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 33f feet wide by 129 feet in length, so 

 that 9 rows could be planted 3f 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), 1 was used 

 as a basis for indicating the dates of irrigation. The term " wilting 

 coefficient" of a soil is denned 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. 



