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All of the many varieties of sorghum (Andropogon sorghum) are annuals. They have a well-developed root system similar to that of corn but generally finer and more fibrous. Shortly after germination and the growth of the radicle or seed root, 14 the first node develops below the surface and from this node the first permanent roots develop. After the permanent roots begin to function, the temporary root [radicle] soon decays." 187
Mature Root Habit of Black Amber Sorghum.--The Black Amber variety of the sorgo group was grown in both upland and lowland soil at Lincoln. Three months after sowing, when the plants were full grown and the seeds were maturing, the root habit was studied. The tough fibrous roots were 3 to 4 millimeters in diameter at their origin and often 1 millimeter thick at a depth of 4 feet. They were of a grayish-white color, those near the surface being strongly tinged with red, but the deeper, younger roots were glistening white. They were very numerous and completely occupied the hard, dry soil beneath the plants. Branches from ½ inch to 3 inches long were exceedingly abundant and were quite well rebranched. In the more mellow surface soil, these branches spread somewhat widely in all directions, but in the hard, jointed subsoil, the branching was confined largely to the crevices and was in one plane. The abundant ultimate branches were hair-like, shining white, and exceedingly delicate. Not infrequently, they occurred in clusters of three to five on a millimeter of root length. Often, they formed cobweb-like mats covered with root hairs in the deeper soil crevices. As a whole, the absorbing system of sorghum is very efficient and so well distributed throughout the soil that it can thoroughly exhaust it of the water available for plant growth. In both plots, the roots reached a working depth of 4 feet, although the average height of tops, 4.4 feet, of the more vigorous plants in the lowland exceeded that in the upland by 1.4 feet. A maximum depth of about 4.6 feet was attained.
Mature Root Habit of Folger Sorghum.--The Folger variety of sorgo, was examined in a compact, upland, sandy loam soil at Manhattan, Kan. 204 Plants 2.5 months old had roots which resembled those of corn, although the fibrous growth of roots near the surface was much less prominent. Midway between the hills, which were 3.5 feet apart, the roots were 6 inches below the surface, and at the hill, the average depth was nearly 3 inches, which was fully as deep as the seeds were planted. Some roots reached depths of 3 feet. At maturity, the surface soil was much more thoroughly occupied, often to the ground line; the working depth was also greater, some roots extending to the 3.5-foot level.
Mature Root Habit of Kafir.--Kafir corn of the Blackhulled white variety, grown at the same Station, 204 had roots penetrating to a depth of nearly 3 feet when the plants were 4 feet tall and nearly 2.5 months old. But the greater number did not penetrate below 18 inches. The roots, though similar to those of corn, were finer and more fibrous, but coarser than those of the sorgo roots just described. The vertical growth was much less strongly developed than in corn, but roots in the surface soil were more strongly developed. At maturity, the greater portion of the roots did not exceed 3 feet in depth, although some were found at 3.5 feet.
Root Development of Blackhull. Kafir and Dwarf Milo.--The root habits of the grain sorghums, Blackhull kafir and Dwarf milo, have been thoroughly studied in sandy loam soil at Garden City, Kan. 140 The crops were grown in rows 3.7 feet apart, the plants being, 8 to 18 inches apart in the row. The plots were well irrigated the preceding autumn but not cultivated after planting, the weeds being kept out by scraping the surface with. a hoe. Roots were examined at four stages of development. When the tops were 1 foot tall and in the 6- to 8-leaf stage, 12 to 15 roots from each plant penetrated to a depth of 1 foot, although some of them were 1.5 feet deep. The lateral spread was 3 feet and the upper soil was well filled with roots to within ¼ inch of the surface (Fig. 88).
Fig. 88.--Root system of a Dwarf milo plant on June 24 at the age of 4 weeks. Maximum lateral spread is 3 feet. (After Edwin C. Miller.)
When the plants were 7 weeks old and 2.5 feet tall, the wide lateral: spread of nearly all of the roots was very marked. Although they reached out laterally 3 feet in milo, yet none were over 2.8 feet deep. The kafir was only 2.5 feet deep but had a lateral spread of 4 feet. Branching was profuse throughout, an average of 30 laterals occurring per inch of main root.
Further studies were made when the milo and kafir were 3 and 4 feet tall, respectively, the former in the seed-forming stage and the latter just heading. The lateral spread had increased to about 3.5 feet for milo, and both varieties had greatly increased their absorbing area in the deeper soil. A maximum depth of 4 feet was attained.
A final examination, when the plants were maturing seed and the height of the milo and kafir was 3 and 5 feet, respectively, showed still further root growth. Both had a very profusely branched root system reaching 3 to 4 feet on all sides of the plant and penetrating to a depth of 6 feet (Fig. 89).
Fig. 89.--Root systems of two Dwarf milo plants On Sept. 3 when the seed was in the milk stage. (After Edwin C. Miller.)
Relation of Root Habit to Drought Resistance and Growth in Poor Soil.--The preceding study was undertaken primarily to determine the fundamental characteristics possessed by the sorghums which enable them to withstand severe climatic conditions better than corn. In these experiments, Pride of Saline dent corn was grown in alternate rows with the sorghums. It is worthy of note that the primary and secondary roots of both sorghums at all stages of growth were more fibrous than those of corn. Each of the three kinds of plants at any period of growth possessed the same number of primary roots, and the general extent of these roots in both a horizontal and a vertical direction was the same for all. The length of the secondary roots was also found to be approximately the same at any examination, but it was ascertained that the number of secondary roots per unit of length of primary root was approximately twice as great for the two sorghums as for corn. Moreover, this root system, which, judging from the number of secondary roots, would be twice as efficient in the absorption of water, supplied a leaf area which was only approximately half as great as the leaf area exposed to evaporation by the corn plants. Thus, the excellent root system, coupled with a relatively small transpiring area and a low water requirement, goes far towards explaining the high degree of resistance of sorghums to drought. Their ability to remain in an almost quiescent state during drought is another important characteristic. The sorghums remain fresh and green during periods of dry weather which would be extremely harmful to corn. In fact, the plants may even cease growth for a considerable time, but when revived by a rain, a vigorous growth rate is resumed. The slow growth of the aboveground parts until an extensive root system has been established is also an important characteristic in resisting drought. Drought resistance has done much to make sorghum the leading crop in the drier parts of the south and west portions of the grassland region. 41
Sorghums are also adapted to a wide range of soils and will thrive under conditions where other crops, like corn, do poorly. This is due in a large measure to their exceedingly well-developed root systems. Even on land that has become too poor and thin to raise corn and small grains, two or three good crops, of sorghum may be grown, often without the addition of fertilizer. This may be due largely to a more thorough occupancy of the soil, especially of the deeper soil, by the sorghum plants than by other crops.
Relation to Tillage and Crop Rotations.--As for corn, methods of cultivation should be practiced with continual reference to the degree of development of the root system. Since sorghum is usually grown in regions where available moisture is nearly always limited, its conservation by a surface mulch and the removal of weeds is imperative. This may be accomplished by frequent and thorough cultivation. But such cultivation should not be too near the young plants. Later cultivations must be shallow. Practically all tests show that deep cultivation does more harm than good, as measured in yield, after the roots have attained their normal spread. As in the case of corn, listed sorghum stands dry weather better and gives better yields than a surface-planted crop, when moisture is deficient during the latter part of the growing season. This is probably due in part, to the more deeply placed root system which is further covered by each tillage operation and is, therefore, not so subject to drought as are the roots of a surface-planted crop.
It is common knowledge in sorghum-growing regions that crops following sorghum are often less thrifty and less productive than the same crops following corn or wheat. Sorghums are believed to be "hard on the land." Sometimes, this injurious effect, is observed for more than one season. For example, in Kansas, winter wheat grown after a crop of kafir has yielded, on an average, 3 bushels less per acre per year during a period of 6 years than wheat grown after corn. This depressing effect upon yield has been thought to be due to the fact that the extensive root system of the heavy yielding sorghum depletes the soil so thoroughly of nutrients and water that it leaves it in a very poor condition as regards texture and as an abode for nitrifying bacteria. Recent investigations indicate that the depression in wheat yield is due to the harmful effects of decomposition products arising from the decay of stubble and roots after the crop is harvested. 182, 17
The root habit of sorghum, though varying somewhat in the different varieties, is very similar to that of corn. The roots are finer and more fibrous, and often have twice as many branches as those of corn in a similar stage of development. The early superficial rooting habit is marked, plants only in the 6- to 8-leaf stage having a lateral spread of 3 feet, with a network of roots extending, even to the soil surface, although the entire root system may be confined to the surface 1.6 feet of soil. Later in development, the roots penetrate the deeper soils, working levels of 3 to 4 feet being common and maximum depths of 4.5 to 6 feet frequent. As a whole, the absorbing system is very efficient and so widely distributed and so profusely branched and rebranched that it may exhaust thoroughly a large volume of the soil of its water available for plant growth.