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-mutter production by 
Pima cotton on plats 1 and 3 in 1919. 
Month, 1919. 
Water los 
Inches. 
t per acre. 
Tons per day. 
| Dry matter pro- 
duced per acre 
(pounds). 
Field water re- 
quirement. 
Plat 1. 
Plat 3. 
Platl. 
Plat 3. 
Plat 1. 
Plat 3. 
Plat 1. 
Plat 3. 
May 
1.2 
5.5 
5.6 
6.4 
2.7 
0.9 
2.4 
4.5 
6.0 
1.3 
4.4 
20.8 
20.5 
23.4 
10.2 
3.3 
9.1 
16.5 
21.9 
4.9 
t 5,426 
4,013 
894.8 
June 
July 
853.6 
August 
September 
