ROOT DEVELOPMENT OF CROP-PLANT ECADS. 129 



feet, upon the death and decay of older alfalfa roots, are very important in 

 aiding soil aeration (c/. Weaver, 1915: 243). It would seem that the excellent 

 development of other crops upon old fields of alfalfa, sweet clover, and red 

 clover, the roots of which all penetrate deeply, aside from the increased nitrogen 

 supply in the soil, must be benefited also by its better aeration. The fertilizing 

 effect of the deeper portions of these root systems must be below the reach of 

 most crops. 



One part of this field of 2-year-old alfalfa extended over a rather steep hill- 

 side and to the crest of the hill. For the purpose of comparison, a large num- 

 ber of roots were excavated on the hilltop. It was discovered, after consider- 

 able labor had been expended in excavation, that subsoil conditions were far 

 from typical for the region, but nevertheless the results were of much interest. 



The surface 1.2 feet of clay-loam was dark in color and rather rich in humus. 

 Below this was a subsoil of stiff yellow to slate-colored clay about 2 feet thick. 

 This was somewhat intermixed with streaks of decomposed Dakota sandstone 

 which modified its tenacity. Below 3.2 feet the clay became very hard and 

 much jointed, roots being largely confined to these joints. It was intermixed 

 with pockets and streaks of chalk, the latter sometimes being an inch in 

 diameter. The soil was so hard, and especially the deeper soil, that it was 

 removed with considerable difficulty. Its glacial origin was shown by pebbles, 

 often 2 or 3 inches in diameter, which occurred throughout; however, these 

 were nowhere very abundant. A large number of plants from the hilltop and 

 valley were compared. The tap-roots had about the same diameters, varying 

 from 5 to 10 mm., but a marked difference in branching habit was at once 

 apparent ; the plants on the hilltop were very much more abundantly supplied 

 with both large and small laterals in the surface 1 to 1.5 feet of soil, while 

 those on the lowland rarely had laterals more than 1 to 2 mm. in diameter (in 

 fact, branches more than a millimeter in diameter were rather rare) ; the plants 

 on the upland were abundantly supplied with branches 1 to 4 mm. thick and 

 much more numerous smaller ones. In the latter habitat the first 6 to 8 

 inches of the tap-roots gave rise to branches two or three times more abundant 

 than did the plants in the valley. These differences are shown in plate 13, c, 

 where typical specimens were selected from a very large number excavated 

 at each place of examination. 



While the smaller, well-branched rootlets in the drier upland soil ran off 

 laterally to a distance of only a few inches, some of the larger ones ran off 

 obliquely in the surface 1.5 feet of soil to horizontal distances of about 1.5 feet, 

 where they turned downward. Some of these larger laterals ultimately 

 reached depths of 5 or 6 feet. Not infrequently they divided up into large 

 branches, which, with numerous small ones, often only a few inches in length, 

 furnished the plant with an absorbing system especially well developed in the 

 surface 2 feet of soil. The deeper roots pursued a much more tortuous course 

 than those in the valley. Many roots were found at a depth of 5.5 feet, which 

 was the average working depth. Not a few extended to depths of 6 to 7.5 

 feet and one specimen was traced to 8.8 feet. All but the deepest roots were 

 very well branched, especially in the last 1.5 feet of their course, with both long 

 and short branches. The latter were mostly 1 to 4 inches in extent. In the 

 deeper soil, branching was largely confined to one plane in the joints of the clay. 

 This root habit should not be confused with rather rare cases in the lowland 

 soil, where the tap-root had been destroyed, or for other reasons was not so 

 much in evidence. At a depth of a few inches to several feet it may divide into 

 two or more branches which take on more or less the character of the tap-root, 

 and, spreading very little, pursue an almost vertically downward course. The 

 plants on the hilltop were in a good, even stand and were almost as thick as 



