72 IRRIGATION PRACTICE 



volume the point of optimum water content as dis- 

 cussed is the point at which heat moves most readily 

 through the soil. The growing season in the irrigated 

 region is usually very warm, and it might be of considera- 

 ble importance in hastening maturity, or in aiding plant- 

 growth, to enable the soil to absorb much heat and to 

 conduct it readily into the lower layers, where the plant 

 roots are working. 



This is of special importance in districts where the 

 irrigation water is taken from the cold mountain streams 

 that are often only a few degrees above the freezing point. 

 Under such conditions, the ready absorption and con- 

 duction of heat by the soil may determine the rate of 

 growth and length of the growing season, both of which 

 are often of vital importance. All in all, our knowledge 

 of the relation of water to the physical properties of soils 

 would indicate that the wise irrigation farmer will apply 

 to the soil only moderate quantities o water. Too little 

 or too much water at a time are equally dangerous, and 

 threaten loss to the farmer. 



50. Water a universal solvent. Practically every 

 known substance is soluble to some degree in pure water. 

 The rocks and minerals, the fragments of which consti- 

 tute soil are, therefore, partly dissolved in the soil water. 

 Many of the common minerals of chief occurence in soils, 

 such as apatite, clay, mica, hornblend and serpentine, 

 dissolve in water to the amount of 4 per cent to 1 per 

 cent of their total weight. The solvent power of water 

 depends on a number of conditions, the most important 

 of which under field conditions are (1) temperature, (2) 

 dissolved carbon dioxide, (3) dissolved inorganic salts, (4) 

 dissolved organic compounds, and (5) living organisms. 



The higher the temperature, the more rapidly does 



