WATER OF SOILS 187 



Specific Heat. The amount of heat required to raise one unit 

 mass of a substance one degree in temperature as compared to that 

 of the same weight of water is the specific heat of a substance. The 

 specific heat of water exceeds that of every other substance and is 

 used as the standard or 1.0. Substances with low specific heat 

 change temperature rapidly. 



Viscosity. The fluidity or viscosity of water varies with tem- 

 perature and with substances in solution. If at degrees C., the 

 viscosity is 100; at 50 degrees C. it is 31. 



Some substances when dissolved in water increase the viscosity, 

 while others decrease it. 



Uses of Water. Green plants contain a very large percentage 

 of water, as stated above. A constant stream is passing in through 

 the roots and transpired by the leaves. From 300 to 500 pounds 

 of water are required for every pound of dry matter of the plant. 

 The soil then must contain a sullicient amount and must be of 

 such character that it can deliver to the plant this enormous amount. 

 If this is not supplied, the plant is stunted or may wilt and die 

 before maturity. The uses of water in soils are as follows : 



(1) Directly as a plant food, taking part in the building up 

 of tissues, either as water or indirectly by being used in combina- 

 tion with other elements. 



(2) As a solvent for various substances in the soil that may be 

 used by plants. It was believed by Jethro Tull that plants fed 

 directly upon the very fine soil particles, taking them in through 

 the roots, and that this accounted for the better growth of crops in 

 well pulverized soils. 



(3) As a means by which those nutrient solutions are brought 

 to and taken into the plant, either along with the water or by 

 diffusion. 



(1) As a regulator of certain physical phenomena. 



(5) As an aid to chemical action produced directly or through 

 the agency of bacteria. 



The Amount of Water Required by Plants. Experiments 

 have been carried on in various parts of the world by different 

 investigators to determine the amount of water used in producing a 

 crop. More or less transpiration is going on constantly during 

 growth, and an enormous amount of water passes out through the 

 stomates of the leaves. Tbe following table gives the amount of 

 water required to produce a pound of dry matter, as determined by 

 different investigators. 



