained; in fact, the deeper root system demands more complete drainage of the soil than do many other field crops. If the soil lacks depth due to such factors as a high water table, hardpan, rock ledge, etc., the crop is unable fully to utilize its deeply rooting habit. Although adapting its root habit to a considerable extent, growth will usually be less vigorous and the plant less able to withstand the competition of weeds rooting in the surface soil. Its deeply rooting habit is a character which well adapts it to semiarid regions, where, because of moderate precipitation, the soils are well drained, rich in unleached lime and other nutrients, but often fairly moist to considerable depths. Its ability to withstand drought and to continue growth, often when other plants are wilting or dead, is due, in a large measure, to its deep root system drawing upon water in the moist layers of the deeper soil.
Because of its large root growth, alfalfa exerts a very beneficial effect upon the soil, even compact soils becoming porous and friable. Aside from the fertilizing effect of the decayed roots and the increased nitrogen supply resulting from the activities of the bacteria in the nodules, some of the nutrients obtained from the deeper subsoil are left near the soil surface upon the decay of the roots and stubble and are thus made available for other plants. The roots and crowns decay quickly, leaving the soil mellow and highly productive. It should be noted, however, that alfalfa requires much more of certain other nutrients, notably phosphorus, potassium, and calcium, than equivalent yields of many other crops, and the soil is consequently more depleted of these elements.
It has been found in Colorado and in Canada that alfalfa plants having a branched root system are better able to withstand winter soil heaving than those having only a single taproot. Plants which develop rooting underground stems are able to maintain themselves after the death of the main root. When alfalfa has the habit of spreading by root proliferations, the plant is better able to recuperate from injury and to withstand cold. It has also been ascertained that such plants are generally more drought resistant. 16a, 193
Recent physiological investigations have shown an important interrelationship between the size and composition of the roots of alfalfa and the productivity of tops.
New top growths, especially in their early stages, are initiated largely at the expense of previously deposited organic root reserves. The quality of these storage materials and their relative availability to the early growth requirements of a young shoot influence its subsequent growth and ultimate production. The translocation of the storage products from the root to the young stems results in root-reserve depletion. This deficiency cannot be restored by supplying mineral nutrients to the plant; but, since the reserves appear to be largely organic, they must be elaborated by the plant itself and translocated to the root. Before the partial depletion of the root reserves is fully replenished, the stems must reach a certain degree of maturity. In alfalfa, this is apparently the seed-pod stage. 145
Frequent cutting in an early bud stage checks root growth. In one experiment involving the measurement of 355 plants, the root diameter near the crown was reduced 29 per cent by two cuttings in the early bud stage as against one cutting in full bloom. The growth of new stems is accompanied by an appreciable decrease in the roots of both available carbohydrate and nitrogen reserves.
With favorable soil and climatic conditions, the growth of new shoots and stems of alfalfa is largely dependent on the organic food reserves remaining in the root and crown when the last cutting is made. Since the early growth of alfalfa stems tends to exhaust the root reserves, the primary cause of root exhaustion is a removal of the leaves and stems before they have had opportunity to replenish completely these losses. Cutting the crop too often in an immature stage ultimately robs the root and crown of its stored products, and the plant is left in a weakened and exhausted state. Each successive premature cutting results in increased detrimental effects until finally the plant dies. 145
Prematurely cut alfalfa often exhibits decided pathological symptoms of yellowing and stunted growth. Accompanying decreased vigor and stand is a corresponding increase in weeds and bluegrass infestation. Depleted root reserves are associated with a high water content of the root and a low concentration of dissolved organic constituents, a condition which may be a contributing factor in winterkilling. 145
Alfalfa is a long-lived, very deeply rooted perennial. Upon germination, a strong taproot develops rapidly and penetrates almost vertically downward. It often reaches a depth of 5 to 6 feet the first season, 10 to 12 feet by the end of the second year, and may ultimately extend to depths of 20 feet or more. It is notably a deep feeder. In common alfalfa, practically no branches occur in the surface few inches of soil, and those that originate deeper do not spread widely but turn downward after running a little distance obliquely and usually pursue a course more or less parallel with the taproot. Often, both large and small branches are quite scarce, and the taproot is always the most prominent part of the root system. Under favorable soil conditions, nodules occur at all depths. The root habit shows considerable variation among the different varieties, and that of a given variety varies markedly under different environmental conditions. When depth of penetration is limited, the degree of branching and wide lateral spreading of branches may become very pronounced. The crop makes its best growth in deep, moist soils where the full extent of its deeply penetrating root system may be utilized. A close relation exists between development of tops and growth of roots, too frequent and, especially, too early cutting retarding root growth.