10 



BULLETIN 1018, U. S. DEPARTMENT OF AGRICULTURE. 



The taproots of the large plants were proportionately larger than 

 those of the small plants to a depth of 10 or 12 inches below the sur- 

 face of the ground, but at a greater depth than this they were not 

 larger in diameter than those of smaller plants. Such data would 

 seem to indicate that a limiting root system may have an important 

 bearing on the water-stress behavior of the largest plants, it being a 

 logical conclusion that the root system required for supplying the 

 water demands of a large plant would be larger than that of a small 

 plant. 



In order to have additional data on this point, attention was next 

 directed to the relative quantities of vegetative material produced on 

 the different plats and to the demands made upon the moisture of the 

 soil in supporting this material. 



From the difference in the moisture content of the soil samples, 

 which were taken at weekly intervals and also just before and two 

 days following all irrigations, it was possible to compute the mois- 

 ture lost from the soil during any week or month and for the entire 

 period of development of the plants. The average quantity of water 

 lost per acre each day (Table IV) was determined by dividing the 

 weight of water lost from an acre during the entire month by the 

 number of days in the month. The quantity of dry matter produced 

 per unit area was determined by harvesting the plants when fully 

 matured on an area of 900 square feet near the center of each plat. 

 These samples, which included practically all of the dry matter pro- 

 duced above ground, were placed on canvas sheets and dried by the 

 sun until they had reached a constant weight. Subsamples were 

 then taken and dried at 110° F., and from these results the air-dried 

 weights were corrected and taken to represent the average produc- 

 tion of dry matter on the entire plat. The field water requirement 

 was determined after the method of Briggs and Shantz (10), by di- 

 viding the total weight of the water lost from the soil during the 

 whole period of growth by the total dry weight of the crop. 



Table IV. 



-Comparison of monthly water loss and dry-matter production by 

 Pima cotton on plats 1 and 3 in 1919. 



Month, 1919. 



May 



June 



July 



August 



September 



Water lost per acre. 



Inches. 



Plat 1. Plat 3 



1.2 

 5.5 

 5.6 

 6.4 

 2.7 



0.9 

 2.4 

 4.5 

 6.0 

 1.3 



Tons per day. 



Platl. Plat 3 



4.4 

 20.8 

 20.5 

 23.4 

 10.2 



3.3 

 9.1 



16.5 

 21.9 

 4.9 



Dry matter pro- 

 duced per acre 

 (pounds). 



Platl. Plat 3 



5,426 



4, 013 



FieldWwater re- 

 quirement. 



Plat 1. Plat 3. 



S94.8 



853.6 



