The plant is the most important agent in crop production. Soils, cultivation, fertilizers, irrigation, and other factors, in a sense, are all more or less subsidiary. Soils are modified by cultivation, by adding manure or other fertilizers, by drainage or irrigation, and in other ways with the express purpose of changing the environment so as to stimulate plants to increased productivity.
It clearly seems that a thorough understanding of the activities of plants both aboveground and belowground and the ways in which these activities are favorably or unfavorably modified by various cultural practices should be basic for scientific crop production. Yet almost countless field experiments, selections, breeding, and testing of varieties, etc. have been carried on in all parts of the world with little or no knowledge as to the behavior of that very essential portion of the plant, the absorbing system.
The student of plant production should have a vivid, mental picture of the plant as a whole. It is just as much of a biological unit as is an animal. The animal is visible as an entity and behaves as one. If any part is injured, reactions and disturbance of the whole organism are expected. But in the plant, our mental conception is blurred by the fact that one of the most important structures is underground. Nor is the plant usually treated as an entity; it is often mutilated by pruning, cutting, and injuring the root system, frequently without much regard to the effect upon the remaining portion. 61
Modifying the Root Environment
In both field and garden the part of the plant environment that lies beneath the surface of the soil is more under the control of the plant grower than is the part which lies above. He can do relatively little toward changing the composition, temperature, or humidity of the air, or the amount of light. But much may be done by proper cultivation, fertilizing, irrigating, draining, etc. to influence the structure, fertility, aeration, and temperature of the soil. Thus, a thorough understanding of the roots of plants and the ways in which they are affected by the properties of the soil in which they grow is of the utmost practical importance.
Adaptation of Roots to Environment
By more or less profound modifications of their root system, many plants become adapted to different soil environments, others are much less susceptible to change. Among forest trees, for example, the initial or juvenile root system of each species follows a fixed course of development and maintains a characteristic form for a rather definite period of time following germination. The tendency to change when subjected to different external conditions becomes more pronounced as the seedlings become older. But some species thus subjected exhibit much earlier tendencies to change than others and a widely different degree of flexibility is also shown.
Continued study has shown that many field crops, although governed first of all by the hereditary growth characters of the species or variety, are usually subject to change. Certain varieties are able to adapt their root systems to unfavorable conditions much more readily than others.174,86,87
The wide range in soil and agricultural conditions under which vegetable crops are grown renders them particularly suitable both for the investigations of their root habits and also for a study of the agricultural significance of the differences encountered.
Root Adaptation and Crop Production
Enough work has been done to show clearly that among garden crops root adaptations frequently occur. These will be pointed out in the following chapters, although a mere beginning has been made. The field is enormously large and difficult but one rich in possibilities.
The agriculture in many of these areas is ancient, there have been few innovations, and the soil conditions have had time to impress themselves on the varieties of crops cultivated. A condition of equilibrium between the type of plant and the soil has been obtained, as there has been ample time for the operation of natural selection. When we compare the root system of linseed from the black-soil areas with that of the varieties grown on the Gangetic alluvium, striking differences appear.
Thus the differences between the root systems of varieties have been clearly shown and the economic significance illustrated.
Vegetable growing is an important phase of agriculture and one that is increasing at a rapid rate. In the United States practically every kind of soil is used for growing vegetables. Among the dozens or sometimes hundreds of varieties of the various vegetable crops, some undoubtedly have root systems more suited to a given soil environment than others.
The more I learn about cultivated plants the more I am convinced that the future lies in the root-soils relation and in matters which influence it.60
An intimate knowledge of the habits of growth of the root systems of vegetable crops will enable the grower to space plants to better advantage. It should also permit him to intercrop or grow in succession such crops or mixtures that the soil volume will have a better distribution of roots and thus permit of methods of more intensive cultivation. Similarly, by means of proper crop rotations and occasionally cultivating very deeply rooting crops, the subsoil may be kept in good condition and the effects of drought mitigated. 126
Interrelations of Plant, Soil, and Climate
In considering the importance of root relations in crop production, it should be clearly kept in mind that the plant, the soil, and the climate form a closely interlocking system of which no part should be overlooked or overemphasized. It is now rather generally recognized that climate and vegetation are the most important factors determining the character of the mature soil. 148
The features assumed by the soil in its development from infancy, through youth, maturity, and old age, vary with the environment, especially with the climate and the vegetation. 102 The effect of both climate and soil on the growth of aboveground plant parts has long been known.
The complex relationship of plant, soil, and climate may be further illustrated in the use of fertilizers. They modify the habit of growth as well as the composition of the plant. For example, phosphates, when applied to soil upon which wheat or certain root crops are grown, promote deeper root penetration. This results in a greater water and nutrient supply for the plant. Earlier development and ripening may be promoted or drought mitigated.
An adequate discussion of the environment of roots (the soil), how roots are built to perform their work, and root habits in relation to crop production has been so recently given in "Root Development of Field Crops" that further statement seems unnecessary. For a general discussion of the effects of irrigation, drainage, water content, aeration, temperature, nutrients, tillage practice, plant disease, and related phenomena upon root habit and their significance in crop production, the reader is referred to the same volume.
Activities of Roots in Subsoil
The great extent of the root systems of most vegetable crops and their usual thorough occupancy of the subsoil may at once arouse interest concerning the importance of the deeper soil layers. Experiments have shown that the roots of crops are active in the absorption of both water and nutrients even to the maximum depth of penetration. 174,165
Method of Root Study
In the present studies the direct method of root examination has been employed. It has been used by the writer and his coworkers in the excavation of hundred of root systems during the past 14 years and has proved very satisfactory. By the side of the plants to, be examined, a long trench is dug to a depth of about 5 feet and of convenient width. This affords an open face into which one may dig with hand pick and ice pick and thus uncover and make a careful examination of the entire root system.
Upon excavating the roots, detailed notes and careful measurements were made in the field. After several plants were examined, these notes were studied and any point that remained indefinite was at once clarified by further study. This method leads to a high degree of accuracy. Drawings of the root systems were made in the field on a large drawing sheet with pencil and later retraced with India ink. They were made simultaneously with the excavation of the roots and always by exact measurements.
Condition for growth
Since roots, like aboveground parts of plants, are greatly modified by environment, a brief statement will be given of the conditions under which the crops were grown.
The vegetable crops at Lincoln were grown in a fertile, dark-brown, Carrington silt loam. Preceding them white clover had been raised for 2 years. This was followed by a crop of maize. The soil was not only in an excellent condition as regards tilth but also was well supplied with humus. In general, it may be said that the soils of eastern Nebraska have a sufficient supply of all the essential nutrients to insure good crop yields, except that, owing to the systems of cropping, there may be a deficiency in available nitrates.
The surface 12 to 14 inches was a mellow, very dark silt loam from which the roots were readily removed. At greater depths it intergraded into a dark-colored, clay subsoil. This became quite sticky when wet and hard when dry. It exhibited a columnar or jointed structure, especially below 18 inches, cracking badly when it shrank upon drying. Roots were removed with much more difficulty from this soil layer.
Mechanical analyses of the soil at the several depths to 5 feet, together with the hygroscopic coefficients are given in Table 1.
Mechanical analyses and hygroscopic coefficients of soil from Lincoln, Nebraska
|Very fine sand
The fine texture of the soil is reflected in the rather high hygroscopic coefficients at the several depths. This, of course, denotes also a high water-holding capacity.
Number and Size of Plants
The crops, in nearly all cases, were grown in triplicate plats so that early, midsummer, and later examinations could be made without disturbing the areas in which the plants were to make further growth. The plats, although somewhat variable in size, were in all cases large enough to permit of normal field development, each group of plants being entirely surrounded by plants of its kind.
The field was plowed 8 inches deep early in the spring after all cornstalks, stubble, and dried weeds had been carefully removed. This was followed by repeated harrowing until an excellent, firmly compacted, moist seed bed of good soil structure was formed. On areas not immediately planted, weeds were removed by raking and hoeing.
The rainfall of 19.2 inches during the 6 months of the growing season was 2.3 inches below the normal, with periods of moderate drought occurring in May and again in July.
April with a deficiency of 1.2 inches had three well-distributed showers. In May efficient rains occurred on the eighth and fifteenth, with a monthly deficiency of 3 inches. June was a wet month with a total of 6.6 inches of precipitation, mostly in seven well-distributed rains. This was 2.3 inches above the mean. July had four rains during the latter half of the month and a total precipitation 1.8 inches below the mean. Five well-distributed showers fell in August, an excess of 1.2 over the mean. September had only 0.2 inch above normal, the rains also being well distributed.
As a whole the growing season was typical for eastern Nebraska. The spring was cool, frost occurring until about May 12.
The loss of water through transpiration is directly controlled in a large measure by the amount of moisture already in the air. This is also an important factor governing the evaporation of water directly from the surface soil. Consequently, a record of relative humidity was obtained by means of a hygrograph appropriately sheltered with the recording apparatus about 5 inches above the soil surface. Table 4 gives the average day humidity (6 a.m. to 6 p.m., inclusive) and the average daily humidity (24 hours) for the several weeks from May 13 to Aug. 25.
Relative humidity in per cent, 1925
|Week ending||May 13||May 20||May 27||June 3||June 10||June 17||June 24|
|Week ending||July 1||July 15||July 22||July 29||Aug 5||Aug 12||Aug 19||Aug 25|
The rather low relative humidities promoted high rates of transpiration, which in turn undoubtedly stimulated an active growth of roots.
Evaporation was measured by Livingston's standardized, non-absorbing, white, cylindrical atmometers. These were placed in a bare area in the field with the evaporating surface at a height of 2 to 4 inches above the surface of the soil.
Table 5 gives the average daily evaporation by weeks from Apr. 16 to Aug. 29. These data clearly show that during the last half of May and periods in June and July the evaporation was quite high.
Average daily evaporation in cubic centimeters, 1925
23 CANNON, W. A. Experimental studies on roots. Carnegie Inst. Wash., Year Book, 24: 289--298. 1925.
60 HOWARD, A., Director, Institute of Plant Industry, Indore, Central India. Correspondence. 1926.
86 KRAUS, C. Untersuchungen fiber die Bewurzelung der Kulturpflanzen in physiologischer und kultureller Beziehung. Forschungen auf dem Gebiete der Agrikultur-Physik, 18: 113-166 1895.
87 Ibid. 19: 80-129. 1896.
131 SAHUT, F. De Finfluence directe du sujec; sur le greffon. Rev. Hort., 57: 305-309. 1885.
148 TANSLEY, A. G., and T. F. CHIPP. Classification of mature soils according to climatic soil types. In "Aims and Methods in the Study of Vegetation," pp. 131-135. Published by The British Empire Vegetation Committee and The Crown Agents for the Colonies, 4. Millbank, Westminster, S. W., London. 1926. Access Online
160 Toumey, J., W. Initial root habit in American trees and its bearing on regeneration. Address at the Fourth International Botanical Congress at Cornell University, Ithaca, N. Y., Aug. 17, 1926. Unpublished.
165 VOROB'EV, S. I. On the study of the root systems of cereals and forage plants. Selsk. Khoz. Liesov., 251: 477. 1916.
Activate one of the layers to get more information.
Amout above the hygroscopic coefficient:
It is a well-established fact that rainfall is only a very general indicator of soil moisture, since many other factors both climatic and edaphic intervene between precipitation and water available for plant growth. Hence, a study of the soil moisture in several of the plats was made from time to time