igig] ALWAY, McDOLE, &> TRUMBULL— SUBSOIL 203 



1 2 ft. or more after prolonged droughts, and even in wetter seasons 

 is commonly found in one or more foot levels within the first 6 ft. 

 That the lack of dryness in the deeper subsoil of the humid prairies 

 is not due to any peculiarity of the subsoil is evident from the 

 observation that a fair stand of alfalfa may in the course of a few 

 years reduce the moisture content almost to the hygroscopic 

 coefiicient to a depth of 20 ft. or more. 



In our deep cyhnder experiments (i) the exhaustion of free 

 water was observed only within the zone of root development, and 

 in our recently reported study of the movement of water in the 

 absence of plants (3) we found no appreciable transfer of water from 

 a moister to a drier portion of a soil when the ratio in the former 

 was as low as i . 5 and that in the latter between i . 5 and i . o. 



If we assume that movement of water through a soil ceases 

 when the ratio in the moistest portion has fallen as low as 1.5; 

 that the deeper subsoil loses water through upward movement 

 only when it is penetrated by plant roots; and, lastly, that plants 

 are able to develop roots into a soil layer only when this has 

 a moisture content above the computed wilting coefficient (5), 

 the ratio 1.5, we must conclude that the roots responsible for the 

 dry condition (indicated by ratios of 1.1-1.4) encountered in 

 any subsoil level either will be found surviving or that they have 

 died only since this level of the subsoil was last reduced to the 

 dry condition. 



In order to explain how the dry condition of the deeper subsoil 

 is first established and how it is renewed after wet periods, it seems 

 necessary to assume that among the shallow rooted grasses there 

 are distributed a considerable number of very deep rooted peren- 

 nials. After this dry condition of the deeper subsoil has once been 

 established it may be maintained through a dry period of several 

 successive years without the presence of any roots in it, the moisture 

 from the rains and snow being held near the surface until it either 

 evaporates or is transpired by the shallow rooted plants, while the 

 upward movement of water from the moist zone beyond the extreme 

 reach of plant roots is at least too slight to show a distinct effect. 

 The absence of the dry condition in the deeper subsoil after pro- 

 longed droughts, such as illustrated by field E at Wauneta (table 



