70 Elementary Principles of Agriculture 



when the soil contained only 4 per cent moisture. In 

 some soils 20 per cent moisture caused injury, while 

 in others only 10 per cent moisture acted injuriously 

 on the plants. These figures indicate approximate 

 amounts only. While the range from the "dry" to "wet" 

 seems narrow, it should be remembered that 1 per cent 

 difference in water in the first foot of soil would amount to 

 a rainfall of only about 0.41 inch for clay soil and 0.57 

 inch for sand, allowing 80 pounds per cubic foot for clay 

 soil and 110 pounds for sand. Water weighs 62.31 pounds 

 per cubic foot. One inch of rainfall completely absorbed 

 would increase the percentage of moisture about six 

 per cent. 



103. In Irrigation it is important to know how much 

 water to apply. Injury may be done by applying too 

 much water, besides causing undue expense in handling 

 the water. 



103a. How much water should be applied to a sandy loam 

 soil weighing 90 pounds per cubic foot to raise the moisture from 

 3% to 20%? 



104. What Becomes of the Rain? The average annual 

 rainfall at Washington, D. C., is about forty-four inches; 

 that is, in a year's time, the rain, snow, and sleet would 

 be sufficient to cover the surface forty-four inches deep 

 in water. In some parts of the United States the rain- 

 fall is fifty inches, and in other sections only about 

 fifteen. What becomes of this large amount of water? 

 Some of it runs off into the creeks before it can be 

 absorbed by the soil. This is called the "surface run-off," 

 or simply surface water. This water is lost for the use 

 of the plants. When the surface layers are hard and 

 compact, the water can not be absorbed quickly, and 

 may even flow off while the roots in the deeper layers are 



