SOIL CHANGES DUE TO IRRIGATION 73 



solution go on. In districts where irrigation is indispen- 

 sable, the average temperature during the growing season 

 is generally high, and the solution of soil in the applied 

 water goes on rapidly. In many places the irrigation 

 water itself, taken from comparatively large rivers, is 

 very warm, which, added to the high average daily tem- 

 perature, accelerates greatly the rate of solution. In 

 otiher places, however, the water, as it issues from the 

 mountain canyons, is almost immediately spread over the 

 soil. Such water, fresh from the melting snow-banks, is 

 of low temperature and chills the soil considerably, and 

 in all probability retards the rate of solution of the soils. 



In practically all natural waters there is an abundance 

 of the gas carbon dioxid obtained by the water from 

 decaying organic remains in the soils through which it 

 passes. Such carbonated waters exert a strongly solvent 

 action upon the minerals of the soil ; indeed, carbon dioxid 

 is by far the most important of the factors that influence 

 the solubility of the soil in water. Natural waters gen- 

 erally contain also a large proportion of inorganic salts 

 which, as a rule, increase the solvent action of water. 

 Likewise, solutions of the organic substances formed from 

 the decomposition of plant and animal residues exert a 

 strongly solvent effect on soils. Finally, the presence of 

 living organisms in irrigation water or in soil have much 

 to do with the rate at which the soil constituents are 

 dissolved. 



51. Humid and arid soils contrasted. The solvent 

 power of water modifies so deeply the composition and 

 properties of soil that it is one of the most important 

 factors in the establishment of a rational system of irri- 

 gation practice. The soil-making forces, from the begin- 

 ning, have tended to make soils more soluble, that is, to 



