118 Plant Physiology 



In all cases the amount of water given is a considerable 

 part (averaging about one fifth in the central United 

 States) of the annual rainfall. Considering the run-off 

 and the evaporation from the soil, both during and outside 

 of the growing season, it is essential to study carefully 

 the water requirements of crops, even in regions where 

 the rainfall seems generally adequate. 



An abundant or optimum supply of water in part 

 obviates the necessity of maximum cultivation, since 

 cultivation may be very considerably concerned with con- 

 servation of water. Nevertheless, there are factors of aera- 

 tion, proper conditions for certain types of bacterial action, 

 texture of soil, and the like which require cultivation, 

 wholly aside from the water relation. 



67. Precipitation and crop growth. Under ordinary cir- 

 cumstances the greater part of the precipitation water 

 is not conserved by the soil. Of the total annual rainfall 

 only a certain percentage is available to vegetation or to 

 the crop. Some of the water is lost in the immediate 

 surface run-off, a small part may be lost by percolation, 

 and there is further a considerable amount represented by 

 evaporation. When the water-table is low, plants are, of 

 course, wholly dependent upon the water which is con- 

 served in relatively superficial strata. 



Practically speaking, no section of Europe or of the 

 United States is wholly free from droughty periods. This 

 implies the well-recognized fact that precipitation during 

 the growing season is demanded by the great majority 

 of crops and types of vegetation. Nevertheless, when 

 proper measures are taken for the conservation of water 

 which may fall outside of (as well as during) the growing 



