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Corn (Zea mays indentata) is a summer annual. It has a coarse fibrous root system which spreads widely and penetrates deeply. The depth of planting apparently bears no relation to the depth of rooting. The first whorl of roots usually arises within an inch or two of the soil surface, even if the seed is planted several inches deep. In this respect, it is similar to other cereals. All of the roots, except those arising directly from the seed (usually three in number), come from the nodes in whorls. But since the internodes are very short, careful examination-is often necessary to ascertain the origin of the 2 to 10 or more roots from each node. The entire group of whorls constitutes the root crown.
Fig. 83.--lowa Silver Mine corn 36 days old.
Early Development.--The general root habit is well illustrated by Iowa Silver Mine corn grown in loess soil in eastern Nebraska. This crop was planted on May 9, after the soil had been shallowly plowed and harrowed. The rows were 3 feet apart, and the kernels were drilled 3.5 inches deep and 1.4 feet apart. When the corn was, 5 weeks old, it had the root habit shown in Fig. 83. Both the seminal and adventitious roots were confined almost entirely to the surface foot of soil. The roots were coarse, from 10 to 15 in number, about 1.5 millimeters thick, and ranged in length from 1 inch to over 2.5 feet. Throughout their length, except near the tip, they were profusely branched, 33 rootlets sometimes occurring on a single inch of the main root. The branches varied from a few millimeters to over 4 inches in length and were themselves rebranched. Only 8 to 12 inches of each of the younger, rapidly growing root ends were free from laterals.
Midsummer Growth.--When the crop was 4 feet tall and the stalks had about 12 leaves each, a second examination was made. This was on July 5, 8 weeks after planting. During the 3 weeks since the first examination, a remarkable extension of the root system had taken place. The main lateral roots had extended even more widely, some of them to 4 feet from the base of the stalk. Many of them had turned downward rather abruptly, penetrating into the second and third and even into the fourth foot of soil (Fig. 84). In addition to these, an entirely new group of roots had arisen. These penetrated more or less vertically downward 3 to 4 feet and filled the soil beneath the plant. This region had not been penetrated by the main lateral root system. Thus, the main vertical roots supplemented the main lateral ones. The longest of the vertically penetrating roots had grown at the rate of over 2 inches per day. That growth was not yet complete was shown by the abundance of new roots, some of which were only a few inches long and had succulent turgid ends 3 to 4 millimeters thick. Moreover, it was clear, from the absence of root hairs and branches near the ends of even the oldest roots, that they were still elongating. The longest branches were confined to the first foot of soil. Many extended to within half an inch of the surface. These, with their many ramifications, formed an intricately dense and efficient absorbing system.
Fig. 84.--Root system of corn 8 weeks old.
At this time, the roots of neighboring stalks, which were only 16 inches apart, had greatly overlapped. Indeed, roots of plants in adjoining rows, 3 feet apart, were drawing upon the same soil area for water and nutrients. Since many roots grew so near the surface, the field was cultivated shallowly with a hoe in order not to disturb them.
The large quantities of water transpired by corn in eastern Nebraska have been measured, 275 pounds or 34 gallons (over a barrel of water) usually being needed to mature a single stalk of corn.
Mature Root System.--The root system of maturing plants was excavated and examined on Sept. 2. The stalks were 8 to 9 feet high, and though a few leaves had dried. most of them were still green. The husks on the ears were beginning to dry, and the kernels were dented. The crop had completed its root growth.
Fig. 85.--Mature root system of corn.
The main lateral root system (Fig. 85) had scarcely increased over that found early in July. Most of the roots of this type were found in the first foot or two of soil. Some retained a nearly horizontal position throughout their entire course. Others ran at various angles from a few inches to 3 feet or more and then turned downward either abruptly or with a gentle curve. The longest extended into the third and fourth foot of soil, or even deeper. Branching throughout was even more profuse than before. Unlike the shallower portion of the root system, the more deeply penetrating part had made a marked development. The number of roots varied from 20 to 35. They either ran straight downward from the base of the plant or obliquely outward to a distance of 2 feet or more and then, with a graceful curve, took the perpendicular line of growth. A few were short and did not grow deeper than 1 to 2 feet, many extended 6 to 7 feet deep, and a few to over 8 feet. All were profusely branched with laterals ranging from less than an inch to 15 inches in length. Usually 10 to 12 of these branches with their sublaterals occurred on an inch of the main root. Thus, over 200 cubic feet of soil and subsoil were quite thoroughly drawn upon for water and nutrients by the roots of a single plant.
A part of these deeply penetrating roots originated as. "prop" or brace roots. These arose in whorls from the lower nodes aboveground. The aerial roots extended obliquely downward until they entered the soil. They were covered with a mucilaginous substance which protected them from drying. The portion in the air stratum was unbranched, but after entering the soil, they could not be distinguished from the other roots. They performed the double rôle of anchorage and absorption.
Summarizing, the shallower part of the root system (main lateral roots) completed its development early, so far as lateral spread was concerned. The main vertical roots developed later and continued to increase in number and extent until well towards the time of maturity. The lateral spread on all sides of the plant was approximately 4 feet and the maximum penetration 8 feet. Thus, corn is furnished with a remarkably extensive and efficient root system.
Relation of Root Habits to Tillage Practice.--A study of the root habit shows clearly why corn does best on a deep, well-drained soil which has an abundant and uniform supply of water throughout the growing season. If the soil is well prepared before planting, the main benefits of cultivation are derived from keeping own weeds, preventing the crusting of the surface, and keeping the soil receptive to rainfall. The superficial position of the roots shows clearly why deep cultivation is harmful. Fortunately, weeds are most easily destroyed when coming through the surface of the soil by shallow cultivation such as harrowing surface-planted corn. This also breaks the soil crust, giving a drier and warmer soil and more vigorous crop growth. The harmful effect of letting weeds grow for a time is not entirely due to their rapid removal of water and plant food materials from the soil, but to the breaking of the corn roots due to the deeper cultivation necessary for weed eradication.
An examination of the half-grown root system explains why late tillage, except for weed eradication, is of little value. The roots are so well distributed through the soil that little moisture can escape even from uncultivated land. Hilling at the last cultivation not only acts as a mechanical support to the stem but also encourages the development of brace roots which are an additional aid in holding up the plant against strong winds. But if hilled early, later cultivation partly removes the hill and exposes a portion of the root system. Even shallow cultivation cuts off many of the roots, and deep cultivation is very harmful and greatly decreases the yield. Cultivation to a depth of 4 inches during a period of 9 years in Ohio gave a decreased yield every season but one, as compared with similar cultivation to a depth of 1.5 inches. The average decrease per acre was 4 bushels of grain and 183 pounds of fodder. 230 In Indiana, 127 very similar results have been obtained. In Missouri, deep cultivation compared with shallow reduced the yield 6.5 to 13 bushels per acre. 216, 86 The harmful effects of deep cultivation are always more pronounced during years of drought. In Illinois where the roots were pruned to a depth of 4 inches at a distance of 6 inches from the hill, the yield, was decreased 17 bushels per acre. 142 For the highest yields, cultivation should never be deep enough to injure the roots seriously. They should be allowed to occupy fully the richest portion of the soil, which is usually the furrow slice. The proper type of cultivation is one which is deep enough to kill the weeds but shallow enough to reduce root injury to a minimum.
Why listed corn stands dry weather better and why this method of growing corn is more successful in regions of limited rainfall and on comparatively light types of soil may be explained in part by root development. The root system begins its growth deeper in the soil, is further covered by each tillage, and is, therefore, not so subject to drought as is that of surface-planted corn. Listed corn stands up better and is very rarely blown down on account of the roots pulling out. This is probably due in part to the more favorable conditions stimulating the growth of prop roots. A comparison of the rooting habit of listed and surface-planted corn is worthy of detailed study.
The wide spread of corn roots is an important factor in competition. Where the soil will stand a heavy rate of planting, as moist bottom land, undoubtedly the increased yield of drilled over hill-planted corn is due in part to the better distribution of the root system. Another reason, aside from better light, may be due to the fact that the roots are damaged less than under the hill method where cross cultivation is practiced. 86 It has been clearly demonstrated that the results obtained by the "ear to the row" method in variety selections, etc., are often vitiated by the competition of roots and tops of plants growing in close proximity. 119 Why the rate of planting should be decreased in drier and poorer soils may be readily understood by examining the extensive root system and the large amount of water absorbed by the corn plant.
Variations in Root Habit under Different Degrees of Irrigation.--Like most other plants, corn shows marked variations in root habit when grown in soils of different texture and chemical composition, since these not only affect available. nutrients but water content and aeration. Some illuminating experiments were conducted at Greeley, Colo. 104 Here, in fine sandy loam soils of very similar physical and chemical composition, crops were grown under different amounts of, irrigation. The annual rainfall is only 13 inches and irrigation is widely practiced. A yellow dent corn, Minnesota No. 13, a variety considered the best adapted to this region, was used. It was grown in plots which were all treated alike as regards seed-bed preparation, time and rate of seeding, shallow cultivation, etc. One of the plots received no irrigation water, another was irrigated only lightly, and the third was fully irrigated. Moreover, the irrigated plots had been fertilized uniformly with 5 tons of barnyard manure per acre.
Early Development.--At the end of 6 weeks, when the crop was 12 to 15 inches tall, the root systems were examined. Marked differences were apparent. In the fully irrigated soil, where both water and nutrients were plentiful, the roots, as is normally the case, were almost all in the surface foot. Nearly all were quite parallel with the soil surface, and in fact, the bulk of them were in the surface 6 inches of soil. Only a few ran obliquely downward and entered the second-foot layer (Fig. 86). But in the dry land, where only 4 to 5 per cent of available water was present, the direction of growth was different. Although a few roots ran rather horizontally, most of them grew outward and downward 1 to 2 feet. The longest extended 2.5 feet, where they turned downward into the second foot of soil. In fact, this latter tendency was marked, roots seeming to seek moister areas. Some of the more nearly vertical roots even extended a little into the third foot. Root habit in the lightly irrigated plot was intermediate. Although the number of roots was the same in all three plots, there were great differences in both number and length of branches. In the fully irrigated soil, they averaged 12 per inch of main root as compared with 27 in dry land. Branches in the fully irrigated soil were only 2 inches long, with 9 branchlets per inch on an average; in dry land, they averaged twice as long and also had twice as many sublaterals.
Fig. 86.--Root system of 6-weeks-old corn: A, fully irrigated soil; B, lightly irrigated; and C, dry land.
Midsummer Root Habits.--Further studies on July 11, 17 days later, showed even greater differences. No efficient rain had fallen, and only 2 to 4 per cent of available, moisture occurred at any depth in the dry land. Water in the surface 6 inches was entirely exhausted. The corn was beginning to tassel in all the plots, although it was only 2 feet tall in the dry land, where it showed the effects of continued drought. The leaves were tightly rolled during the hottest part of the day, and sufficient water was not available for them to recover even during the cool nights.
In response to the low water content, the roots had made a more extensive growth in the dry land than in either of the other plots. The lateral spread had been increased to a maximum of 42 inches, furnishing water from a region hitherto unoccupied. This exceeded that of the fully irrigated plants by 14 inches. The vertically penetrating main roots were well developed, reaching a working depth of 30 inches and a maximum penetration of 46 inches. This was very different from those in the moister manured soil (Fig. 87) . Here, although the stalks were a foot taller, three-fourths of the root system was still limited to the surface foot. The working level was only 24 inches and the maximum penetration 4 inches greater. Branching, as at the preceding examination, was much less pronounced and the laterals were very much shorter. The soil of intermediate water and air content contained roots that were intermediate in many respects to those described.
Fig. 87.--Root system of corn about 8 weeks old: A, dry land; B, fully irrigated soil.
Mature Root Systems.--During September, the mature plants were examined. In the dry land, drought was so severe that the corn had " fired " and lost most of its leaves during July. It was only 3.5 feet tall, and the ears were very poorly developed. The irrigated corn was 7 feet tall and indicated a heavy yield.
Practically no change had occurred in the roots in dry land, except a more thorough occupation of the soil directly below the plant. Branches now occurred on all the roots to their very tips. The roots were unable to furnish sufficient water to keep the leaves turgid and these in turn failed to supply the materials needed for further root growth. But in the fully irrigated plot, marked changes had occurred. The roots, formerly confined to the first foot or two of soil, now extended into the fifth and sixth foot, and many of the horizontal ones had likewise turned downward and penetrated deeply. A working depth of 40 inches and a maximum penetration of nearly 6 feet were found. Seepage water occurred at the 6- foot level. This fine root system was surpassed, however, by that in the drier and somewhat more sandy soil of the lightly irrigated plot. As in the former case, there were 50 to 70 major roots. The root habit was practically identical with that grown in loess soil (Fig. 85), the working depth and maximum penetration being about a foot less.
It is interesting to note that the yield was in direct relation to the extent of the mature root system. This was at the rate of 25 bushels per acre in dry land, 102 bushels in fully irrigated (later, water-logged) soil, and 115 bushels in the lightly irrigated plot.
Root Development under Increased Precipitation.--These studies were continued during the following year, which was one of abnormally high rainfall. The showers were not only heavier than usual but quite well distributed, thus promoting a good crop growth even on the dry land. Owing to the better watered soil, the young roots showed a more normal surface spread with less tendency to turn downward. The branches were also fewer and shorter than during the preceding year. The. top growth was far more luxuriant (yield, 51 bushels per acre) and the roots too were much more extensive. They had no greater lateral spread, but the working level (about 4 feet) was 16 inches deeper and the maximum penetration was 5.5 feet. The length of branches was again greater than in the fully irrigated plot where root behavior was almost identical with that of the preceding year. Moreover, owing to smaller differences in water content than formerly, root habit in the lightly watered plot was more nearly like that in the fully irrigated one. These results show clearly the profound modifications that are brought about by differences in environment, Since, in these experiments, the aerial environment, i.e., temperature, humidity, wind, Aevaporation, etc., was nearly the same in the several plots, the differences were due to edaphic or soil factors. Among these, as has been pointed out, physical and chemical composition of soil was very much the same (except for manuring), and temperatures at all corresponding depths were almost identical. Hence, the controlling factor in root variation was water content.
Other Investigations on Corn.--The root habits of corn have been examined more or less thoroughly, usually in connection with tillage experiments, in widely separated areas in the United States. At Geneva, N. Y., the lateral spread was found to be 3.7 feet, 14 but the roots were traced to a depth of only 2.8 feet. 162 In stiff clay soil in Pennsylvania, corn seemed to be a shallow-rooted plant. 87 In Illinois , root habits have been observed which were very similar to those described, the plants having a depth of 6 feet or more. 95 At Madison , Wis., it has been found that corn roots grow 4 feet deep and in well-drained soils even deeper, 121 under good cultivation drawing upon soil water in considerable quantities at depths greater than 7 feet. 122 Corn roots commonly reach a depth of 3.5 to 4 feet at Fargo, N. D., 202 and at Manhattan, Kan., 204 they have been found at depths of 4 to 5 feet. Corn grown at Garden City, Kan., in sandy loam soil which was irrigated in the fall, after plowing, with 8 to 10 inches of water, had much deeper roots. 140 A lateral spread of 3.7 feet and a depth of 6 feet were found. Root development at various stages of growth were in all essentials similar to those described.
In comparing these results, it should be kept in mind that disagreements are due, in part, to environment and also to varietal differences in the crops examined. 47 Moreover, in some cases, the roots were not traced to their extremities. Experimental evidence indicates that there are fundamental differences in root systems of various inbred strains of corn. 95 Reduced root systems occur among strains susceptible to root rot and leaf firing. Not only do they have significantly smaller numbers of main roots than plants of good strains, but the number and length of lateral branches are also much less. In some cases, there seems to be an actual deficiency in the root system compared with the vegetative growth above ground. Certain strains have such limited and inefficient root systems that they are unable to function normally during July and August, when soil moisture is low. Plants with reduced root systems are much more susceptible to lodging and give a lower grain production. Differences in the character of the root system are heritable (Fig. 19). 95
In conclusion, it may be said that corn has a remarkably widely spreading, deeply penetrating, and profusely branching root system. A lateral spread of 3.5 feet on all sides of the plant is not uncommon even early in its development, and a depth of penetration of 5 to 6 feet is usual. The degree spreading as well as the depth varies somewhat with soil and other conditions, the root system probably reaching its greatest development in deep, mellow soils only moderately well supplied with water. But even in stiff clay soils, the roots are by no means superficial. Although much more study is needed, it is already certain that extent and distribution of root systems are very important factors in determining the economic value of different strains of corn and their adaptability to varying conditions.