THE WATER OF SOILS. 



235 



when subjected to frequent alternate wetting and drying, it 

 becomes fissured and crumbly, so as to resemble in its texture 

 a tilled soil. This frequent alternation of wetting and drying 

 is precisely what, in the course of time, is brought about by 

 underdrains; rendering clay subsoils pervious both to air and 

 water. The consequence is that even heavy rains can be fully 

 absorbed by the soil mass lying above the drains, the surplus 

 draining off readily in a short time. Roots therefore can not 

 only penetrate, but exercise their vegetative functions perfectly 

 at the full depth of the drains. They are still at liberty to pene- 

 trate as much deeper as their demands for moisture may re- 

 quire; but the depth of four to four and a half feet is already 

 so much greater than in the humid region would usually be 

 reached by them in undrained clay soils, that commonly the 

 moisture successively retained within that mass is as much as is 

 required by them during the growing season. At the same 

 time, their feeding roots are so far below the surface, that 

 ordinary short droughts do not reach them at all ; while the 

 underdrains prevent any injurious stagnation of water around 

 them. It need hardly be added that the entire task of cultiva- 

 tion is also greatly facilitated ; not only because drained soils 

 can be plowed within a few hours after the cessation of rains, 

 as against the same number of days that would have to elapse 

 in the undrained areas; but because tillage is easier, and less 

 draft is required, even when it is carried to a much greater 

 depth. 



Under drainage, then, must be counted as being among the 

 most effective means both of utilizing the rainfall so as to pre- 

 vent loss from runoff and injury from washing, and of creat- 

 ing a deep, loose, pervious soil mass, well adapted to root pene- 

 tration as well as to the conservation of moisture; rendering 

 possible timely tillage and cultivation, and early development 

 of crops fully supplied with moisture and therefore secure 

 against loss from drought. The safety and improvement of 

 crops thus secured corresponds in the humid region to that 

 brought about by the command of irrigation water in the arid 

 countries. But it by no means follows that underdrainage can 

 therefore be dispensed with in the latter, or irrigation in the 

 former. Both have their proper place in both regions; but 

 from special causes underdrainage, as has already been stated, 



