264 SOILS 



irrigate from 70 to 100 acres of land during an 

 irrigating season of about ninety days. 



DUTY OF WATER 



This is the amount of land that it should 

 irrigate. No definite rules can be made on this 

 point, owing to the varying capacity of different 

 soils to hold water and the varying demands of 

 different crops. For example, nearly twice as 

 much water is needed in Arizona as in Montana, 

 largely because the season is longer and the loss by 

 evaporation much heavier. The actual amount 

 of water used by the crop itself is small compared 

 with the amount needed to saturate the soil so that 

 conditions favourable for plant growth are pro- 

 duced. Then there is always a considerable 

 amount of seepage and evaporation which cannot 

 be measured. Much also depends upon the 

 capillary power of the soil, or its ability to draw 

 water upward, and especially upon the water- 

 holding and water-moving power of the subsoil. 



In regard to the actual amount of water needed 

 by plants, Professor King has determined that it 

 takes from 300 to 500 pounds of water to make 

 one pound of dry matter in the crop. Since an 

 inch of water covering an acre weighs about 113 

 tons it would require 3 to 5 inches of water to 

 make one ton of nay, corn fodder or other dried 

 crop. This is about the minimum figure and does 

 not allow for serious loss by seepage and evapora- 

 tion. In southern California excellent results have 

 been secured with 6 or 7 inches per year in years 

 when water was low, but only when the water was 

 used with great economy, and the irrigation was 

 supplemented with excellent tillage. Ordinarily 



