CHAPTER V 



SOIL CHANGES DUE TO IRRIGATION WATER 



THE soil cannot, directly, be greatly changed by the 

 farmer. As it is, so, in a large measure, it must remain. 

 Tillage implements modify only slightly the upper layer 

 of the soil. Water, however, may cause fairly large 

 changes in the soil to the full depth to which it pene- 

 trates. Irrigation, therefore, with its power of regula- 

 ting the quantity of water applied, may be made a means 

 of modifying soil properties. Physical, chemical and 

 biological soil changes are induced by irrigation, and 

 some of the most important principles of a permanent 

 system of irrigation agriculture, depend upon the effects 

 of water upon soil. 



43. Contraction and moisture film. If a camePs-hair 

 brush be dipped in water, and then removed, the hairs 

 cling together to form a narrow and rather hard brush 

 suitable for use in painting. If a trifle of the water in the 

 brush be squeezed out, the brush becomes rather stiffer 

 than it was before, but if more water be removed, the 

 brush become looser and looser until it is dry and fluffy. 

 This adhesion of the hairs is due (1) to the contraction 

 of the films surrounding each little hair, and (2) to the 

 contraction of the water film enveloping the whole brush. 

 (Fig. 15.) 



In like manner, the particles of a soil, when wetted or 

 dried, tend to move either more closely together or farther 

 apart, and the soil becomes more or less rigid. When 



(64) 



