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Barley (Hordeum vulgare) is grown either as a summer or winter annual. The root system is very much like that of oats and spring wheat. Manchuria barley was grown in experimental plots in both upland and lowland silt loam soil at Lincoln. Root habit was studied several times during the development of the crop.
Early Development.--When 3 inches tall and in the second-leaf stage, a maximum depth of penetration of 10 inches was attained on the upland. Unbranched laterals 0.3 to 1 inch in length were fairly abundant except near the root ends. The general root habit as regards fineness of roots, branching, and lateral spread was almost identical with that of spring wheat and oats, being somewhat intermediate.
Fifteen days later (May 15), when the plants were 4.5 inches high and possessed 3 or 4 leaves and 2 or 3 tillers, the roots were 5 to 11 in number. The working level was 9 inches but some roots were 2.2 feet long. A maximum lateral spread of 8 inches had been attained. Short secondary branches occurred on some of the older roots. As among the other cereals, the glistening white, deeper roots -often ran' several inches without branching.
Half -grown Plants.--By June 3, when the plants were in the sixth-leaf stage, the area to be occupied by the mature root system was fairly well delimited except in depth. The shallower roots, which spread 5 to 8 inches on all sides of the plants, were somewhat nearer the surface than those of wheat or oats of the same age. Of the 10 to 17 main roots, the shallowest ended in the surface 3 inches of soil. Many reached depths of 1.5 to 2.5 feet and a few were 3.2 feet deep. The best developed branches were in the surface 1.5 feet of soil. Here, often as many as 15 laterals per inch occurred, but they were only an inch or two long and secondary laterals were nowhere abundant. The last 6 to 12 inches of the rapidly growing deeper roots were entirely devoid of branches. The working depth was 1.8 feet.
Mature Root System.--At the time of blossoming, when the plants were 2.3 feet tall and only fairly well tillered, a great tangle of well-branched roots spread laterally from medium-sized plants to distances of 7 to 10 inches and occupied the soil thoroughly to the working level at 2.7 feet. Roots were quite abundant 8 to 10 inches deeper, the longest reaching depths of 4.4 feet. They were more abundant in the surface 3 inches of soil than were those of either wheat or oats. The development of secondary rootlets was very similar to that of wheat or oats. Wheat, however, was more abundantly supplied with finer rootlets than either oats or barley.
When the grain was ripe, 22 days later, the working level had reached nearly 3 feet and the maximum depth 4.7 feet. The volume of soil under the plants was even more completely filled with great masses of finely branched roots, the whole forming an exceedingly efficient absorbing system. On the lowland where the tops were more luxuriant, both working level and maximum root penetration were about half a foot greater. Similar root relations were found during subsequent years.
Other investigations, where barley was grown in large pots, indicate that maximum root development, as regards weight of roots, is reached at about the time that fertilization takes place. Here, root growth culminated with the final stage of preparation of the plant for grain formation. The physiological explanation of this is that during the period of vegetative growth, the plant needs large supplies of nitrogen and ash constituents to aid in building up a strong shoot in readiness for grain formation, and the root constantly increases in order to be able adequately to meet this demand. During the reproductive phase, on the other hand, vegetative development is reduced to a minimum and the whole energy of the plant is diverted towards the grain. Although nitrogen and ash constituents are just as essential as before, the area of supply is increased as migration of these substances from the straw into the grain goes on from the outset. 19 If the water supply is limited, however, these conditions may be somewhat modified.
Root Variations under Different Soils and Climates.--In loess soil in northcentral Kansas, lateral spread of roots and degree of branching were very similar to that at Lincoln, but the depth of penetration was somewhat greater (maximum 6.7 feet). In the short-grass plains, root penetration was limited by dry subsoil to the surface 2 to 2.5 feet. Under these drought conditions, the roots extended even more widely in the surface soils than those of wheat. Great mats of branches occurred in the surface 6 to 12 inches, forming a profusely developed absorbing system on all sides of the plant, even to a distance of 1 to 1.2 feet.
Shoot development and yield were correspondingly reduced with depth of root penetration. It was clearly demonstrated that this shallow root habit was due to lack of moisture in the subsoil and not to soil nutrients. During another season, an unusual amount of soil moisture, partly due to the accumulation of drifting snow, moistened the subsoil. The crop, which was 2.5 feet tall, had a working depth of 4.2 feet and a maximum root penetration of 5.7 feet as compared with a maximum penetration of 2.5 feet in the dry soil.
At Fargo, N. D., barley reached depths of at least 4 feet, 186 and at Manhattan, Kan., a root penetration of over 4.5 feet has been found. 204
In the deep, mellow loess soil of southeastern Nebraska, Manchuria barley, when 54 days old, had a slightly greater height growth than 63-day-old plants at Lincoln. The roots were 1.3 feet deeper, and the differentiation of the root system into a shallower and deeper portion was clearly indicated (Fig. 82). The deeply penetrating roots varied from two to four in number, ran vertically or obliquely downward and were profusely branched, sometimes as many as 20 branches occurring on a single inch. On mature plants, the shallow portion was scarcely more extensive than before, extending from 8 to 16 inches on all sides of the plant. The primary root system constituted the part which penetrated deeply. Occasionally, however, a root which had developed later from a node on the stem turned downward and penetrated deeply into the subsoil. Many roots penetrated to over 5 feet and a few were found at 6.3 feet. Branching extended to the root tips and showed that growth was complete.
Fig. 82.--A, Manchuria barley 20 days old; B, 54 days old.
Many investigators have 'found that the presence of fertilizers modifies root development of barley. In containers large enough so that the roots can develop normally and under field conditions, nitrate fertilizers at any level lessen root penetration but greatly increase branching. 225 Potassium salts and phosphates, on the other hand, greatly promote root extent. When liberally fertilized with these salts, plants in moist soil make a more vigorous growth both above and belowground, the roots extending farther into the substratum. For example, at Rothamsted, England, in a very shallow, heavy clay soil in poor tilth, with a compact clayey subsoil, the roots of barley plants were mostly confined to the top 2 inches, and none were found below 6 inches. Deficient aeration was indicated by the decay of some of the roots. But when superphosphates were used, the roots were somewhat deeper, and in the plots treated with barnyard manure, a depth of 9 inches was attained. The plants responded readily to the favorable conditions of penetrability and aeration under pot cultures. They soon reached the bottom of the pots, which were 14 inches deep, where they curved about and formed an extensive growth. 19
Barley, when grown in rich deep soil, has a root habit very similar to that of spring wheat and oats; the fineness of the roots, degree of branching, and lateral spread often being intermediate. As in these cereals, the soil volume to be occupied is early delimited, except in depth, by the widely spreading roots. A lateral spread of 6 to 12 inches is usual, great masses of well-branched roots frequently filling the soil to a depth of 3 to 3.5 feet and maximum depths of 4.5 to 6.5 feet are frequently attained. Sometimes, the differentiation of the root system into a shallower portion and a deeply penetrating portion is very distinct. Barley roots often occur nearer the surface than those of oats or wheat. The root system is very plastic, and where dry soil prevents normal penetration, lateral spread and degree of branching are greatly emphasized. Poorly aerated, heavy clay soils may cause the roots to be very superficial. Addition of fertilizers promotes root development.