JOURNAL OF AOaamRAL RESEARCH 
Vol. XXIII Washington, D. C., March 24, 1923 No. 12 
SUMMER IRRIGATION OF PIMA COTTON 1 
By R. D. Martin and H. F. Loomis, Assistants in Crop Acclimatization, Office of Crop 
Acclimatization and Adaptation Investigations , Bureau of Plant Industry , United 
States Department of Agriculture 
INTRODUCTION 
Summer irrigation problems need to be distinguished from those of 
the spring months. In the spring the problem is chiefly one of controlling 
the growth of the young plants and their habits of branching so as to avoid 
excessive vegetative development. After these early stages are passed, 
and the plants are flowering and fruiting, the question is how often to 
apply water to the best advantage for the development of a maximum 
crop. The answer will depend upon differences in soil and other condi¬ 
tions that affect water requirements, but treatment can be adjusted to 
the needs of the crop, as indicated by the behavior of the plants. 
A first experiment with summer irrigation, recognized as a distinct 
problem, was made with Pima 2 cotton at the Cooperative Testing Sta¬ 
tion, 3 Sacaton, Ariz., in 1920, 4 and yielded some interesting and entirely 
unexpected results. The plan was to compare the behavior of plants, to 
which different frequencies of irrigation had been applied, after they had 
reached a normal early fruiting stage. In a series of such comparisons 
the results were alike, with no such differences of growth as have been 
observed in many experiments that included earlier stages of plant de¬ 
velopment. The general indication is to confirm and emphasize the 
importance of controlling the early development of cotton in order to 
bring the plants to a normal early fruiting stage. When this condition 
is established summer irrigation problems are simplified. 
RECOGNITION OF A NORMAL FRUITING STAGE 
Early in July, when the experimental treatment began, the plants were 
about 18 inches high and had from five to eight fruiting branches, were 
mostly without vegetative branches, and just beginning to flower. This 
form of plant is recognized as the early fruiting stage of a normal devel¬ 
opment. The plants had been brought to this condition by withholding 
water in the early growing period and by delaying thinning until the 
plants were from 10 to 14 inches high. This was done in order to check 
any tendency of the plants to grow rank, or put out an excessive number 
1 Accepted for publication July 2, 1921. 
* Pima is a variety of Egyptian cotton bred and developed by the Department of Agriculture and is 
grown extensively in the Salt River Valley of Arizona and the Imperial and San Joaquin Valleys of Cali¬ 
fornia. 
* The Cooperative Testing Station is located in the Gila River Valley at the Indian village of Sacaton and 
is approximately 30 miles southeast of Phoenix, Ariz. The soil and climatic conditions are similar in many 
respects to those in the Salt River Valley. 
4 An effort was made to repeat the experiment in 1921 on the same plots with as nearly as possible the same 
treatments, but spring conditions were very unfavorable so that regular stands were not secured and rains 
in July and August interfered with the control of growth by irrigation, so that comparable results were 
not obtained. 
Journal of Agricultural Research, Vol. XXIII, No. 12 
Washington, D. C. Mar. 24,1923 
aaz Key No. G-291 
27976—23 - 1 
(9*7) 
