CHAPTER XIII. 



WATER OF SOILS (Continued). 



THE REGULATION AND CONSERVATION OF SOIL MOISTURE. 



IN view of the commanding importance of an adequate 

 supply of water to vegetation, the possible and available means 

 of assuring such supply by utilizing to the best advantage both 

 rainfall and irrigation water, require the closest consideration. 



Loosening of the Surface. The first thing needful, of 

 course, is to allow the water free opportunity to soak into the 

 soil, so as to moisten the land as deeply as possible. That to 

 this end the surface should be kept loose and pervious by till- 

 age, breaking up crusts that may have been formed by the beat- 

 ing of rains, has already been discussed. In the case of heavy 

 clay soils, however, this alone is not always sufficient. The 

 most effectual way to loosen the land to greater depths than 

 can be reached by tillage, is by means of underdrains laid at 

 the greatest depth that is practically admissible. 



Effects of Underdrains. That drain tiles laid for the ex- 

 press purpose of carrying off surplus water should help to con- 

 serve soil moisture, seems at first sight to be a paradox. Yet 

 the explanation of the fact, which has been demonstrated by 

 long experience, is not difficult. The effect is most striking in 

 clay soils, for sandy soils are commonly naturally underdrained 

 already. 



In discussing the changes of volume which soils undergo in 

 wetting and drying, the fundamental points in the premises 

 have already been mentioned (see chap. 7, p. 112). Clay soils 

 in drying shrink considerably, and re-expand on wetting, but 

 rather slowly : moreover, some clays crumble when wetted 

 after drying, while others, very plastic when wet, crumble on 

 drying (see chap. /, p. 116). 



It follows that while a clay subsoil when kept permanently 

 wet, will form a uniform, p?.sty, difficultly penetrable mass: 



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