RESISTANCE OF PLANTS 375 



Frequent irrigations are most favorable for the development of 

 plants, but the amount of water applied at every irrigation must 

 be decreased, because an excess of irrigation water above that 

 indispensable for the plant produces a harmful effect. It com- 

 pacts the soil excessively and sometimes even creates a condition 

 of waterlogging that decreases the development of the roots. 

 The chief harm from excessive irrigation is the percolation of the 

 irrigation water into the deeper soil layers, which are frequently 

 rich in soluble mineral salts. The rise of the water table accom- 

 panying overirrigation promotes the capillary movement of 

 water together with the soluble salts toward the upper soil 

 layers, thus increasing a concentration of alkali in them. Irregu- 

 lar use of irrigation water very often is the cause of too much 

 alkali in soils. Many examples may be mentioned where large 

 tracts of fertile soils with expensive irrigation systems have been 

 abandoned because of an excessive concentration of alkali in 

 the top soil. 



The time of irrigation, its frequency, and the amount of irri- 

 gation water must be planned in such a way as to allow the plants 

 to consume all the water applied, during the interval between the 

 irrigations. The application of water, however, must not be 

 planned purely mechanically according to the calendar; it must 

 ])e coordinated with the development of the plants, so as to 

 provide sufficient water during the critical periods. For spring 

 wheat, for example, it is indispensable to supply the plant with 

 abundant water during the period of shooting and later during 

 the filling of the grain. Wheat is usually irrigated twice: the 

 first time, toward the end of the tillering phase, before shooting; 

 and the second time, soon after flowering, before the beginning 

 of filling of the grain. 



Very desirable from a physiological pohit of view is the new 

 method of spray irrigation. The water is directed onto the field 

 through portable pipes terminating in spray nozzles, which dis- 

 charge water over a large surface area in fine drops similar to 

 rain. This method makes it possible to apply water in small 

 amounts, corresponding to 2 to 3 cm. of precipitation, while 

 the usual methods of surface irrigation require a layer of water 

 no less than 8 to 10 cm. to guarantee a uniform distribution of 

 water on the field. Spray irrigation thus makes it possible to 

 maintain the soil in a sufficiently moist state, eliminating the 



