2 BULLETIN 1018, U. S. DEPARTMENT OF AGRICULTURE. 
fairly accurately the amount of moisture in the soil by applying 
measured quantities of water at times when it was indicated by soil- 
moisture determinations that the moisture content had been depleted 
to the minimum point desired. 
Although the experiments were not carried to the point of demon- 
strating a satisfactory method that could be recommended for gen- 
eral practice, it is believed that some of the data are of sufficient 
importance to be made available for other experimenters and for the 
interested public. As a result of these and other experiments it is 
now believed that the behavior of the plants will serve as a practical 
guide in judging the need of irrigation, making unnecessary the 
determinations of the moisture content of the soil except in critical 
cases. 
Certain phenomena were noted in regard to the water relations of 
the different sizes of plants produced under varied soil-moisture con- 
ditions, indicating a retardation of growth and fruiting activities 
during the late development of the large plants. Also there is in- 
cluded a discussion of the period of maturation for Pima bolls, data 
for the shedding of immature bolls, distribution and depth of pene- 
tration of the roots of Pima cotton, and other phases of the irrigation 
problem. 
LOCATION AND PLAN OF THE EXPERIMENTS. 
The investigations were conducted in a large commercial planting 
about 1 mile west of the Capitol Building, Phoenix, Ariz. The soil 
is a heavy clay loam in the first 18 inches, closely resembling adobe 
in properties. Beneath this is a layer of coarse sandy loam, which 
at a depth of 5 or 6 feet is interspersed with concretions of calcareous 
material. 
Seed of the Pima variety of American Egyptian cotton was planted 
on April 5, 1919, on four 0.1-acre plats, which were separated from 
each other and from the rest of the field by substantial dikes. The 
plats were approximately 33| feet wide by 129 feet in length, so 
that 9 rows could be planted 3} feet apart. 
When the plants were about 10 inches in height they were thinned 
by hand pulling to intervals of 12 inches. A string with markers 
attached was used to insure exactness of spacing. 
For controlling the content of moisture in the different plats the 
" wilting coefficient," as defined by Briggs and Shantz (9) ,* was used 
as a basis for indicating the dates of irrigation. The term " wilting 
coefficient" of a soil is defined by Briggs and Shantz (9) as the 
moisture content of the soil (expressed as a percentage of the dry 
weight) at the time when the leaves of the plant growing in that 
1 The serial numbers (italic) in parentheses refer to " Literature cited," at the end of 
this bulletin. 
