THE PHYSICAL PROPERTIES OF SOILS 



either side by depriving it of the necessary moisture. 

 Plants take up their food from the soil in very dilute solu- 

 tions, and, in order to obtain sufficient, a large amount of 

 water must be transpired. 



Numerous attempts have been made to estimate the 

 amount of water transpired by crops during their period of 

 growth. The methods have been generally alike and the 

 results have been given as pounds of water transpired for 

 each pound of organic matter formed. ,The results obtained 

 by different observers do not agree very closely, but it is 

 evident that the amount of water evaporated depends upon 

 the kind of plant and the amount of water and of available 

 plant foods in the soil. Some idea of the amount of water 

 transpired by plants may be formed from the following 

 figures : 



It will be seen that on the average the crops transpire 

 about 400 times their dry weight of water. A crop which 

 yields 4,000 Ibs. of dry matter per acre at harvest would 

 thus transpire about 1,600,000 Ibs. of water, or more than 

 7 inches 1 of rain, during its period of growth. The quantity 

 of dry matter in a crop of meadow hay is generally less 

 and in a turnip crop more than that given above. Cal- 



1 Taking the mass of the soil at 70 pounds per cubic foot or three 

 million pounds per acre 12 inches deep, 1 per cent, of moisture is 

 equal to about 13'39 tons of water per acre or 0'133 inches of rain. 



