92 Development and Activities of Roots of Crop Plants. 



VI. DEPTHS AT WHICH PLANTS ABSORB WATER 

 AND NUTRIENTS. 



Repeated examination of developing root systems of plants during the past 

 six years (Weaver, 1915, 1917, 1919, 1920), together with the usual findings 

 of great depth of penetration where growth conditions were favorable, have 

 led the writers to gravely doubt the accuracy of statements current in the 

 literature on soils regarding the depth at which plants absorb. The common 

 viewpoint is well stated as follows in one of our best modern works (Russell, 

 1917 : 56). 



"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." 



It seems certain that in many of our native grassland species which possess 

 strong tap-roots with little or no branching in the surface foot or two of soil, 

 Httle if any absorption occurs in these soil layers, because of the cutinized 

 or suberized root cortex. Rosa arkansana, Kuhnia glutinosa, Liatris punctata, 

 and Amorpha canescens are common examples. Among cultivated crops, 

 fife-history studies indicate that older alfalfa and sweet-clover plants, for 

 example, are very similar to the preceding in this regard. Even among 

 fibrous-rooted species, including cereals, it must be kept distinctly in mind that 

 the number of roots in any given area of soil is no criterion of their activity. 

 It seems probable that with increasing age the older roots are cutinized or 

 suberized, and unless new branches are put out, the seat of maximum absorbing 

 activity is transferred to soil layers of ever-increasing depth, inhabited by the 

 younger parts of the root system. While it is clear that many new roots 

 develop in the surface soils, especially from plants that tiller, and that older 

 ones normally develop abundant laterals, yet it seems equally certain that the 

 bulk of the surface roots are developed earfier in the fife of the plant and 

 because of age and consequent structural changes must be less active absorbers 

 at a time when rapid development of a network of new roots is occurring at 

 greater depths. A casual perusal of the preceding pages should impress the 

 reader with the great depth to which certain crops penetrate and with the 

 abundance of roots below the level of cultivation (6 to 8 inches). Extended 

 examinations of the root distribution of winter wheat and rye at 10 different 

 stations in true-prairie and mixed-prairie areas of Nebraska, Kansas, and 

 South Dakota show conclusively their deep-rooting habits. The average 

 maximum depth attained by these cereals was 5.0 and 5.1 feet respectively, 

 while the average working depth (the soil-level to which the bulk of the roots 

 penetrate and consequently where they are very abundant) was 3.7 feet for 

 wheat and 3.9 feet for rye. Other crops, such as red clover and alfalfa, were 

 found to be rooted very much deeper (Weaver, 1920). In fact, in nearly all 

 cases where the roots of crop plants were excavated, the total development 

 below the cultivated soil-layer was as great and usually much greater than 

 that in the surface soil. Among native plants, the bulk of the root system in 

 the great majority of cases fies below the surface foot, and the same holds true 

 for many crop plants, including especially the fall-planted cereals. The de- 

 pendence of plants upon the deeper-seated portion of their root systems is 



