296 ELEMENTS OF GENERAL SCIENCE 



do these tilings signify ? Soil which is very wet changes tem- 

 perature much more slowly than dry soil (sect. 158). The 

 water of the soil, being at a lower temperature than the air 

 near the ground on a sunny spring day, makes the soil feel 

 cold. If in the hot summer weather the soil may be culti- 

 vated in such a manner that the water will be held in it, 

 the soil will not become so hot and will therefore be a better 

 living place for plants. 



299. Amount of water used by crops. From the calcula- 

 tions given it is evident that some soils may contain much 

 water, but it is true that large amounts of water pass off 

 from the soil by evaporation and that plants take large 

 quantities of water from the soil. Growing plants are con- 

 stantly carrying water through their roots, stems, and leaves, 

 using some of it in food manufacture but allowing most of 

 it to pass off into the air. The following data show the aver- 

 age quantity of water used by three common farm crops : 



Corn : 50 bushels per acre require 1,500,000 pounds of water. 

 Potatoes : 200 bushels per acre require 1,268,000 pounds of water. 

 Oats : 29 bushels per acre require 1,192,000 pounds of water. 



It has been estimated that in producing a ton of clover 

 hay, which after drying is 85 per cent dry matter, 375 tons 

 of water are carried into the air during the growing season, 

 either through the plant or by evaporation directly from the 

 soil. There may often be a yield of more than 2 tons of 

 clover hay to the acre, but assuming it to be 2 tons, the 

 amount of water taken from the soil to make the crop is 

 equal to more than 6 inches of rainfall. It is estimated that 

 water equal to a rainfall of almost 10 inches is used in pro- 

 ducing a crop of corn of 50 bushels per acre. What amount 

 of rainfall is there in your locality during the growing season ? 



300. Soil water as affected by tillage. The ways in which 

 farm and garden soils are cultivated have much to do with 

 their water-holding capacity. When water evaporates from 



