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Wheat (Triticum) is an annual. But under our climate and cultural conditions, there are two seasonal forms, viz., spring wheat, which is a summer annual, and fall or winter wheat, a winter annual. Both have a fibrous root system which penetrates deeply into the subsoil. That of winter wheat, perhaps because of the longer season for growth, is more extensive. Upon germination of the grain, the primary root takes the lead, but very, soon, two other roots appear on opposite sides of the first. To this whorl of three, still others may be added, and together they constitute the primary root system. In some cases, there may be as many as eight roots. 160 Early in the development of the plant, roots of the secondary root system grow from nodes above the primary one. The first whorl of roots of the secondary root system always develops within an inch or two of the soil surface. The number of roots increases somewhat in proportion to the number of tillers.
The development of Marquis spring wheat (Triticum aestivum) has been studied in detail both in upland and lowland silt loam soil in eastern Nebraska.
Early Development.--On May 1, a month after planting and when the second leaf was half grown, a typical root system wasdrawn (Fig. 56). The number of roots varied from three to eight. Lateral roots were fairly abundant but entirely unbranched. The greatest lateral spread was 5 inches and the working depth or working level (i.e., a depth to which many roots penetrate and to which depth considerable absorption must take place), 6.5 inches. Compared with the shoot development, the plant had made an excellent growth underground.
Fig. 56.--Marquis spring wheat 31 days old.
A plant 45 days old is shown in Fig. 57. During the 15-day interval since the first examination, two tillers and four or five new roots had developed on most of the plants. Young roots only 1 to 2 inches long were frequent. Thus, the balance between transpiration and absorption was well maintained. Lateral spread had increased 4 inches and working level about 3.5 inches. Moreover, lateral branches were much longer and secondary branches were beginning to appear. The slow rate of growth is shown by the fact that wheat planted in the same field on May 5 had a more advanced growth when only 25 days old.
Fig. 57.--Wheat 45 days old.
Half-grown Plants.--Fifteen days later when the plants were 2 months old, the root habit was again examined (Fig. 58). The crop was now 8 inches high and the parent stalks had four to six leaves. Tillering had increased, and about five new roots, on an average, had been added to the secondary root system. The lateral spread had increased but slightly. Many roots had penetrated deeper, however, and others had spread obliquely downward and, with the increase both in number and length of laterals, had begun to fill in the soil volume already delimited at the earlier stage. The roots had deepened the working level to 1.5 feet, an increase in depth of about 10 inches.
Fig. 58.--Wheat 60 days old.
Mature Root System.--Further studies on June 20, when the crop was in blossom, revealed marked differences. The plants were 2.2 feet tall, but the root system reached a maximum depth (4.8 feet) which was more than twice as great as the height of the shoot. During these last 20 days, there had occurred a marked development of roots (Fig. 59). The working level was at 3 feet; the lateral spread had increased to a maximum of 12 inches. A vast network of rebranched laterals occupied a volume of soil extending approximately 10 inches on all sides of the plant and to a depth of 2 to 3 feet. The total number of roots varied from 20 to 25 according to the number of tillers. Many of these were more superficially placed than those in the earlier stages of development, running rather horizontally or obliquely and ending in the surface 3 to 8 inches of soil. In the surface 2 feet especially, laterals were exceedingly abundant, usually 5 to 9 occurring on an inch of root length. Many of these were short, and few exceeded a length of 4 inches. Secondary laterals were not abundant. In the second foot, the branches were mostly less than 1 inch in length. Below 2 feet, branching was somewhat less pronounced, and in the fourth and fifth foot, numerous roots were characterized by unbranched, very short laterals. The fact that some root ends were without branches for a distance of several inches from the tips showed that growth was not yet complete.
Fig. 59.--Wheat at the time of blossoming.
A final examination, a few days after harvesting on July 15, showed no great change in root development. The roots, except the deepest ones, were somewhat shrivelled and more brittle. Depth and lateral spread had increased only slightly. The crop on the lowland was 4 inches taller and better developed than that on the upland and the working level of the roots 6 inches deeper. No marked differences were found in the branching habit or extent of lateral spread. As a whole, the root system of wheat is a little finer and somewhat more extensive than that of oats (See Chapter VII).
Root Variations under Different Soils and Climates.--A crop from the same lot of seed was grown in mellow, fine sandy loam soil at Phillipsburg, in north central Kansas. Here, the annual precipitation is only 23 inches. But probably owing to 11 inches excess precipitation the preceding year, the mellow loess subsoil was quite moist beyond the maximum root penetration, 5.8 feet. The working level, lateral spread, degree of branching, etc., were about the same as described for Lincoln, in eastern Nebraska. The following season, both working level and maximum penetration were somewhat less.
Wheat was also grown in the hard, fine sandy loam soil of the short-grass plains at Burlington, Colo. Here, the 17 inches of annual precipitation moistens the soil to a depth seldom greater than 2.5 feet. Moreover, cold nights in early spring delay crop development, while later drought dwarfs the plants. Marquis spring wheat, grown on land that had been broken for 2 years, reached a height of only 1.7 feet, notwithstanding that the season was unusually favorable for crop growth. The mature root system was confined entirely to the first 2.7 feet of soil, since no available water occurred deeper. Not infrequently, roots extended laterally 10 to 12 inches only 6 inches below the 'surface. The maximum lateral spread exceeded that of plants grown further east, and the entire root system was more profusely branched. Thus, the roots, although more shallow than normal, were well adapted to extract water and solutes from these surface soil layers of low water content. The marked difference in the degree of branching here and in eastern Nebraska is shown in Fig. 60. During the next season, these findings were confirmed, the root system being slightly less extensive.
Fig. 60.--Wheat roots showing normal differences in branching at Lincoln, L; and Burlington, B; 1, at depth of 1.5 feet; 2, root ends.
Investigations at Limon, Colo., another station in the shortgrass plains, gave similar results. The dry soil, watered by light showers, stimulated the development of an intricately branched and extensive surface-absorbing system at the expense of depth of penetration. In this widely spreading, surface-rooting habit, the crop behaved like the native grama and buffalo grasses.
A variety of red spring wheat on the short-grass plains at Flagler, Colo., was found to have a root system almost identical with that of the Marquis variety. The tops were 2.5 feet tall. The roots extended to a similar depth, where dry soil prevented their further development. Lateral spread was marked and branching was profuse.
In the mellow loess soil along the Missouri River at Peru, Nebr., the shallower portion of the root system of Marquis wheat was not highly developed. But the part fitted to absorb in the deeper soil made a vigorous growth, having a working level of 4 feet. Many roots penetrated deeper, a few to 6.7 feet..
Durum wheat (Triticum durum) was also grown at Peru. Compared to other cereals, it has a rather meager surface-feeding system at maturity. Usually, this consisted of six to eight (rarely more) roots that extended out in an almost horizontal direction from 2 to 14 inches. They usually ended only 4 to 7 inches below the soil surface. The primary roots, accompanied by others, however, ran vertically downward or downward and outward, pursuing a more or less zigzag course. The soil was especially well filled with roots to the fourth foot, many also occurred in the fifth and sixth foot, and several extended even deeper. Maximum penetration was 7.4 feet. Examinations at several periods during its growth showed clearly that the root system developed in correlation with the aboveground parts, for it was only in this way that the increasing demands of the developing shoot for water and nutrients could be met.
Variations in Root Habit under Irrigation.--Marquis spring wheat was grown in dry land and in irrigated soil at Greeley, Colo. 104 Since the precipitation is only 13 inches annually, irrigation is widely practiced. The fine sandy loam soil in the several plots was of very similar physical and chemical composition. The wheat plots were treated alike as regards seed-bed preparation, time and rate of seeding, etc., except that the irrigated plots had been fertilized uniformly with 5 tons of barnyard manure per acre.
Early Development.--Root development on May 10, when the crop was about 6 weeks I old and in the fourth leaf stage, is shown in Fig. 61. The roots in irrigated soil developed quite normally. In the dry land, the available water in the second foot of soil was almost exhausted. Hence, the roots were mostly confined in their distribution to the surface 12-inch layer. The number of roots and branches was about the same in both cases, but the branches averaged considerably longer in the dry land.
Fig. 61.--Root system of Marquis spring wheat 6 weeks old: A, in dry land; B, under irrigation.
Half-grown Plants.--A month later, although a small amount of water was available in the second foot of soil, the dry-land crop was only 13 inches high and showed distinct signs of suffering from drought. The plants had only one or two tillers each. The irrigated plants were 21 inches tall and had about twice as many tillers. Differences in root habit were very striking (Fig. 62). The wider spread, longer primary branches, and the much greater number of secondary and tertiary laterals in the drier soil are clearly evident. The network of roots just beneath the soil surface afforded an efficient means of securing water furnished by light showers. But many of these roots had died from drought, and growth was greatly retarded. Under irrigation, lateral spread was much less developed, but the root system extended very much deeper. Maximum penetration in the two cases was 31 and 65 inches, respectively.
Fig. 62.--Roots of wheat plants 2.5 months old: A, in dry land; B, in irrigated soil.
Mature Root Systems.--A final examination was made when the crop was nearly ripe. The very meager rainfall during the interval since the last examination was entirely dissipated in several light showers, and the soil in the dry land had become progressively drier. Here, the wheat was only 15 inches tall.
Fig. 63.--Roots of mature wheat plants: A, in dry land; B, in irrigated soil.
Only about half of the plants were furnished with a tiller, very few of which had headed. The yield was scarcely 3 bushels per acre. A fine crop, 43 inches tall and yielding at the rate of 29 bushels per acre, had developed under irrigation. Differences in root habit were quite as marked as before. A comparison of Figs. 62 and 63 shows that the roots in the dry land had grown relatively little. The chief differences were a more thorough occupation of the second and, to a slight extent, the third foot of soil. The working depth was 24 inches as compared with 52 in the irrigated plots. In the same sequence, maximum penetration was 37 and 75 inches. The furrow slice in both plots was thoroughly occupied by a large number of remarkably branched superficial roots. Probably owing to the death by drought of many of these in the drier land and to continued growth under irrigation, they were now more abundant and also longer in the watered soil. These, with the profusely branched older roots, formed a wonderfully efficient absorbing system. The greater length and degree of branching of laterals were, as before, very conspicuous in the drier soils.
Root Development under Increased Rainfall.--The following season, when the soils were equally moist, due to increased rainfall, the first examination revealed no differences in root habit. Later, the area occupied by the root system of the dry-land crop was much greater than during the preceding season, owing to a better shoot development, more tillers, and a subsoil with available moisture in which none of the roots died. The lateral spread was as great as formerly, and the working depth was over a foot deeper. In fact, the root habit was more nearly like that in the irrigated soil than that in dry land. The crop was 3 feet high and the yield 25 bushels per acre. Root development in the irrigated soil was approximately the same as the preceding year. A third plot, where light irrigation was practiced both years, gave results intermediate to those just described.
Development of winter wheat under measured environmental conditions has been thoroughly studied at Lincoln, Nebr. 226 A strain of Turkey Red winter wheat (Triticum aestivum), known as Kanred, was grown. It was drilled 2 inches deep in fertile silt loam soil on Sept. 20, and the growth both above and belowground recorded at 10- or 15-day intervals. Growth conditions were very favorable during both years of the experiment, and the crop developed normally.
Early Development.--Ten days after sowing, when the second leaf was about half grown, the roots were excavated (Fig. 64). The number of roots varied from two to five, but nearly all of the plants had three. The primary roots were deepest, extending to maximum depths of 8 to 9 inches. While these roots took a rather vertically downward course, the others usually ran obliquely outward, often later turning downward. The fairly abundant supply of laterals was scattered quite irregularly, the best-branched portions of the root giving rise to 12 or more per inch.
Fig. 64.--Primary root system of 10-day-old plant of winter wheat.
Ten days later, the plants had four leaves, and nearly all had one tiller extending an inch or more from the axil of the first leaf. The leaves were rapidly losing their vertical position, some already being nearly horizontal. A second tiller, originating either from the axil of the second leaf or more commonly belowground near the grain, was also found on most plants. Nearly every plant had a new root in addition to those of the primary root system. These roots of the secondary system were about a millimeter thick, turgid, white, and entirely unbranched but densely clothed with root hairs. They originated from the stem near the grain and ran mostly in a horizontal position or turned only a little downward. None exceeded 2.5 inches in length. The roots of the primary system had extended into the second foot of soil, elongating at the rate of over half an inch per day (Fig. 65). They were more frequently branched, and the branches were longer than before, but no laterals of the second order had appeared.
Fig. 65.--Wheat plant 20 days old. The first root of the secondary system has appeared.
When 30 days old (Oct. 20), most of the parent plants had the fifth leaf about half developed. Each was furnished on an average with four tillers. The largest of these had three leaves and stood as high as the parent plant. The prostrate habit, due to the outward curving of the short stems, was well initiated, the plants having a spread of 3.5 inches on each side of the drill row. This rapid growth of tops was correlated with a pronounced root development. The primary root system had reached a working level of 16 inches and a maximum penetration of 2.8 feet was attained by some plants (Fig. 66). The long, thick, unbranched root ends indicated rapid growth. Not only was the lateral spread greater, but the branches were much longer, and the older portions of the roots possessed many more of them. Branohes of the second order were found only on the oldest laterals from the main roots, but here they were abundant.
Fig. 66.--Wheat plant 30 days old. The secondary root system now furnished 18 per cent of the absorbing area.
The secondary root system, moreover, had made a marked growth. Each plant now had a total of 4 to 10 roots, an average of 4 in addition to the seminal ones. They varied from 1/8 inch to 6 inches in length and turned downward from horizontal to nearly vertical. They had about twice the diameter (1 millimeter) of the seminal roots in the surface soil, and only in the deeper layers was the diameter of the latter equal to that of the roots of the secondary system. All were densely clothed with root hairs, and short laterals occurred on the older roots.
Fig. 67.--A view in the wheat field on October 30, forty days after planting.
The next 10-day period revealed a marked growth. The number of tillers had increased to 7 per plant. These were so well developed that a plant of average size had a total of 20 leaves, more than half of which were fully grown (Fig. 67). To provide water and nutrients for such a large shoot, an extensive root system was imperative. An examination of the latter showed that the roots of the secondary root system averaged 9 per plant. While some were only a small fraction of an inch long, others had a length of 19 inches. In general, they ran rather obliquely outward and downward with an average spread of about 5 inches from the base of the plant (Fig. 68). A few extended into the second foot of soil. The older and longer ones were irregularly branched with short laterals at the rate of 5 to 10 per inch. All the main roots were so densely clothed with root hairs, to, which the soil clung tenaciously, that the smooth, white, root ends .stood out in marked contrast. The primary root system had increased both in working depth (now about 1.7 feet) and maximum extent, a few of the deepest roots having penetrated to 3 feet. The oldest portions of the roots, especially the first foot of the deeper ones, appeared shriveled, and microscopic examination showed a deterioration of the cortex. But the abundant root hairs on the deeper main roots and their branches, together with their bright, turgid appearance, showed plainly that they were functioning vigorously as absorbing organs. Primary branches were longer than before, and on some, secondary laterals were much more abundant. Thus, the efficiency of this portion of the root system was greatly increased.
Fig. 68.--Root system of 40-day-old wheat plant.
Late Autumn Development.--After a 15-day interval, further examinations were made, the crop now being 55 days old. Growth had been very good so that, although the drill rows were 8 inches apart, over much of the field the soil was practically concealed by the plants. The height was only 3 inches, owing to strong curving of the short stems which gave the plants their favorable prostrate winter habit. Tillers had increased rapidly from 7 per plant, 15 days earlier, to 11. On an average, each plant now had 32 leaves, an increase of 11. The photosynthetic area showed a gain of 141 per cent, and dry weight of tops, 160 per cent during the 15-day interval. The extensive tops furnished abundant food for the growth of an elaborate root system.
Fig. 69.--Root ystem of 55-day-old wheat plant.
Roots of the primary system often reached depths of over 3 feet, and a few were found at the 4-foot level (Fig. 69). Branching habit of the primary system had changed but little in the surface 2 feet except that the branches were somewhat longer. The younger portions, as the roots deepened, became clothed with laterals similar in number and in secondary branching to the older parts above. An average of 10 roots of the secondary system was found. They ran almost horizontally or so slightly obliquely downward that few or none occupied the area under the plant where the roots of the primary root system were absorbing. The working level was at 8 inches, but some of the longer roots penetrated to a depth of 20 inches. About half were unbranched or nearly so; others were profusely branched throughout with laterals averaging an inch in length. Moreover, a few secondary branches were beginning to appear.
An examination on Nov. 29, the interval again being 15 days, revealed approximately the condition in which the roots passed the winter. Although some of the tips of the older leaves were frozen, the plants had made a good growth. Tillers had increased to 14 per plant, and the total number of leaves, to about 40. The height and spread of tops had not changed measurably. The primary root system now occupied the soil to a depth of 3 feet (Fig. 70). A few roots reached 4 feet. Thus, some growth in depth had occurred, and branching had increased considerably. Only the stele of these roots remained intact in the surface foot.
Fig. 70.--Root system of wheat 70 days old, showing the extent of root growth before the period of winter dormancy.
Early deterioration of the cortex was probably due to low water content of soil. In number, the secondary roots had increased from 10 to 11, the chief development being in the elongation of those already formed. A comparison of Figs. 69 and 70 shows the considerable increase in branching and the much more thorough occupation of the soil.
Relations of the Development of Roots and Tops.--The relative growth of the tops, including photosynthetic area and dry weight, at the end of the several intervals is shown in Table 6. Here, also, is given the growth of both the primary and secondary root systems. The table includes measurements throughout the period of winter dormancy until growth was resumed in the spring. These data can best be interpreted when plotted with the temperature (Fig. 71), since temperature was the limiting factor to growth, soil moisture and other conditions being quite favorable.
TABLE 6.--GROWTH OF WHEAT FROM SEPT. 20, 1921, TO MAR. 29, 1922 Average Working Number length, Num- Num- Photo- Dry depth roots, roots, Date ber ber synthetic weight, primary second- sec- of of area, shoots, root ary ondary leaves tillers sq. cm. grams system, system system, inches inches Sept. 30 1.5 0.0 8.28 0.013 6.01 0.0 0.0 Oct. 10 3.5 1.6 21.50 0.046 11.0 0.8 0.7 Oct. 20 13.0 4.4 59.46 0.146 16.0 3.9 1.8 Oct. 30 20.6 7.2 93.98 0.239 20.0 8.7 2.1 Nov. 14 31.5 10.5 226.39 0.621 30.0 10.0 4.7 Nov. 29 39.7 13.8 0.835 36.0 11.0 5.4 Dec. 14 42.5 15.1 0.882 36.0 11.0 7.0 Jan. 13 40.0 0.697 Feb. 12 40.5 14.8 0.556 Mar. 14 45.0 15.3 0.490 Mar. 29 61.0 17.7 0.727 36.0
It may be noted that tiller production started about 15 days after planting and was kept up continuously until the middle of December. Leaf output paralleled the growth of tillers, and growth rate, based on dry weight, was very similar. Table 6 shows that the primary root system increased its working level quite uniformly. This was at the rate of 0.55 inch per day during the first 55 days, the extent of branching correlating with root elongation. The secondary root system began to develop simultaneously with the appearance of tillers. On an average, a new root and a new tiller appeared every 4 or 5 days until the middle of November, after which the rate of tillering exceeded that of root production. However, the increase in length and branching of the secondary root system continued with the formation and growth of tillers and at an undiminished rate until the middle of December. Here, growth both above and belowground ended abruptly when the air temperatures averaged almost continuously below freezing and the soil was frozen to a depth of several inches. The number of leaves and tillers decreased slightly due to repeated freezing and thawing, wind whipping, etc., and consequently, the dry weight of tops also decreased. The greater decrease in the latter (44 per cent) was due to the fact that many of the leaves were only partly injured, causing a marked decrease in dry weight but not in numbers. But the root system, even that part subjected to the greatest temperature changes, was apparently uninjured.
Fig. 71.--Graphs showing the growth rate of winter wheat and the temperature of soil and air. The upper temperature line is that of the soil at 6 inches depth, the lower one that of the air temperature.
During the second week in March, both roots and shoots resumed growth. Frost disappeared from the soil as the air temperature became higher (Fig. 71), and the plants developed slowly at an average temperature of 40°F. Rains replenished the water content of the surface soil, and with increasing temperature, the crop made a steady growth. The primary root system was apparently functioning as in late fall. Many new roots of the secondary system appeared. The culms began to grow 'erect and the spikes to develop. In less than 2 weeks, at an average temperature of 42°F., the crop regained 60 per cent of the loss in dry weight which had occurred during the 90 days of winter dormancy. This vigorous early spring growth was due largely to the well-developed root system and culms stored with food.
Absorbing and Transpiring Areas.--Determinations were also made of the relation of the actual absorbing area (exclusive of root hairs) to that of the transpiring area of leaves and stems. The work was done during the following season when conditions for growth were very similar' to those described. This was accomplished by carefully washing the soil from the roots with a gentle water spray, thus securing the root systems in their entirety and floating them in shallow trays of water while measurements were being made. These data are given in Table 7.
TABLE 7.--LENGTH AND ABSORBING AREA OF ROOT SYSTEM (EXCLUSIVE OF ROOT HAIRS) AND PHOTOSYNTHETIc AREA OF TOPS, 1922 Area Area Total Photo- Length Length Total primary second- area of synthetic primary of sec- length Date root ary root area root ondary of system, system, system, tops, system, root sys- roots, sq. cm. sq. cm. sq. cm. sq. cm. cm. tem, cm. Sep. 30 9.63 0.00 9.615 7.68 95.05 0.00 95.05 Oct. 10 31.02 0.37 31.39 22.50 323.87 1.50 325.37 Oct. 20 50.31 11.20 61.51 50.50 490.40 40.70 531.10 Oct. 30 62.80 42.32 105.12 88.98 632.70 207.45 840.15 Nov' 14 115.97 121.54 237.51 215.30 1,509.70 1,171.80 2,681.50 Nov. 29 151.76 157.92 309.68 280.00 2,004.70 1,234.70 3,239.40
The absorbing area of roots increased progressively with that of tops and was uniformly 10 to 35 per cent greater in extent (Fig. 72). Since microscopic examination indicated that practically all of the roots and their branches were clothed with functioning root hairs, except at the growing tips and on the oldest parts of the primary roots, the absorbing area was actually probably eight to ten times greater than the transpiring area. Deterioration of the cortex on the oldest portions of the roots of the primary root system began about the middle of November. By the end of the month, the epidermis on about the first 10 inches was either destitute of root hairs or sloughed off with the cortex, leaving only the stele intact. This reduced the absorbing root area, however, by less than 1 per cent. On Oct. 20, the secondary root system already furnished 18 per cent of the total absorbing area. This had increased to 40 per cent 10 days later, and by the middle of November, it was 51 per cent, notwithstanding the great increase of the area of the primary root system. Owing to the relatively finer roots of the primary system coupled with more profuse branching, its total length exceeded that of the secondary root system. On Nov. 29, it made up 62 per cent of the 32 meters of root length possessed by an average-sized plant. The absorbing area of the roots, exclusive of root hairs, exceeded the photosynthetic area, which was actually about the size of this page, by nearly 30 square, centimeters. The striking parallelism of the graphs of areas of roots and tops, together with the constantly greater area of the former (10 to 35 per cent) shows clearly the great importance of extensive root development in the economy of the plant.
Fig. 72.--Graph showing: 1, the absorbing area of winter wheat (exclusive of root hairs); 2, photosynthetic and transpiring area; 3, the absorbing area of the Pnmary root system; 4, the absorbing area of, the secondary root system.
Mature Root System.--At maturity, winter wheat has a very extensive root system. As with other cereals, the abundance of roots, lateral spread, and amount and length of branching, as well as the depth of penetration, are quite variable in different kinds of soil and under different climates. A representative specimen of the Turkey Red variety is shown in Fig. 73. It was grown in moist, rich, silt loam soil near Lincoln. The tops were 3.8 feet high and the heads were well filled. Most of the numerous thread-like roots penetrated rather vertically downward, others ran obliquely downward but seldom reached a greater spread than 6 to 8 inches from the base of the plant. Still others ran out parallel with the soil surface for short distances before turning downward. The working depth was found at approximately 4.4 feet, and the maximum root depth was 6.2 feet. Beginning just below the surface and extending to a depth of 4 feet, numerous profusely branched laterals filled the soil. These light-colored roots showed very plainly in the black earth. They were covered with dense mats of root hairs, the rootlets intercrossing in the jointed subsoil in such a manner as to give a cobwebby appearance. It is quite impossible to show these finer roots and all their branches in the most detailed drawing. Below 4 feet, the roots were less abundant but still well branched and supplied with root hairs.' The last 6 inches of the deeper ones were poorly branched with laterals which were only a few millimeters in length.
Fig. 73.--Mature root system of winter wheat.
Root Variations under Different Soils and Climates.--A field only 2 miles distant from that last mentioned, where silt loam intergraded at a depth of 2.3 feet into a very hard tenacious subsoil of clay intermixed with streaks and spots of chalk, gave marked differences in root extent. The crop was 3.3 feet high and of excellent quality. The working depth was only 3.2 feet and the maximum root penetration 4.7 feet, depths approximately 12 to 18 inches less than in the deep silt loam soil. However, in a third field, an equal distance from the first and also examined the same season, much greater root extent was found. Here, the silt, loam soil gradually gave way at a depth of about 1.5 feet to a very deep, rather mellow, loess subsoil. Like that in the other fields, it was moist to great depths. The mature crop was 3.5 feet high. The lateral spread was similar to that described, but roots were fairly abundant to the working level at 4.9 feet. Not a few penetrated to 7 feet, and a maximum depth of 7.3 feet was attained.
Quite in contrast to this excellent growth was that on the short-grass plains. At Flagler, Colo., a field of the Turkey Red variety was rooted entirely in the surface 16 inches of soil. The roots were developed very much as if grown in a large flowerpot, because the soil was moist only to a depth of 15 inches where a very tenacious hardpan, 7 inches thick, occurred. Below this, the soil was less compact but very dry. Such a limited root development was correlated with a poor growth of tops which scarcely exceeded a foot in height.
Relation of Roots to Tops under Different Climates.--Data on the development of roots and tops of wheat at many stations throughout a wide range of climatic and edaphic conditions are tabulated in Table 8. Even a casual examination of the table shows clearly the close correlation between the growth of tops and roots and the better development of both under an increased water content of soil and the presence of moisture in the subsoil as well as a more humid atmosphere. These relations are clearly shown in Fig. 74.
TARLE 8.--DEVELOPMENT OF WHEAT AT VARIOUS STATIONS IN THE GRASS LAND FORMATION Variety Soil Height Work Maxi- Station of crop of tops, ing mum feet depth, depth feet feet Short-grass plains: Yuma, Colo.: Turkey Red Very fine sandy loam 2.1 2.1 2.3 Sterling, Colo Turkey Red Very fine sandy loam 2.0 2.7 2.8 Flagler, Colo Red Spring Very fine sandy loam 2.5 2.5 2.8 Flagler, Colo Turkey Red Very fine sandy loam 1.0 1.4 1.5 Burlington, Colo Turkey Red Very fine sandy loam 2.5 3.8 5.4 Colby, Kan Kanred Very fine sandy loam 3.2 2.0 2.3 Limon, Colo Turkey Red Very fine sandy loam 1.8 2.0 2.8 Limon, Colo Spring Very fine sandy loam 1.7 2.0 2.0 Averages ................................... 2.1 2.3 2.7 Mixed prairie: Union, Colo Turkey Red Very sandy loam 1.8 3.0 4.0 Ardmore, S. Dak Turkey Red Pierre clay 2.6 3.3 4.1 Phillipsb'g, Kan Turkey Red Very fine sandy loam 3.8 4.8 5.7 Mankato, Kan Turkey Red Very fine sandy loam 2.8 3.2 3.7 Averages ................................... 2.8 3.6 4.4 Tall-grass prairie: Lincoln, Nebr Turkey Red Silt loam 3.3 3.2 4.7 Lincoln, Nebr Turkey Red Alluvial silt loam 3.8 4.4 6.2 Lincoln, Nebr Turkey Red Silt loam 3.5 4.9 7.3 Fairbury, Nebr Turkey Red Clay loam 3.0 3.0 4.1 Wahoo, Nebr Turkey Red Silt loam 3.0 3.6 5.0 Wahoo, Nebr Turkey Red Silt loam 3.0 3.8 5.0 Averages ................................. 3.3 3.8 5.4
Fig. 74.--Diagram showing the growth of roots and shoots of winter wheat in rich silt loam or very fine sandy loam under different climates.
At St. Paul, Minn., isolated clumps of spring wheat were found to have roots which spread throughout a radius of 16 inches and had a depth of penetration of more than 4 feet. 82 Scotch Fife, a spring variety grown at Fargo, N. D., had many main roots, most of which ran almost vertically downward, sending out numerous small feeders, which practically occupied the soil to a depth of 4 feet, many roots presumably penetrating a foot or two deeper. A lateral spread of 9 inches was found. 202 Red winter wheat at Manhattan, Kan., has been shown to form a network of fine fibrous roots quite to the surface of the ground. The roots were recovered to a depth of 4 feet, although they probably extended deeper. 204
In root studies of cereals on the Coastal Plain soils of New Jersey, it was found that very little root growth extended beyond a depth of about 8 inches, root development of these crops being almost invariably confined to the area above subsoil. 144 In unproductive, heavy, very poorly aerated clay soils at the Rothamsted Experiment Station, England, few roots of wheat or barley penetrated below the surface 2 to 4 inches and none appeared below the 6-inch level. Where applications of barnyard manure had improved the soil structure, the roots were well branched and several penetrated to a depth of 9 inches. 19
A well-prepared, firm seed bed is essential in growing the smaller cereals. It not only furnishes better conditions for water absorption by the seed but also gives the young roots better soil contact and thus promotes their efficiency in absorbing water. and nutrients. Grain that is drilled at a uniform depth in a firm seed bed that is well compacted beneath will germinate better, and the roots will have a more favorable environment I for growth than grain that is broadcast and worked into a loose soil. A loose crumbly surface soil, however, is best for retaining the water about the roots. Extremely adverse physical conditions, such as packing of the soil by heavy rains or drying and crusting of the surface, may prevent or delay the development of the secondary root system. 130 It has been fully demonstrated that early fall plowing for wheat promotes nitrification and thus furnishes a greater supply of nitrates to the wheat seedlings. 28 This results in increased yields.
A root system that is well established before the beginning of winter is better able to withstand the tearing or breaking of roots sometimes resulting from alternate thawing and freezing and heaving of the soil. It would seem equally important to sow spring grain early enough so that it may develop a deep root system before the advent of the hot dry weather which frequently occurs during the last few weeks before the crop matures. Even casual examination of the root system shows clearly that all of the surface soil is fully occupied with the roots of the crop and that there is no room for weeds. Their roots come into direct competition for water and nutrients with those of the cereal, and if weeds are permitted to grow, the yield of grain will be reduced.
The addition of fertilizers has a marked effect. Nitrogen promotes root branching and retards elongation. In New South Wales, where roots of spring wheat regularly penetrate to depths of about 4 feet, the effect of adding superphosphates is marked. In addition to other useful effects, they encourage rapid growth and deep root penetration, thus enabling the crop to draw upon moisture and nutrient supplies from deeper layers of the subsoil than in the case of land receiving no fertilizer. 53 In one experiment, an increased depth of 8.5 inches was ascertained; and in another the depth of penetration was almost doubled. 217, 128
Both spring and winter wheat are annuals with deeply penetrating, widely spreading, and profusely branching, fine, fibrous roots. Probably because of their shorter period of growth, the roots of the spring varieties are less extensive. The primary root system, consisting usually of a whorl of three roots with their branches, originates from the embryo and is the first to appear. But soon a secondary root system develops from the nodes above, the number and branching of roots increasing in correlation with the number of tillers. Root elongation, under favorable conditions, is very rapid; sometimes, a growth rate of over half an inch a day is maintained for 6.0 to 70 days in the primary root system of winter wheat. Although there is apparently some variation among varieties, the primary root system of spring wheat rather regularly reaches depths of 4 to 5 feet. Roots of the secondary system ramify the soil near the surface 6 to 9 inches on all sides and, likewise, fill the 2 to 3 feet below this area with a network of well-branched roots. Winter varieties are similar in general habit but more deeply rooted. Crops planted early during seasons favorable for growth form a secondary root system which rather thoroughly fills the surface 12 to 20 inches of soil, while the primary roots extend well into the third and fourth foot. The mature root system has a working level of 3.5 to 4 feet and a maximum depth of 5 to 7 feet. Pronounced modifications in root habit occur under different soil environments. Where the subsoil is dry, root depth is greatly abbreviated, and lateral spread, degree of branching, and absorption from surface soils are greatly increased. These differences occur in the same kind of soil. if one portion is irrigated and the other unwatered. Variation in depth and degree of branching also occurs in response to fertilizers. Moreover, in stiff clay soils where aeration is very poor, roots do not penetrate so deeply.