REGULATING THE WATER SUPPLY 153 



Most of this water is required during a period of five or 

 six weeks, ranging from about July 10 to tlie end of August. 

 On p. 65 it was pointed out that evaporation from the soil 

 and loss from run-off probably equals or nearly equals 

 the requirements of the plants in making a 50-bushel crop, 

 or the total requirement by the crop, and evaporation 

 from the soil, etc., for July and August probably amounts 

 to 12 inches. This is twice the storage capacity of the 

 soil and perhaps three times the amount usually available 

 early in July. After the water stored in the soil is ex- 

 hausted, if rains are delayed, the crop suffers, being 

 greatly reduced, and this often happens even when abun- 

 dant rains come later. The seasonal requirements of corn 

 are illustrated bj^ Fig. 24. 



107. Three ways are open for regulating the water 

 supply : (a) increasing the water-holding capacity of the 

 soil; (6) conservation by preventing evaporation; (c) 

 decreasing run-off during the growing season. 



Since the water-storage capacity of soil is closely related 

 to its physical composition, httle can be done to improve 

 this condition in a practical way. The addition of vege- 

 table matter helps only to a limited extent. 



A certain amount of evaporation can be prevented by 

 cultivation, but how much has never been satisfactorily 

 determined under field conditions. It is probable, how- 

 ever, that loss by evaporation of water that has reached a 

 depth of 12 inches in the soil is very small, and that culti- 

 vation serves principally to prevent evaporation of mois- 

 ture from rains that penetrate no deeper than 6 to 10 

 inches. Experimental results under field conditions to 

 show the effect of cultivation give extraordinary variation. 

 For example, at the Illinois Agricultural Experiment 

 Station, plats of corn that were not cultivated but merely 



