CHAPTER XIV 

 WATER OF SOILS 



PLANT growth cannot take place without moisture. Plants con- 

 sist of from 60 to over 90 per cent of water. This represents only 

 a very small part of the water used, since many plants transpire in 

 twenty-four hours an amount equal to their weight. Water is of 

 primary importance in many physical and all chemical changes 

 that take place in the soil. 



Some Physical Characteristics of Water. Water has certain 

 physical characteristics that should be noted here. Its volume 

 changes with temperature. Its maximum density is attained at 4 

 degrees C., 39.2 F., and expansion takes place either above or below 

 this temperature, and at 15 degrees C., 59 F., the density as com- 

 pared with water at 4 degrees C. is 0.99. At the freezing point the 

 density of water is 0.99988, while the density of ice at the .same 

 temperature is 0.928. In the melting of ice a large amount of heat 

 is used, but it does not raise the temperature. This heat becomes 

 latent or is used in changing the condition of the water from a 

 solid to a liquid. It requires more heat to melt (fuse) ice than to 

 fuse metals. To melt one gram of ice requires 80 calories,* whereas 

 metals require from 5 to 77 calories. Expressed in Fahrenheit- 

 pounds, the English system, the figures are 144 for ice and from 

 9 to 138.6 heat units for metals. 



In the evaporation of water a similar phenomenon is observed. 

 When the boiling point is reached no further change in temperature 

 of water occurs, but the heat is used in changing the water from 

 a liquid to a gas. To effect this change in one gram 537 calories 

 are necessary, or, with the Fahrenheit-pound, 966.6 heat units. It 

 must be remembered that when water evaporates, an amount of 

 heat equivalent to the above is used, regardless of the temperature. 

 In changing from a higher to a lower temperature, or from gaseous 

 to liquid form, or from liquid to solid, equivalent amounts of heat 

 are liberated. 



* A calorie, in general, is the amount of heat required to raise the tem- 

 perature of a gram of water one degree centigrade. A heat unit or a British 

 thermal unit is the heat required to raise one pound of water one degree 

 Fahrenheit. 

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