26 



subject to shock when their roots are disturbed or partly removed. 

 The size of tanks is a limiting factor in the root distribution 

 of tank growth, especially roots of the spreading tjrpe. It has 

 been observed that roots of tules growing in tanks of small diam- 

 eter become greatly crowded after the first year, and the plant 

 growth becomes more or less stunted as the investigation proceeds. 



Limitation of the amount of soil as affecting fertility is 

 likewise important in determining the use of water by tank crops. 

 Investigations running over extended periods in which the rela- 

 tively small volume of soil is used continuously are likely to 

 result in a stunted growth. The effect of continued cropping of 

 tank soil has been noticed in cotton investigations extending 

 over a 3-year period. Beckett and Dunshee (1) say: "A comparison 

 of the size of plants grown in the tanks with those grown in the 

 field plots under similar irrigation treatments, showed that 

 smaller plants were produced in the tanks each year than were 

 obtained in the plots. These smaller plants, however, used from 

 40 to 53 per cent more water than the plants growing in the field. 

 This increased use of water might be explained by the probable 

 root concentration in the limited soil mass of the tanks, a higher 

 soil temperature in the tanks, and the higher temperatures and 

 lower humidities surrounding the individual plants in the tanks." 



Crop overhang of tank growth also presents a serious ques- 

 tion in tank Investigations. The aerial portions of som.e crops, 

 such as tules and cattails, grow naturally stiff and erect and 

 occupy approximately the same horizontal area as the tank. In 

 other growth, such as sweetclover, the stems droop over a much 

 greater area than that occupied by the tank. The interception 

 of insolation under such conditions is greater than the tank 

 intercept, and it is incorrect to compute the water loss on a 

 basis of tank area. 



Protection of soil tanks during periods of precipitation 

 to prevent entrance of rain water into the soil has been generally 

 condenuied, yet for some purposes prevention of rainfall on the 



