HOME AG LIBRARY CATALOG TABLE OF CONTENTS NEXT CHAPTER
The roots of plants are the absorbers of water and mineral matter from the soil. These, with the carbon dioxide taken from the air, are the materials out of which not only the food for plants but the world's food supply is manufactured by plants. Green plants, only, of all living organisms, are able to build up organic food from the relatively raw materials of the inorganic world. (Certain bacteria, e.g. nitrifying and sulphur bacteria, are also able to build up organic materials. They obtain their energy by oxidizing ammonia and hydrogen sulphide and obtain carbon from the carbon dioxide of the air without the agency of sunlight. The organic substances produced are relatively negligible in amount.) Cultivated plants almost always have a sufficient supply of carbon dioxide for normal growth. Oxygen for respiration and light are likewise usually plentiful. The temperature of the air and soil, except in high latitudes, is seldom seriously unfavorable for plant development, and it is unusual to find the physical or chemical nature of the substratum unsuitable for plant production. But in the principal agricultural sections of the United States, especially in the more and or semiarid parts, water is the chief limiting factor. Insufficiency of water, perhaps more often than any other environmental factor, lessens crop yield. In fact, more than half of the surface of the earth receives insufficient precipitation for the most favorable growth of crops. A method of supplementing this inadequate water supply is by the application of water by irrigation, and the normal supply may be conserved by systems of tillage and crop management which aid in getting as much of the precipitation, as possible into the soil and keeping it there until it is absorbed by plants. The great importance of the seedling establishing contact with a water supply is shown by the vigorous growth of the rootlet which usually reaches a considerable length and often begins to branch before the leaves are unfolded. In fact, the rate of this early root growth may determine the success or failure of establishment in dry situations.
In some agricultural areas, on the other hand, soil water is so abundant that most crops will not grow until the wet soil is drained, often at great trouble and expense, and a supply of fresh air for the roots is admitted. Keeping the soil in proper physical and chemical condition as regards water content, aeration, productivity, temperature, etc., so as to promote vigorous plant growth, is the purpose of tillage even in moderately moist soil where rainfall is approximately optimum. The functioning of the part of the plant aboveground is conditioned very much by the distribution and activities of the root system. The latter may be controlled to a considerable extent by various cultural practices. Thus, it is obvious that an exact knowledge of root development of cultivated plants, of their position, extent, and activities as absorbers of water and nutrients at various stages of growth. is of paramount importance to all who are engaged in crop production. Likewise, a knowledge of modifications that are produced by variations in the soil, whether due to soil structure, to excessive water content or drought, tillage, fertilizers, or other causes, is of no less importance.
The chief activities of roots, aside from growth and anchorage of the plant, are the absorption and conduction of water and nutrients. Absorption may occur throughout the entire extent of the root but takes place most actively in the younger and usually deeper parts.
Extent of Root Systems.--Roots of cultivated plants, contrary to current opinion, are not superficial but deeply seated. Casual examination of the figures on the following pages shows the remarkable extent and great variation in root habits of crops and the great importance of both soil and subsoil in their development.
Studies on the rooting habits of crops, giving a clear understanding of their great extent, have been so recent and the old idea of roots being superficial is so commonly believed that many current textbooks still speak of them as "shallow." The old viewpoint is well stated as follows in one of our best modern works on soils.
It is well known that only the top 6 or 8 inches of the soil is suited to plant life and that the, lower part, or subsoil, plays only an indirect part in plant nutrition- We shall, therefore, confine our attention almost exclusively to the surface layer. 169
Absorption of Water from the Subsoil.--That crops absorb water in large quantities from the subsoil and that both quantity and quality of yield can be greatly modified by the addition of fertilizers to the subsoil have been conclusively demonstrated. 225, 49
Fig. 17.--Barley grown in container with wax seals at 6-inch intervals. The upper portion of the container has been removed.
In One experiment, barley was grown in the field in large cylindrical containers 18 inches in diameter and 3.5 feet deep. The containers were filled in such a, manner that each 6-inch layer occupied the same depth in the container that it had formerly occupied in the field. Each 6-inch layer of soil, of known water content, was separated from the one above and below by thin wax seals. The seals effectually prevented the movement of the water from one soil layer to another but permitted the roots to develop in a normal manner (Figs. 17, 18). When the crop was ripe, it was found that the water had been absorbed from the several levels, beginning at the surface, in the following amounts: 20, 19, 16, 16, 14, 12, and 11 per cent, respectively, based on the dry weight of the soil. Moreover, it was further ascertained, from barley grown in other containers examined at various intervals, that during the period of heading and ripening of the grain, the bulk of absorption was carried on by the younger portions of the roots in the deeper soils, the surface layers by that time being quite dry.
Fig. 18.--Wax seal at a depth of 2 feet showing the penetration of the seal by roots and their abundance under field conditions.
Similar results have been obtained with rye. 213 During the period of ripening, maximum absorption occurred at a depth of 2.5 feet and considerable quantities of water were removed at greater depths. Winter wheat behaved in a similar manner, absorbing most vigorously late in life from the deeper soil layers (2.5 feet or deeper) and reducing the water content to a much, greater degree than in the moister layers above. These results show clearly that absorption by a plant during different stages of its life does not depend upon total root surface or root mass but is determined mainly by the area of the functioning parts of the root system.
Potatoes absorbed water to depths of 2.5 feet. Corn, which uses water extravagantly, absorbed large quantities from the third and fourth foot and smaller amounts from the fifth when grown in containers 3 feet in diameter and 5 feet deep.
Absorption of Nutrients from the Subsoil.--In other experiments, nitrates were placed in the soil in measured amounts (400 parts per million of soil) at various levels. At the time of blossoming, barley had removed 286 parts per million of the nitrates from the third 6-inch level and 135 and 168 parts per million from the next deeper layers, respectively. At maturity, it had absorbed 186 parts per million of nitrates from the 2- to 2.5-foot layer of soil. Potato roots absorbed the nitrates at similar levels, although in slightly smaller amounts, and corn removed 203, 140, and 118 parts per million, respectively, from the third, fourth, and fifth foot of soil. In every case where roots came in contact with a fertilized soil layer, they developed much more abundantly and branched more profusely. Such a layer also retarded normal penetration of the root system into the soil below. Other experiments in which the soil layers were impregnated with monocalcium phosphate showed similar root activities s regards absorption at the several depths.
These experiments show that the roots of crops were active in the absorption of both water and nutrients even at the maximum depth of their penetration. The materials necessary for food manufacture were taken from the deeper soil in considerable quantities, although to a lesser extent than from the soil nearer the surface, which the roots occupy first and, consequently, at least in annual crops, where they absorb for the longest time. It is important to note that these plants received their supply of water and nutrients from the deeper soil layers during the later stages of their development which, in some respects, are the more critical ones.
Effect upon Yield.--To determine the effect of the depth of fertilizers upon the yield, two series of containers were used in which barley was grown. 49 In the first, the soil was fertilized with nitrates or phosphates at different. sealed 6- inch levels. In the second series, the surface foot was fertilized and, in addition, various 6-inch levels of the subsoil. Although the plants used the largest amounts of salts from the surface foot, they also took large additional quantities from the deeper levels when they were available. The absorption of nutrients at levels below the surface affected materially both quantity and protein content of the yield. This effect was pronounced even when the surface foot of soil was abundantly supplied with a similar nutrient. For example, sodium nitrate when applied to the surface foot increased the total dry weight by promoting heavy tillering and also added to the nitrogen content of the grain. But when available at lower levels as well as in the surface foot, it increased still further both the dry weight and the quality of the grain.
Such experiments show the importance of the subsoil as a probable source of nutrients for crops. It seems clear that an attempt should be made to promote, by cultural practices, an ample distribution of the deeper portion of the root system. In the agricultural practices of humid regions, the "rawness" or unproductiveness of subsoil has long been recognized, but it is well known and generally accepted that subsoils of arid regions are not unproductive and that, in them, plants make a good growth. 88, 5, 139a In the semiarid soils of the eastern half of Nebraska, eight successive crops of inoculated alfalfa gave almost as heavy yields on the subsoil as on the corresponding surface soils, although corn and non-inoculated legumes did poorly. 5 That roots absorb phosphorus and potassium as well as other elements from the subsoil is indicated by the fact that plants, when potted in subsoil, will grow if nitrogen is added. 139 In some soils, nitrogen has been shown to leach in rather large amounts to depths of 3 to 6 feet or more. 188
That many native species must absorb their nutrients from the subsoil, i.e., below a depth of 2 to 3 feet, is clearly indicated by the root habit. Little or no branching occurs in the surface layers, while it seems certain that morphological changes in the roots occupying the upper soil are such as to preclude absorption. Differences in the root habits of various cultivated plants may also lead to marked variations in the amount of nutrients removed from the different soil levels. 139 Consequently, knowledge of the extent and distribution of the roots of cultivated plants is of great practical interest. The fact that roots may absorb nutrients at deep levels in the subsoil as well as from the surface layer should be given greater attention by all plant growers. The too prevalent idea that it is mainly the surface layer of soil that supplies the plant with nutrients and that the subsoil is the crop's reservoir for water should give way to the fact that it is the whole soil mass permeated by roots that determines root activity.
The habits of roots, as those of shoots, are more or less characteristic for every kind of plant. They are governed, first of all, by the herieditary growth characters of the species or variety in question. It has been shown that inbred strains of corn, for example, differ greatly in the character and extent of their root systems. 94 "Certain strains . . . have such a limited and inefficient root system that they are unable to function normally during the hot days of July and August, when the soil moisture is low." 95 Other strains have fewer roots and a lower ratio of tops to roots (Fig. 19). Experimental evidence has recently been found which supports the suggestion that selective absorption by individual corn plants may prove to be a very important heritable character. 92, 93
Fig. 19.--Heritable differences in extent and character of root systems. Representative root systems of two inbred strains of dent corn. The one illustrated above is susceptible to leaf firing; the lower one is highly resistant to both leaf firing and root rot. Note difference in character and extent of the two root systems. (After J. R. Holbert et at, Ill. Agr. Exp. Sta., Bull. 255.)
A study of the inheritance of root forms in mangels and sugar beets has shown that, in general, the roots of the F1 generation were intermediate between the parental forms. Sugar beet crosses in which wedge-shaped forms were involved proved to be exceptions. Wedge shape was completely dominant over walnut form and also over long, somewhat slender roots. 83 Recent experiments with peas, where a dwarf variety with a short root system was crossed with a tall variety with a deep root system, indicate that root characters are hereditary and segregate out in the F2 generation according to the Mendelian ratios. 105
Within the species or variety, root modifications are usually brought about by the operation of such factors as water content, aeration, soil structure, and nutrients. In fact, the character of the root system is usually an indicator of soil conditions. Of 28 native grasses and other herbs studied in two or more widely separated habitats, 25 showed very striking changes in their root habits as to depth of penetration, and position and number of branches; one exhibited only moderate differences, and two showed practically no change (Fig. 20). 220, 221 Several shallow-rooted forest trees as well as certain deeply rooted ones do not adapt themselves to changed soil conditions but others belonging to each class show considerable plasticity. 155 Great variability occurs in the rooting habits of fruit trees. For example, the wide adaptation of black walnut to so many soils that it is almost universally used as stock for the English walnut in California is well known. Other fruit trees show much less plasticity, certain varieties failing unless grafted onto other stock, the roots of which adapt themselves to soils underlaid with alkali 63 or containing excessive moisture, 173 or to very exposed or dry situations. 192 As is amply illustrated in the following pages, the roots of many cultivated crops are very plastic, responding readily to environmental changes. Sometimes, the root variation is so great and the growth habit so profoundly changed that the roots are scarcely recognizable as belonging to the same species.
Fig. 20.--Root systems of the false Solomon's seal (Smilacina stellata): A, in dry gravelly soil; B, in moist soil in the shade of a forest. Scale in feet.
Relation of Roots to Soil Moisture.--In studying the moisture relations of a soil, the root extent should furnish the criterion as to the depth to which soil moisture should be studied and also the maximum depth to which samples should be taken. The time, method, and amount of the application of irrigation water should be worked out in connection with their effect upon root distribution. It should also be kept clearly in mind that the ideal root system is not necessarily one with the most extensive branching but one that fully occupies the soil to an adequate depth and throughout a radius sufficient to secure enough water and nutrients at all times.
Influence of Time and Amount of Water Added by Irrigation.-- Keeping the surface soil too moist during the early life of the plant may promote a more shallow rooting habit, and the crop may later suffer from drought, unless watered very frequently. One of the most difficult problems of irrigation is to apply the water in such a way that plants are not made surface feeders. Otherwise, the natural advantages of the roots widely penetrating the subsoil for nutrients are lost. Conversely, delay in time or amount of water used may tend to promote a deeper rooting habit (cf. Fig. 87). The proportion of roots to tops may be definitely increased by lowering the soil moisture. 76 Roots of plants that mature a crop in fairly dry soil must penetrate deeply and spread widely, a distribution hindered by a very moist surface soil early in the life of the plant.
Crops respond to differences in water content and aeration, both in amount and direction of growth. By varying these factors by the application of more or less water, not only the root system but also the aboveground plant parts and yields may be varied, since a close correlation exists between the growth of roots and tops. Two-year-old alfalfa plants grown under irrigation in dry upland soil in New Mexico had roots 3 to 4 feet deep where 2 inches of water were applied at each irrigation, but they were 4.5 to 5 feet deep where 5 inches of water were applied each time. 206 While too much water may produce yellow, shrunken kernels of wheat, it has been shown that cereals are able to utilize water up to the date of full maturity. Too little water, even after the spikelets are losing their color, results in checking the deposit of dry matter in the grain, and a deficiency earlier in the development of the kernel probably determines its size, even before the rate of the deposit of dry matter is checked. 75 Clearly, the necessary water can be applied more effectively if a knowledge of the extent and position of the root system as modified by the chemical and physical nature of the soil is known.
Decreased yields may often be correlated directly with conditions influencing the development of roots. This is especially true in irrigated districts where water supply is the great limiting factor in crop production. In fact, root development often explains the reasons for differences in crop yields that are otherwise obscure.
Experiments have shown that when water is applied at the proper time, two or three irrigations give as good results as a greater number. 77 In some areas, a method of growing wheat with a single irrigation has been worked out under which, the yields are often higher and the harvest is earlier than under the old practice of irrigating six times. 97 Thus, there is a great saving not only of water for use on other arable lands but also of time and labor.
Too much water is frequently injurious to the soil by leaching out nutrients and in other ways. It often delays the maturing of the crop which is, consequently, more liable to rust and attack by other diseases. Frequently, both quality and quantity of yield are decreased. Since crops differ greatly in the amount of water they can profitably use, 124 as well as in their response to adverse aerial environments, a thorough knowledge of the root systems is not only warranted but imperative for an adequate explanation of crop behavior.
Raising the water table even temporarily by irrigation causes the death of the deeper roots in many plants and usually results in a decreased yield. 11 The roots of some species succumb more readily than others. Among many plants, top development depends upon a sufficient root supply, as was clearly illustrated in the case of the cotton plant. The amount of shedding of leaves and bolls was directly proportional to the extent of the root system which was submerged (and died) as a result of a rising water table. But when the water table was again lowered, a new growth of tops took place simultaneously with a new growth of roots into the area thus provided for root extension. 11
The general shape of the root system of trees and other plants may be controlled more or less by regulating, under irrigation, the depth of the water table. 10 If the subsoil is water-logged and thus unaerated, deeper roots will not develop or, if already grown, will soon die as the water table rises. In either case, there is a marked tendency towards the production of an abundance of roots so superficially placed that cultivation results in more or less serious root pruning. Moreover, under such conditions, plants are more sensitive to drought, temperature changes, etc. 89 They require heavier irrigation and greater amounts of fertilizers than those more deeply rooted.
Effects of Drainage upon Root Habit.--The proper drainage of swamps and bog lands for cultivated crops should be determined with reference to root relations. Extensive experiments have shown that the water table, if at a shallow depth, determines the limit of root penetration and, to a large extent, the yield of many common meadow grasses such as timothy, meadow fescue, and bluegrass. Even when the water table is high, only rarely does a root penetrate into the saturated soil, although, in well-drained soils, these grasses are quite deeply rooted. 150 In pasture mixtures, this water relation may be an important factor in determining which species will thrive and become dominant and which will disappear. Many coarse marsh grasses and grass-like plants can thrive in wet situations, since their roots are built anatomically to permit of rapid gaseous exchange. But most cultivated plants require well-aerated soil. For example, corn in well-drained soil frequently penetrates 6 feet or more deep, but in peat marshes, where a system of underdrainage kept the water table almost stationary at about 2.5 feet, it, has been shown that the roots, upon reaching a level 18 inches above the water table, turned aside and failed to penetrate deeper. 57 This inhibition to deep root penetration clearly reflected itself in the dwarfed stature and reduced yields of the aboveground parts. Although this may be found to be an exceptional case, it clearly shows the important relation between root habit and the plan of the drainage system by which the water level in such marshes should be controlled. The maximum depth to which the water table should be lowered depends largely upon the nature of the soil as, well as upon the root habits of the crops to be grown. If the soil is coarse and capillary action consequently low, too great lowering of the water table may result in a soil too dry to afford maximum yields.
Root Responses to Low Water Content.--As is amply illustrated in the following pages, a relatively low water content of soil, within certain limits, stimulates the roots to greater development, resulting in a greatly increased absorbing surface. Corn grown for 5 weeks in a moist, rich, loess soil (available water content 19 per cent) had a total root area which was 1.2 times greater than that of the transpiring surface of stems and leaves. Corn, with similar hereditary characters, grown in like soil, with an available water content of only 9 per cent, had a root area 2.1 times greater than that of the top. 223 Similar results were obtained with 2-months-old alfalfa, although, here, the area of the taproot system was exceeded by that of the tops. Plants grown in rich silt loam soil with an available water content of 22 per cent developed a root area 66 per cent as great as the area of the tops. But in a similar soil with 10 per cent available water, the root system had 83 per cent as much area as the aboveground parts. Thus a low water content, within certain limits, stimulates increased root development, which results in a greatly increased absorbing area.
When a crop is planted too thickly, even under otherwise favorable conditions for growth, the plants do not reach maximum development, because there is not enough light, water, and nutrients for all. Under these conditions of competition, the root system has been found to be more extensive in proportion to tops than those of crops less closely spaced. 223 Where decrease in light is the most important factor, the growth of the shoot is made more and more at the expense of the dry weight of the root. 18 But where the soil is very dry, root development is greatly retarded or even ceases, and the aboveground parts are dwarfed accordingly. For example, on the short-grass plains, roots of alfalfa and wheat, which normally penetrate several feet deep, although more profusely branched, are almost entirely confined to the surface 2 feet of soil because of lack of sufficient water to promote growth in the subsoil. 224
Why some crops are better adapted to semiarid regions than others may sometimes be explained, at least in part, by a study of the root habit, although the ability of a. plant like sorghum to endure drought by remaining relatively quiescent is an exceedingly important character. Sorghums when compared with corn were found to have better developed root systems as regards degree of branching in relation to the extent of tops. 140 These studies were carried on in rather lightly irrigated sandy loam soil in southwestern Kansas. Dwarf milo, Blackhull kafir, and a dent corn were grown in alternate rows. The leaf area of the corn at all stages of its growth was approximately twice as great as that of the Dwarf milo and at least one and a half times that of Blackhull kafir. Both sorghums had, in all stages of their development, a main root system just as extensive as that of the corn and, in addition, possessed twice as many secondary roots.
Relation of Roots to Fertilizers.--A knowledge of root systems is fundamental in the proper application of fertilizers. In fact, it should be an important basis for determining not only the time but also the manner and depth of application. Likewise in investigations of soil productiveness, root extent and activity should determine the portions of the soil and subsoil to be studied.
Effects of Fertilizers upon Root Habit.--Crops grown in rich soil have roots that are shorter, more branched, and more compact than those grown in similar but poorer soil. Sachs, over half a century ago, demonstrated that the more concentrated the nutrient solution, the shorter the roots, 172 and Liebig stated that plants search for food as if they had eyes. It has been known for a long time that plants grown in soils with alternate layers enriched with nutrient solution branch much more profusely in these layers. 146 Experiments so arranged that one-half of the root system of peas, for example, grew in soil rich in nitrates and the other half in poor soil gave similar results. 60 Of course, enriched soil promotes better shoot development which, in turn, furnishes a greater food supply for further root growth. The correlation of the growth of roots and tops is usually pronounced. Injury to one also hinders the growth of the other.
Fig. 21.--Root of sugar beet grown in fine sandy loam soil, showing root stratification in the second and fourth foot of soil where layers of clay were encountered.
When roots enter a soil area enriched by the decay of former roots, the greater degree of branching is often very marked. They frequently follow the path of their predecessors for considerable distances, branching in great profusion. Similar branching, which may be due partly to better aeration, frequently occurs in earthworm burrows.
Fig. 22.--Root stratification of the false boneset (Kuhnia glutinosa). The plant was excavated from an alluvial soil of alternate layers of sand and clay.
Marked contrasts in the degree of ramification of roots as they penetrate different soil strata are often to be attributed to differences in richness of soil. For example, sugar beets grown in fine sandy loam stratified at various depths with layers of clay have been found to branch much more profusely in these clay layers richer in nutrients than in the soil above or below (Fig.21). Frequently, root stratification is due to soil moisture, but the factors of water content and nutrients often operate together (Fig. 22). In tree plantations of green ash on the short-grass plains, the surface 6 inches of soil is literally filled with great masses of finely branched tree rootlets (Fig. 23). Under the light precipitation, most of the absorption must take place from this humus-enriched surface soil. However, many large but very poorly branched roots penetrate to a depth of 8 feet or more into the dry subsoil.
Fig. 23.--Network of absorbing roots of green ash in the 4 inches of surface soil in a grove in the short-grass plains.
Similar root layering has been found among many varieties of grapes. While most of the laterals of 6-year-old vines were present in the surface foot of soil, ranging from 10 to 20 feet from the base of the plant, many penetrated several feet deep and occasionally a root reached a depth of over 7 feet. 46 In dune soils where the organic matter is mainly responsible for the water capacity, the roots of many plants tend to occupy the upper layers where the humus and moisture are most abundant. 174 It has been shown experimentally that in every case where roots came in contact with a soil layer rich in nitrates, they not only developed much more abundantly and branched more profusely but failed to penetrate as far into the deeper soil. On the other hand, it has been shown that wheat and barley seedlings grown in both soil and culture solutions low in nitrates produced remarkably extensive root systems, although the shoots were small. Similar results, but to a less marked degree, were obtained when potassium salts were deficient. 68 Thus, it seems clear that the depth at which the fertilizer is placed in field practice considerably affects root penetration and development.
Significance in Crop Production of the Effects of Fertilizers upon Root Habit.-- Fertilizing the surface layers of soil, especially with nitrates, and thus stimulating surface root production in regions where these layers have very little or no available water during periods of drought appears to be distinctly detrimental to normal crop production. The effect of phosphates in promoting root growth in length and number of branches has long been recognized in agricultural practice.
Dressings of phosphates are particularly valuable whenever greater root development is required than the soil conditions normally bring about . . . Phosphates are needed also for shallow-rooted crops with a short period of growth . . . Further, they are beneficial wherever drought is likely to set in because they induce the young roots to penetrate rapidly into the moister layers of the soil below the surface. 168
Wheat on land treated with phosphates was found at the end of 107 days to be rooted almost twice as deeply as in similar soil to which no phosphates had been applied. 128, 217
Not only the quantity of nutrients but also the time at which they are absorbed affects quantity and quality of yield. In spring wheat and oats, the nitrogen or protein content has been shown to increase continuously as applications of nitrate fertilizer were made later and later in the life of the plant. 66 Winter wheat and rye gave a similar response to applications made in the latest periods of growth. Thus, the amount of nutrients in the subsoil available to the younger and more vigorously absorbing roots is of great importance. The problem of getting the phosphatic and potassic fertilizers, which do not leach extensively, into the deeper soil where they may be more efficient is one with which students of fertilizer practice should be concerned. Likewise, the time and quantity of the applications of nitrates, which leach freely into the deeper soil, in relation to rainfall and root depth is a field needing investigation. It is possible that methods of tillage and cultivation can be modified so as to give, in advance of the crop season, a supply of nitrates which might leach into the deeper subsoil in time for their absorption at the most effective period in the development of the crop. In the semiarid wheat growing regions of eastern Washington, the nitrates developed in the summer fallow of the preceding year are found in the subsoil the following spring at a depth of 4 feet or more (Fig. 24). 188
Fig. 24.--The influence of winter rains upon the movement of nitrate nitrogen in the soil of southeastern Washington. The numbers show the parts per million of nitrate nitrogen at the several depths. (After P. J. Sievers and H. F. Holtz, Wash. Agr. Exp. Sta., Bull. 166.)
Excellent results are reported from the use of subsurface application of fertilizers in growing potatoes in New York. The fertilizer is broadcast and thoroughly mixed with the upper 4 to 5 inches of soil by means of a heavy disk harrow. The field is then replowed 9 to 10 inches deep before planting. Severe drought may so thoroughly dry the upper layer of soil that it is of little use to the plant. However, the fertilizer well mixed in the deeper soil has, by this time, induced such a heavy root development that the crop continues growth unchecked throughout the drought period. 54 In other instances, especially in orchard growing, placing fertilizer deep in the subsoil has been shown to be an excellent practice. Until very recently, however, the deep-rooting habits of crop plants have not been appreciated by students of fertilizer practice nor have the nutrients of the subsoil greatly interested them. 135
Hill fertilizing of corn promotes more vigorous early vegetative development and earlier tasseling and earing. The observation of farmers that corn fertilized in the hill sometimes suffers more from drought than when grown in soil where the fertilizer has been uniformly distributed may be explained by a study of root extent in relation to tops. Although no differences were found in the actual abundance, depth, or lateral spread of the roots, the more luxuriant plants resulting from hill fertilizing had a relatively smaller root system. 138 This may also explain why, in Missouri, applying fertilizer in the hill or row yields good returns during seasons of abundant rainfall, but in dry seasons, there is more danger that the fertilizer may cause the corn to "fire" than when it is applied ahead of the planter with a fertilizer drill. 86 Because of the extensive development of the roots of practically all cultivated plants, it seems probable that the chief effect of hill manuring is to promote vigorous early growth and that the plant receives little benefit from the manure at the time when it is completing its growth and maturing its seed.
It has been shown by means of water cultures that when only a part of the root system is supplied with one mineral element and the entire root system with all of the other necessary elements, the plant does not absorb as much of the one element as it would were this element available to all of the roots. The fewer the roots supplied with the element, the smaller the total amount absorbed, although the amount of the element absorbed per gram of root increases greatly as the number of roots in the complete solution is diminished. This applies when the total amount of the element is equal to or in excess of the needs of the plant. For example, with nitrogen and phosphorus, the total amount absorbed by plants with half their roots in the complete solution was only 76 per cent of that absorbed by plants with all their roots in the complete solution. Nor did increasing the element in question in the complete solution appreciably alter the results. Since a plant is unable to attain a maximum absorption by means of only a portion of its root system, it is obviously important to apply fertilizers in such a way that, as far as possible, all of the roots of the plant will be supplied with all of the fertilizing elements. 69 Since the lateral diffusion of fertilizer salts in the soil is small, this can best be done by distributing the fertilizer uniformly over the whole area occupied by the roots.
Root distribution and development is greatly modified by various cultural practices, but our knowledge is very incomplete and a great deal more experimental work should be done in this field.
Transplanting.--Nurserymen transplant trees and shrubs two or three times in order to force root development near the stem and thus to insure the preservation of more young roots when the plants are lifted for shipment. Hence, they have a better chance for recovery when again set out. This explains why nursery-grown trees and shrubs usually survive transplanting so much better than those secured from places where the roots have made their natural growth. Likewise, market gardeners find that transplanting young plants of cabbage, tomatoes, etc., while growing in cold frames, is a great advantage in assisting them to endure the final removal to open ground.
In the transplanting of trees, both depth and spacing should be given careful attention. If the roots are placed either too deep or too shallow, the plant is at a decided disadvantage. Proper spacing in forest planting is necessary to obtain well-balanced and wind-firm root systems. 232 Not infrequently, orchard and shade trees as well as trees planted for windbreaks are so closely spaced that insufficient room for proper root development results in a marked decrease or actual cessation of growth. 13
Layers of compact soil often play an important part in shaping the root system. 209, 73 Western yellow pine seedlings transplanted in clay loam soils with their roots against one side of a hole showed a marked tendency to grow a one-sided root system, the growth being away from the side of the hole towards the looser soil within. When transplanted by the usual "trencher" method, the roots "invariably develop only in the plane corresponding to the longitudinal axis of the trench." 211 In plowing for cultivated crops on heavy soils, the depth should be varied from year to year so that a too firm "plow sole" will not develop at a certain level and tend to confine root development to the plowed layer. 106
The time of transplanting and of early spring cultivation should be considered in relation to root, development. In the red currant, for example, which is representative of many other plants, root growth started in advance of bud development and before the beginning of the growth of the stem. 72 Root growth in apple trees began a little earlier than did shoot growth and continued for several weeks. This gradually passed into a second period extending over a part of June and July during which shoot growth was still active and root growth relatively inactive, a condition which was most marked about midsummer. Later as shoot growth slackened, root activity greatly increased and continued until late autumn. In fact, much of the larger part of the root growth was made late in the summer and autumn. 12 In climates which are not too cold,
. . . fall-transplanted trees are more likely to give a good stand than corresponding spring-set trees, for during the winter months, new root formation is initiated and water can be absorbed in the spring as fast as the new shoots and leaves use it. The spring-set trees, on the other hand, must wait until new roots are formed before they can take up moisture, and if soil conditions remain unfavorable for their root formation and atmospheric conditions stimulate vegetative growth of the top, the pushing shoots will wilt and die, and the tree will be lost. In the autumn, conditions are favorable for root growth for some time after good growing conditions for the top have passed; in the spring, they frequently become favorable for top growth before or simultaneously with suitable growing conditions for the roots. 62
Correlation between Root and Shoot Development.--The maintenance of a proper balance between root and shoot is of very great importance. If either is too limited or too great in extent, the other will not thrive. The root must be sufficiently widespread to absorb enough water and nutrients for the stem and leaves, which, in turn, must manufacture sufficient food for the maintenance of the root system. It is a well established fact that grasses develop a better root system when they are mowed once or twice a year than when they are closely and frequently grazed. In fact, one of the most important factors in the various systems of range and pasture management is to permit the seedling grasses to become well rooted before the tops are removed by grazing. 175
In transplanting crops of various kinds, many of the roots of the young plants are necessarily destroyed. Hence, the top must be pruned back or the plant protected from excessive water loss until the balance between the absorbing and transpiring systems is reestablished. Conversely, a top too small to manufacture sufficient food to feed an extensive root system creates an unbalanced condition in the plant which, if uncorrected, may retard its development and may even result in death.
Pruning back the vines of sweet potatoes is a practice followed by some growers who believe that reducing the growth of foliage stimulates root development. Actually, root yields are reduced. For example, in New Mexico, hills pruned back to 12 inches in diameter yielded 6,012 pounds per acre, those pruned back to 24 inches produced 8,690 pounds, while the yield, where no pruning occurred, was 16,520 pounds. 61
Experiments with 3-year-old almond trees have shown that the development of both the top and root systems was inversely proportional to the severity of the pruning of the tops. The spread of the roots was over a third greater where the pruning was light than where it was severe. 8 Heavy pruning of the first crop of indigo, leaving a few leaves, is said to have resulted in far less damage to the roots and nodules and in a much more rapid development of the second crop than pursuing the common practice (in Bihar, India) of completely cutting back the first growth. 100
Wheat grown in culture solutions poor in nitrogen develops a large root system which, when nitrogen fertilizer is added, absorbs much more of this nutrient than is needed for normal growth. This results in abundant tillering. More than four times as many tillers may develop as among plants with smaller root systems. This indicates a causal relation between differences in root-absorbing capacity and tillering. 66 Buckwheat, beans, and other plants adapted to shade, sunlight, and an intermediate condition, respectively, have been grown in shade and sunlight. In all cases, plants grown in sunlight had a larger root system than those grown in shade, a result attributed to the increased transpiration. 134
Recent studies have shown that among certain species a
. . . light duration unfavorable to aerial development has caused extensive root growth. Growth of root and shoot, therefore, are not necessarily contemporaneous with respect to season, and arrested development of the exposed portion of the plant caused by suboptimal light duration need not be accompanied by checking of root growth. 64
As regards the roots of seedlings, it has been shown that their development is greatly influenced by the nature of the reserve food supply in the seed. The more nitrogen a seed contains, the greater is its shoot growth as compared to root. 157 An abundance of carbohydrate foods and a somewhat limited nitrogen supply promote rapid root development. 156
Tillage Practice and Root Physiology.--The value of various depths of plowing, listing, or subsoiling, and the preparation of the seed bed, as well as the time, depth, and manner of subsequent cultivation are usually judged entirely by the increase or the decrease in growth and the yield of the aboveground parts. Too little attention has been given to the effect of these practices upon root activities and development. A knowledge of root development under each of the various methods of tillage, systems of mulching, etc., with their resultant effects upon water content, nutrients, aeration, and other edaphic factors, will not only give a logical cause for the results obtained by these practices but will form a scientific basis for the application of other methods or combinations which may result in greater yields. It should be kept clearly in mind, however, that tillage methods, as such, may not influence root habits directly. Differences in growth and distribution of roots in the soil are in response to differences in physical and chemical conditions of the substratum in which they live. Tillage methods are merely the means of bringing about these changes.
Much farming practice, both ancient and modern, finds explanation in root physiology. For example, loosening the soil by plowing results in the storage of water and in better aeration. This not only makes conditions more favorable for seed germination but affords better conditions for the growth of roots and for such soil organisms as nitrate bacteria. The latter produce greater amounts of nitrates which, in turn, affect root growth. Chemical corrosion is promoted; that is, nutrients are more rapidly liberated from the soil particles, run-off is lessened, temperature conditions are changed, etc. All these factors affect root habit. Moreover, the loosened soil makes root penetration easier. Roots tend to develop a shorter and more compact structure in dense than in loose soils. Subsoiling carries the air still deeper and at the same time raises more minerals to the surface soil layers. It modifies in many ways (mostly unfavorably as measured in crop yield) the physical, chemical, and biological factors of the soil. All these changes are ultimately reflected in root habit.
Root Habit and Depth of Intertillage.--The depth of intertillage exerts a marked effect upon root habit and often upon yield. For the highest yields, cultivation should never be deep enough to injure the roots seriously. They should be allowed to occupy the richest portion of the soil, which is usually the furrow slice. The proper type of cultivation is deep enough to kill the weeds but shallow enough to reduce root injury to a minimum. A decrease in the yield of corn of 2 to 8 bushels per acre was brought about by deep cultivation in Illinois and a decrease of 13 bushels in Missouri. 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 Similar results have been obtained in New York.
When soil adjacent to hedge rows of Osage orange is not cultivated, their roots gain possession of a much larger area than where thorough cultivation is practiced. One investigation showed that lateral root extent in the two cases was 44 and 29 feet, respectively. Under conditions of cultivation, the water content of the deeper soil is increased, which, in turn, lessens the necessity for wide root spread. The ability of perennial crops like alfalfa and clover to grow in close proximity to the trees while annual crops do so with difficulty is due in a large measure to the more deeply penetrating roots of the perennial crops. 13
That mulching the soil and lack of tillage result in a marked growth of fibrous roots in the surface layers has been repeatedly demonstrated. Roots of fruit trees are often very superficially placed under a straw mulch. For example, apple trees grown for 8 years under a heavy straw mulch and under cultivation, in Indiana, had very different root habits. Quite in contrast to the depths of roots under cultivation was the shallow root system of the trees under the straw mulch. Here, they came very close to the surface of the soil. In fact, roots half an inch in diameter or larger were found growing on the surface of the soil under the straw mulch, and very many fibrous roots were found on the surface soil and penetrating the decaying material. The roots, while more abundant than in the cultivated plots, averaged much smaller in diameter, and 75 to 80 per cent of the entire root system was in the surface foot of soil. 50
Contrary to popular belief, deep tillage as a means of overcoming drought or of increasing yields has little foundation in fact. Extensive experiments conducted in the Great Basin and the Great Plains under semiarid conditions where the greater part of the precipitation occurs in the winter and in the growing season, respectively, as well as in the more humid climates of Illinois, Pennsylvania, and Mississippi, all lead to this conclusion.
Plowing does not increase the water-holding capacity of the soil nor the area in which the roots may develop or from which plants may obtain food . . . Yields cannot be increased nor the effects of drought mitigated by tillage below the depth of ordinary plowing. 40
In these studies, little or no attention, unfortunately, was paid to root habit.
Experiments in New York show that mixing the surface soil layers with the shallower, subsoil promotes deeper root penetration. Where the soil was thus mixed in the first 6, 12, 18, 24, and 30 inches, respectively, in the different plots, marked differences were found in the case of corn. Although some roots in all the plots reached a depth of 2.5 feet, the number was larger in the soil that had been worked deeply. The proportion of shallow roots, on the other hand, was greatest in the plots where the shallower soil alone had been mixed. The soil volume occupied by the roots in the deeply mixed soil was similar to a short cone buried just beneath the surface in its upright position, but in the shallow-worked plots, the cone-shaped volume was inverted. In the earlier part of the moist summer, the plants on the shallow-worked plots grew most rapidly. During a period of severe drought in August, the others overtook and exceeded them in growth. "The rich green foliage and vigorous appearance of these plants presented a striking contrast with the rolled and withering leaves of the plants on the shallow-worked soil." 205 The corn on the most deeply-worked plot was tallest and ripened last but gave the greatest yield. Aside from distributing the richer surface soil to greater depths, the deep-tillage methods also afforded much better aeration in the deeper soil. Although subsoiling sometimes increases yields, it is well known to be a poor practice to turn up a considerable amount of unweathered soil to the surface at any one time.
Relation to Crop Rotations, Cover Crops, and Intercropping.--Crop rotations on different types of soil and under different climatic conditions should be worked out with reference to root relations. That "knowledge of root systems is the basis of agriculture" should be given more than casual consideration. It may be found practicable, especially in semiarid regions, to grow short-rooted and densely rooted crops alternately with those of longer and more spreading root systems. European investigators have emphasized the fact that crop rotations should be made with reference to root change. In the semiarid regions of Russia, which, in many ways, are similar to our Great Plains, definite rotations involving this principle have been worked out. For example, deeply rooted and densely rooted winter wheat or rye is followed by the more meager- and shallow-rooted potato crop, and this by barley or oats, which, in turn, is succeeded by the shallower and more poorly rooted flax. This is considered to be an effective means for contending against drought, since it averts the perennial drying up of the root-inhabited soil layer. 164
In humid regions, under intensive agricultural conditions, two differently rooted crops may be grown in the same field at the same time. Nurse crops of oats for clover are common; pumpkins or soy beans are often grown with corn; and the use of cow peas with corn is an old practice, especially over the southern part of the corn-growing region. Growing mixed cultures is a common practice in India, where they usually outyield pure ones. 153 The selection, breeding, and adaptation of crops for and and semiarid regions should logically center about their efficiency as absorbers and conservers of water. Plasticity of root systems as to depth, lateral spread, degree of branching, etc., goes far towards determining the ability of a crop to make sufficient growth and yield to warrant its cultivation in dry lands. Moreover, under these conditions, root competition is an important factor in determining the rate of seeding.
It seems not improbable that some of our best-yielding crops may be able to outstrip others, largely because of their greater efficiency in securing a larger and more constant supply of water and nutrients. Why certain artificial mixtures of grasses and other herbs may thrive in pastures and meadows, while others do less well, must depend to a large degree upon competition of root systems. This is the case in native grassland, where it is usual for 200 to 250 individuals or groups of individual plants to grow in a single square yard, due to lessened competition resulting from absorption at different soil levels and to different periods of maximum aboveground activity during the growing season.
Depth of rooting can be controlled to a considerable extent by the use of cover crops or by intercropping. If the surface soil is depleted of its moisture by absorption, roots of both crops penetrate deeper. The effect that one kind of plant may have upon the root habit of another by modifying soil conditions is well illustrated in the case of chaparral and Monterey pine at Carmel, Cal. Here, the trees growing in an open stand among the shrubs died when the latter were cleared away. However, a new growth of pine flourished on the same area. The chaparral had shaded the soil and lessened evaporation from its surface, and the dense layers of rootlets and accumulated humus held the moisture in the surface soil. Consequently, the trees were shallow rooted and died of drought when the protecting cover was removed and the soil desiccated. Seedling trees in the changed habitat evidently rooted more deeply. 30
In fruit orchards of Oregon, it was found that the cultural treatment to which an orchard has been subjected has a strong influence upon the location of the major portion of the fibrous roots of the trees. Where clean culture had been practiced without the use of the plough, a thick mat of fibrous roots was found immediately below the soil mulch. Few roots extended to depths of 8 to 12 inches below the mulch. But in a few restricted areas that received neither cultivation nor irrigation, the roots were found to be distributed from near the surface to a depth of 12 to 16 inches. Under sod and irrigation, the roots were quite uniformly distributed from near the surface to 2.5 feet in depth. Under the loose surface soil of the cultivated area, there had been formed an impervious hardpan which was entirely absent in the untilled and unirrigated land. 3
It has been observed by several investigators that trees growing in competition with grass have a relatively heavier root system in proportion to tops as compared with those under cultivation. This relationship has been reversed, however, by adding nitrate fertilizer to the soil about the trees in the sod. 85
The presence of the roots of a previous crop in the soil where another crop is growing exerts an influence upon it in several ways. Upon their decay, they enrich the subsoil to a depth of several feet. A network of tortuous channels, formerly occupied by the roots and about which the soil has been more or less compacted by lateral pressure exerted in their growth, fills soil and subsoil. This permits better aeration which may be an important factor in the oxidation of harmful substances originating from the decay of the roots. The ancient practice of letting a soil lie fallow for a time may restore its productivity, either by allowing time for the formation and diffusion of more soluble mineral salts, or through the removal by leaching or oxidation of injurious substances formed by roots. It has been clearly demonstrated that the presence of sorghum roots and stubble has a distinctly depressing effect upon the yield of wheat. 182 Preliminary field and pot tests with tobacco indicate that the injurious effects of preceding crop plants come mostly from the roots rather than the tops of these plants. Roots of potatoes, hairy vetch, and corn retarded the growth of tobacco even when their aboveground parts were removed from the field in harvesting. 64a
Rotation of crops may derive its value both from the different demands made by various crops upon the nutrient supply of the soil and from the fact that organic materials added by decaying roots are often less injurious to other kinds of plants than to the crops producing them. Indeed, it is held by some that the major benefit afforded by the addition of fertilizers is not the replacement of mineral salts removed by the crop but their beneficial effect, in neutralizing unfavorable soil conditions introduced by root excretion and decay.
There are many other ways in which root habits of plants are related to problems connected with crop production. Prominent among these are problems of soil erosion and weed eradication, as well as crop production on alkali or acid soils. An other important subject is the relation of root development to disease in plants.
Soil Erosion.--Closely connected with crop rotations is the problem of soil erosion, whether by wind or water. This holding of the soil for cultivation applies equally to plants of pasture and meadow lands and to annual crops. Nearly half of the United States--the hilly half--is being seriously impaired by water erosion. Indeed, erosion is one of the most serious dangers that threaten the agricultural and pasture lands of the nation.
Fig. 25.--Three feet of grass roots exposed on an eroding bank.
The roots of plants are efficient soil binders, the effect being more continuous with perennials. Among cultivated crops, this is especially true of meadow and pasture grasses. Even though they may not form a continuous sod, the soil particles are held in place by the extensive and minutely branched fibrous root systems. Grasses that spread by rhizomes and form a compact sod are especially efficient. Not only do the underground stems and roots hold the soil, but the tops form a complete surface cover which efficiently shields the soil from the destructive action of wind and water (Fig. 25). Removal of the cover of vegetation, whether by overgrazing or plowing, results in root decay and is frequently followed by soil erosion. Even after death and decay, roots and tops exert an important effect in cementing together the soil particles as well as by absorbing and holding several times their own weight of water. But once the soil is exposed, every drop of rain that falls has the power of removing soil particles and with them the soluble salts essential to plant growth. Some of the most important grasses of sandy areas are of little or no forage value but exert a pronounced effect upon stabilizing the loose soils and thus permitting the growth of other vegetation (Fig. 26). Certain crops. like rye are also efficient soil binders (Fig. 27).
Fig. 26.--Roots of the sand reed grass (Calamovilfa longifolia) in natural position at a depth of 2 to 4 feet.
Fig. 27.--A crop of rye planted to prevent sand blowing.
Weed Eradication.--In keeping the soil free from noxious weeds, especially perennials, a knowledge of the position, extent, and growth habits of the underground parts is very important. Here as among most groups of plants, however, few data are available. Why some species are more detrimental to crops than are others may be due in part to their absorbing water and nutrients from the same level as the cultivated plants. Marked differences occur in different species. The roots of the dogbane (Apocynum androsaemifolium), for example, fill the soil rather completely to a depth of 3 to 4 feet (Fig. 28). The vervain (Verbena stricta), common in old pastures, occupies the soil in a similar manner but the cord-like roots of the ironweed (Vernonia baldwinii), also frequent in grassland, reach depths of 9 to 11 feet and branch rarely or not at all until they enter the third or fourth foot of soil.
Fig. 28.--The underground parts of a common weed, the dogbane (Apocynum androsaemifolium). When the plants are pulled in a corn or oat field, the vertical stems break and new plants arise from underground buds.
Relation to Alkali and Acid Soils.--Studies of root systems in soils impregnated with alkali point out clearly that the adaptation of plant to habitat is often largely one of root distribution above or below the layers of greatest salt concentration. Shallow-rooted native species which cannot endure alkali may grow upon land which contains alkali at depths below those of root penetration. Orchards and vineyards are sometimes planted in soils of rather high salt content, and the root systems may become thoroughly established in a non-toxic lower layer of soil which is less alkaline than the surface layers. 78 Shallow-rooted crops may fail to give a satisfactory yield because the alkali tends to concentrate near the surface if evaporation is great. "This accumulation makes the salts very strong throughout the feeding zone of the plant and, therefore, toxic even when the total quantity of salts in the upper 3 or 4 feet is rather small." 78 Winter wheat is a poor crop on alkali land, because it is seeded in the fall when the alkali is usually concentrated near the surface, although experiments show it to be more tolerant than many spring crops which, in practice, are better adapted to alkali lands than is wheat.
Deep-rooted plants, like alfalfa and trees, may penetrate the alkali strata by growing in the upper soil while the alkali is beneath and gradually feeding lower as the alkali accumulates at the surface. In this way, some plants not exceptionally tolerant may withstand what seem to be excessive quantities when the whole feeding zone is not considered. Where alfalfa, cotton, and other deep-rooted plants get a good start but encounter a strong alkali stratum at a short distance below, these plants may prove less resistant than the cereals which may feed in the upper less alkaline soil. 78
The sensitiveness of seedlings of sugar beets and alfalfa to alkali and the relative alkali tolerance of well-established plants have long been known. Heavy rains or irrigation during germination and establishment may so dilute the harmful salts that the seedling stage may be safely passed. Both crops soon develop a deep root system. The alfalfa plants shade the soil and thus hinder surface evaporation and salt concentration, Moreover absorption from the deeper soil layers also retards the upward movement of water and solutes. The high resistance of alfalfa to alkali, once the plants are established, is due in a very large measure to its deep root system which often absorbs in the deeper soils below the salts. In the case of sugar beets and other intertilled crops, the surface mulch resulting from cultivation, aided by the shade produced by the crop, tends to check the rise of the salts. Beet roots absorb at all depths to 6 feet, and the plants thrive even when the alkali present in the surface soil is very great. It has been demonstrated that plants with extensive root systems are less subject to injury from harmful salts than those whose root systems are more poorly developed.
In California, soils so impregnated with alkali that two successive plantings of peaches were killed, a third lot of trees grafted on the roots of an alkali-enduring variety proved successful in the same soil. 63 Experiments in Australia indicate that the sour orange is the best stock for orange and lemon in sections where the irrigation water may contain considerable alkali. 212 In California, it has been found that the roots of lemon are unusually susceptible to alkali. 117 With adequate knowledge of the variability of the root systems of different varieties of cultivated plants, selection of those most suitable for alkali areas should be less difficult.
In problems of crop production on acid soils, if attacked from the more modern and logical viewpoint of lime requirement of the plant rather than that of the soil, the root system again plays a decisive rôle The lime requirement of the plant is determined not only by its lime content and rate of growth but especially by its feeding power for lime. The latter is in proportion to the character and extent of the root system, the internal acidity of roots, and their excretion of carbonic acid. 210
Extensive investigations on the effect of soil acidity on root development have been made on moor soils in Europe. The roots of cultivated plants were found to penetrate into the soil only as deeply as the addition of basic materials had sufficiently freed the soil of free acids. 200 Certain native species were found to be less sensitive, their roots occurring in the deeper soil. Experiments with potted soils showed that the length of the root system agreed very closely with the depth of the acid-freed root bed. Studies of root development in the field, where the deeper soil layers were freed from acid by the addition of lime, confirmed the pot experiments.
Wheat seedlings grown in nutrient solutions with a high H-ion concentration developed root systems which were abnormal in being short, stubby, and much branched. The protoplasm of the root hairs was found to be coagulated and flocculated and the hairs were probably rendered ineffective as absorbing organs.1 Excessive acidity affects the roots of crops by partially or wholly retarding growth in length. The roots often thicken and soon become dull white in color. Sometimes, as in the case of root rot of conifers, injury to the roots resulting from acidity of the soil leads to infection by fungi which cause root decay. 7
Aeration and Soil Temperature.--Some fundamental relations have been recently worked out between rate of root growth, aeration, and soil temperature. 37 Under normal conditions of aeration, the rate of root growth is known to be influenced by soil temperatures in such a manner that there are three welldefined temperatures for growth. These are the maximum or highest temperature at which root growth is possible; the optimum, at which temperature growth is most rapid; and the minimum, below which it ceases. But under a diminished oxygen supply, these cardinal temperatures seem to be greatly modified. As the oxygen supply in the soil air is decreased, rate of growth diminishes in a soil with a high temperature. For example, corn roots, in a soil atmosphere of 96.4 per cent nitrogen and only 3.6 per cent oxygen, at a temperature of 30°C. grow about one-third as rapidly as at the same temperature under normal conditions of aeration. But at 18°C., growth is increased to about two-thirds the normal rate at that temperature when the soil is well aerated. Similar results hold for cotton and other species. Such crops, to attain a fair rate of growth at a time of high soil temperatures, must be in a well-aerated soil; otherwise, the rate of growth is considerably reduced.
The roots of cactus (Opuntia) are of two kinds, namely those which grow directly downward from the base of the shoot and apparently serve the purpose of anchoring the plant, and those which spread laterally in the shallower soil and appear to function mainly as absorbers of water and nutrients (See Figs. 44 and 45 in the next chapter). An experiment was so arranged that cuttings of Opuntia versicolor grew for two years in an adobe soil, or in sand filling a large pit dug in the adobe, or on the edge of the pit in both adobe and sand. A suitable water content was maintained in both soils during the growing' season. Owing to the differences in the soil texture the sand was better aerated at all times than the adobe. Temperature differences at corresponding depths probably also occurred. The plants which were in the adobe soil had a normal type of root development, the absorbing roots reaching out far beyond the shoot but lying near the surface of the soil. On plants grown in the sand the differentiation of the roots into the two systems was obscured. There were no proper superficial roots and no proper anchoring roots but the roots extended downward at many angles and divided the soil mass encompassed by them fairly equally. But the roots of the plants situated on the line separating sand and adobe were of both types. Those in the adobe were either superficial and horizontal or extended rather directly downward, and those on the sand side developed after the manner of the roots wholly within the sand. This shows that the roots of Opuntia are exceedingly plastic and are directly affected by aeration and soil temperature. 38a
Experiments with the development of nodules on legumes have shown that they become much larger when soil temperatures are most favorable. In the soy bean, a consistent increase in dry weight of nodules occurred as the soil temperature increased from 15° to 24°C. At higher temperatures, a progressive decrease occurred. Alfalfa, red clover, and field peas likewise gave a maximum nodule production at a soil temperature of about 24°C. 114
Relation to Plant Disease.--The relation of root development to disease resistance of crops is important and requires intensive study. Recent investigations have demonstrated a close relation between the vigor of root growth and disease resistance. 101 "Water stress" in cotton affects most seriously the plants with the greatest vegetative growth. They remain longer in a wilted condition between irrigations and show an earlier recurrence of wilting after irrigation. Little difference has been found in the size of the root system between large and small plants, 120 and this seems to explain the behavior of those with too large aboveground parts. In some species, similar conditions promote shedding of leaves. For example, the pine needle shedding disease can be, cured by promoting the development of a good root system.
Where the disease is physiological, resulting from malnutrition or insufficient aeration, this relation is at once apparent. In some cases, defective soil aeration causes disease. The wilt disease of Java indigo and other monsoon crops of India is due to damage of the root system resulting from defective aeration. This is brought about by the rise of the water table, combined with the decrease of the porosity of the surface soil by heavy rains. Wilted plants possess few fine roots and nodules in an active condition, most of them being dead or discolored. Deeply rooted varieties of crops are especially subject to wilt, while shallow-rooted ones are little affected. The root tips often show marked aerotropism and grow upwards towards the air. In some cases, they abandon the soil and grow over the surface of the ground. Sometimes, insects and fungi attack the crops and cause disease, the actual attack following the operation of some factor such as poor soil aeration or soil temperatures unfavorable for good growth and other functions of the root system. This is thought to be due to change in cell sap, arising from root damage, which prepares the way for the parasite. 101a
Certain plant diseases that cause enormous economic losses are due to organisms that enter the root and cause it to decay entirely or in part, thus bringing about reduced yields or total loss of aboveground parts. In the root rot of clover, tobacco, and other crops, the root system is partially or entirely destroyed by various soil-inhabiting fungi. Sometimes, as in club root of cabbage due to the invasions of a slime mould, the parts belowground are greatly deformed. 125 The root rot of tobacco is marked by the stunting of plants in various degrees due to a reduced root system. 108 The extent of the damage is determined in a large measure by the environmental conditions surrounding the roots of the host.
Diseases such as flax wilt and cabbage yellows are caused by fungi entering the root hairs, pushing back through the cortical tissues, and growing throughout the vascular system. 207 These invasions of root and stem result in diminution of water and supplies of food materials from the soil which, in turn, give rise to stunted plants. The host may be killed in the seedling stage or wilt and die at any time during its growth. In cabbage, the leaves have a pale, lifeless, yellow color. Sometimes, only one side of the root system is seriously attacked. Then the opposite side of the plant grows more rapidly and brings about a curving of stem and leaves. The invaded plants begin early to shed their lower leaves while making a weak attempt to continue growth above. Both root and stem are greatly dwarfed. The majority of the diseased plants continue a sickly existence for a month or more and then succumb. 112
The Texas root rot of cotton, which also affects many other plants, is almost entirely confined to the roots. The plants look normal and healthy, but wilting occurs suddenly. The entire foliage droops and dies and soon falls. An examination of the roots of freshly wilted cotton plants shows that the woody portions lying immediately beneath the cambium are deeply discolored where the fungus has killed the tissue, probably by the secretion of a toxic substance of enzymic origin. The organism spreads underground by contact of infected roots of one plant with adjoining healthy ones of another. Thus, early planting and close planting of susceptible hosts greatly influence the amount of Texas root rot during a favorable season, because of better contact of roots underground. A well-developed root system is a factor which greatly influences the summer spread of root rot. 201 More recent investigations, however, where the root systems of cotton and alfalfa of both healthy and diseased plants were thoroughly examined, lead to a different conclusion. It was found that the fungus causing the disease does not spread from plant to plant by underground contact of diseased roots with healthy ones. It progresses through the soil, and the plants are attacked at any point on the taproot within the first foot of soil, and the laterals rot at the point of attachment with the taproot. 152
These illustrations are sufficient to indicate the need of a thorough knowledge of the root habits of cultivated plants in combating plant disease produced by soil-borne parasites. A more complete knowledge of the soil environment as it affects root growth and the growth of organisms attacking roots is of great scientific and economic importance. Increased attention to the relation of environment to disease inception and development is recognized as imperative to progress in plant disease control. Disease resistance and predisposition to disease may be largely dependent upon environmental conditions under which the plant is developing. 111 Selecting plants whose roots are resistant to fungus attack or ensuing injury is a very promising method of procedure. Why they are immune is a problem that will require a thorough acquaintance with the structural changes and chemical composition of the roots. Our present knowledge of these subjects is quite limited. 55
Green plants only are able to make organic food from the water and inorganic nutrients of the soil and carbon dioxide of the air. Since roots absorb water and nutrients, a knowledge of their development, extent, and activities and how these are modified by the changes in the environment are necessary for a scientific understanding of plant production. The roots of most cultivated plants are very widely spread and deeply seated, often exceeding the extent of the aboveground parts. They absorb large quantities of water from the subsoil, even at depths of 4 to 7 feet, the younger, deeper portions of root systems being particularly active as the crop approaches maturity. Nutrients are likewise absorbed at deep soil levels when they are available and produce a pronounced effect both upon the quantity and quality of the crop yield. Hence, the nature of the subsoil is an important factor in crop production. Although the general form of the root is governed by heredity, it is very responsive to environmental conditions.
Wet soil and consequent poor aeration, especially early in the life of the plant, promote a shallow root system, and moderately dry soil, a deeper one. The latter is more branched and has a much greater absorbing surface. A desirable type of root system is one that fully occupies the soil to an adequate depth and throughout a sufficient radius to secure enough water and nutrients to promote a good growth at all times. By proper methods of tillage and especially by irrigation and drainage, moderately moist soil may be maintained to considerable depths, good rooting habits promoted, and yields increased. Fertilizers also have a marked effect upon root habit. Nitrates inhibit root penetration and promote increased branching. Phosphates are successfully used in promoting deep rooting, a very desirable development especially in regions subject to drought. Successful transplanting depends upon securing a sufficient absorbing area, a result often brought about by previous transplantings during earlier growth.. The maintenance of a proper balance between root and shoot is also very important. Proper methods of tillage bring about conditions in the soil favorable for root growth and distribution. Deep intertillage frequently results in root injury and decreased yields. Mulching and lack of tillage often promote shallow root development. Crop rotations in semiarid regions are sometimes made with reference to root change, deeply and densely rooted crops being alternated with more meager- and shallow-rooted ones. Mixed cultures, so common in nature, often outyield pure ones and may come to be more generally used under systems of intensive agriculture. The lessened root competition may explain, in part, the greater yields. In many agricultural practices, root relations deserve greater attention than has been accorded them. Knowledge of root habits is of value in problems of soil erosion, weed eradication, selection and adaptation of crops to acid or alkali soils, and in many other ways. The interrelation of edaphic factors to root activities is complex. Soil temperature, for example, not only has a direct effect upon root growth but affects it indirectly through soil aeration. At higher temperatures, more oxygen is needed for maintained growth than at lower ones. A close relation exists between the vigor of root growth and disease resistance. Many diseases of great economic importance are caused by soil-borne organisms entering the root and causing it to decay entirely or in part. This results in decreased yields or total loss of the crop. Selecting plants whose roots are resistant to fungus attack and injury is one way of combating disease. Here, as in many other fields of plant production, a thorough knowledge of root habits and root activities is needed.