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Oats (Avena sativa) are annual plants. The varieties studied are all summer annuals. The lesser root extent than that of winter wheat is probably correlated with the shorter period of growth. Root habit is very similar to that of spring wheat. The development of White Kherson oats at five different stages of growth was determined on both upland and lowland silt loam soil near Lincoln. The spring was cold and late and the crop at first grew slowly.
Early Root Development.--On May 1, a month after planting and when the second leaf was just beginning to appear, the root habit was ascertained (Fig. 77). Some of the plants had only 3 roots but most of them had 5 to 7. The primary root extended to a depth of 6 to 8 inches. Short laterals occurred on all but the youngest roots at the rate of 7 to 15 per inch. Where the seed was more than 1.5 inches deep, the secondary root system had begun to develop from a node about an inch below the soil surface. That slow growth was due to unfavorable temperatures was shown by the fact that oats planted on May 15 showed a more advanced development of both root and shoot after 15 days than did these plants at the end of 30 days.
Fig. 77.--White Kherson oats 31 days old.
On May 15, the crop was 4 inches high and mostly in the fourth-leaf stage. The plants had not begun to tiller. The number of roots varied from 7 to 14 with an average of about 8 (Fig. 78). The longer ones reached depths of 1.6 to 2 feet and the maximum lateral spread did not exceed 6 inches. Although the youngest and shortest roots were unbranched, the older ones had branches of the second order, some nearly an inch long, but they were not abundant. The primary laterals, especially in the surface 8 inches of soil, had reached a length of 1 to 3 inches. Plants from deeply planted seed now had a whorl of 4 to 6 roots originating an inch below the soil surface. The slow rate of growth is shown by the fact that the same variety of oats planted on the same date the preceding year in mellow loess soil at Peru, in eastern Nebraska, although only 14 days older, had extended its roots into the fourth foot of soil.
Fig. 78.--Oats 45 days old.
Roots of Half-grown Plants.--Fifteen days later, on May 30, the crop was 9 inches high, and the plants had 7 to 9 leaves each. Tillering was very poor, most of the plants having only a single stalk. Root counts of a large, number of individuals gave a total of 16 to 27 per plant (Fig. 79). Several roots were traced to a depth of 2.7 feet. The working depth was about 1.5 feet. Besides the greater number of main roots, their wider spread, and deeper penetration, there was a greater development of laterals, both in number and length as well as in secondary branching. The widest spreading roots ended in the surface 4 to 8 inches of soil. Below 1 foot, branching was poor and the laterals were short, and the last 6 to 12 inches of the thick, white root ends were devoid of branches.
Fig. 79.--Oats 60 days old.
Mature Root System.--At the age of 80 days, the plants, although scarcely more than 2 feet tall, were beginning to blossom. On an average, there were two tillers per plant. The roots had attained a maximum lateral spread of 0.8 foot and a working depth of 2.3 feet, but a few reached a maximum depth of 4.1 feet. A comparison of Figs. 79 and 80 shows that the chief difference between the older root system and the younger, aside from greater growth in length of most of the roots, is the increase in number and the branching of laterals. In fact, at the early stage (May 30), the volume to be occupied by the mature root system was well blocked out. Later, it had been increased somewhat in width and considerably in depth, but especially it had come to be occupied much more thoroughly in all parts by a fine network of delicate roots. In the surface foot of soil, 7 to 10 laterals, and sometimes as many as 15 to 18, occurred on a single linear inch. On the scale to which the drawings were made, it was very difficult to show all of the multitude of rootlets. The laterals were mostly 1 to 2 inches long, infrequently 3 to 5 inches, and were furnished only poorly with secondary branches. Branchlets of the third order were rare. In the second foot, the branches became shorter, and not infrequently for considerable distances, none were over 0.2 inch long. Below 2.5 feet, the number of roots decreased rapidly, although still rather regularly branched at the rate of 5 to 6 branchlets per inch.
Fig. 80.--Oats at the time of blossoming (80 days old).
Owing to a richer soil as well as to a greater available water supply, especially in the surface 6 inches, the crop on the lowland was better developed. The plants had tillered much more freely; they were 2.6 feet tall and beginning to bloom. Maximum root depth was slightly less than 4 feet, but the working depth was 2.7 feet. Little difference was found either in degree of branching or lateral spread.
When the crop was ripe, the root system was found to be slightly more extensive (4 to 4.8 feet), and the soil mass which had been delimited earlier was more thoroughly occupied.
Root Variations under Different Soils and Climates.--At Phillipsburg, Kan., where fairly moist mellow soil was combined with rather and aboveground conditions, the working depth and maximum penetration were 3.3 feet and 6 feet, respectively, the first season and 3 feet and 5.3 feet, the second. But at Burlington, Colo., in the short-grass plains, quite a different rooting habit was found. Here, drought had so dwarfed the plants that they were only 1.5 feet tall and the mature panicles were less than half as large as at Lincoln. Compared with the growth in more humid regions, the roots, while less extensive, were much more profusely branched. Many young roots which had originated near the surface and were densely woolly with root hairs had dried out and died after reaching a length of only 0.3 to 3 inches. This fact leads one to believe that the excellent development of long, rebranched laterals, especially in the surface 1.5 feet of soil, may be correlated, not only with the prevailing relatively low water content, but also with the incapability of the plant to produce new absorbing organs in the dry surface soils. The plants had partly compensated for shallow penetration by a much more thorough occupancy of the surface soil. The root habit, as regards surface lateral spread, abundance of roots at all angles to the vertical, and profuse branching to the very root ends, was very similar to that of wheat grown in an adjacent plot (Fig. 74).
At Flagler, Colo., where the carbonate layer came within 1.6 feet of the surface and below which dry soil occurred, the roots were even more shallow. The working depth was only 1.7 feet and none extended 2 feet in depth.
The results of a study of the root habits of several varieties of oats under a wide range of soils and climate are given in Table 10.
TABLE 10.--DEVELOPMENT OF OATS AT VARIOUS STATIONS Soil Height Work- Maxi Station Variety of ing mum of crop crops, depth, depth, feet feet feet Short-grass plains: Flagler, Colo Yellow Kherson Very fine sandy loam 2.2 1.7 1.9 Burlington, Colo Yellow Kherson Very fine sandy loam 2.2 4.0 5.3 Colby, Kan Yellow Kherson Very fine sandy loam 2.8 2.5 3.0 Averages ........................... 2.4 2.7 3.4 Mixed prairie: Limon, Colo White Kherson Very sandy loam 1.5 3.2 4.0 Phillipsburg, Texas Red Very fine Kan sandy loam 3.8 4.0 4.8 Ardmore, S. D Sixty-day Pierre clay 2.5 2.7 3.2 Mankato, Kan White Kherson Very fine sandy loam 2.8 3.5 4.6 Averages ........................... 2.7 3.4 4.2 Tall-grass prairie: Lincoln, Nebr White Kherson Alluvial silt loam 3.0 2.6 3.4 Lincoln, Nebr White Kherson Silt loam 2.0 3.1 4.1 Lincoln, Nebr White Kherson Silt loam 2.8 3.2 3.8 Fairbury, Nebr White Kherson Clay loam 3.0 4.2 5.3 Wahoo, Nebr White Kherson Silt loam 3.0 4.1 5.3 Averages ........................... 2.8 3.4 4.4
Variations in Root Habits of Different Varieties.--The root habits of University No. 21 oats, a strain of the White Kherson, were studied in loess soil along the Missouri River in southeastern Nebraska. The differentiation into a rather widely spreading, surface-absorbing system and a deeply penetrating portion was quite marked long before the final examination, 92 days after planting, when the crop was ripening. Of mature plants, the lateral spread was over 12 inches. Roots filled the soil fairly well to a depth of 3.8 feet, and below this to 5.4 feet, they were quite numerous. The crop yielded at the rate of 63 bushels per acre. This extensively developed root system undoubtedly plays no small part in making this strain of Kherson oats one of the best adapted to the semiarid conditions of Kansas and Nebraska.
The root system of Swedish Select oats, grown in an adjacent plot, was very similar, except that at no time did the extent or density of the more shallow portion of the root system even closely approximate that of University No. 21. The depth of penetration, however, was even greater, the working depth being 4.6 feet and the maximum penetration 6.8 feet (Fig. 81).
Fig. 81.--Swedish select oats at maturity.
Data on rooting habits similar to those preceding have been obtained for oats grown at Fargo, N. D., 202, 203 Manhattan, Kan., 204 and elsewhere. 121 Variations in root depth, etc., occur not only in different soils but from year to year in the same soil. Moreover, extensive experiments on the root development of different races of oats grown under similar conditions show. essential differences in root length, etc. It has been found that roots increased in length directly with the lateness of ripening. 128 The relation of cultural practices to root habits discussed in the chapter on wheat applies also to oats and the other smaller cereals.
Oats have a system of profusely branched fibrous roots which are very similar to those of spring wheat. This is true not only at maturity but in all stages of their growth. The roots develop rapidly, those of the primary system reaching a depth of 6 to 8 inches by the time the second leaf begins to appear. Frequently, they are accompanied at this time by 3 or 4 roots of the secondary system. In moist open soils, during favorable seasons, they may extend into the fourth foot by the end of 60 days. The general volume to be occupied by the mature roots is delimited early. Later, it increases a little in diameter and considerably in depth. A lateral spread of 6 to 11 inches, a working depth of 2.5 feet, and a maximum depth of 4 to 5 feet are usual. Great masses of profusely rebranched roots fill the surface 2 feet or more of soil. Root habit varies greatly with soil conditions. The whole root system is sometimes confined by lack of water penetration to the surface 18 to 24 inches of soil, but in deep, mellow, fairly dry soil, some roots penetrate to depths of over 6 feet. Under the latter conditions, a marked separation of the root system into two parts is often apparent. A more shallow, widely spreading part occupies the surface 6 to 8 inches and the remainder of the roots form the deeply penetrating portion. Root branching and depth also vary with the variety.