WATER-STRESS BEHAVIOR OF PIMA COTTON. 
soil first undergo a permanent reduction in their moisture content 
as the result of a deficiency in the soil-moisture supply. By perma- 
nent reduction is meant a condition from which the leaves can not 
recover in an approximately saturated atmosphere without the addi- 
- tion of water to the soil. The water in a soil in excess of its wilting 
* coefficient is called the " growth water," or " available " water ; that 
is, it is the portion available for the growth of the plants. 
The plan of irrigation was as follows : Plat 1 was to be irrigated 
at such frequencies that complete exhaustion of the available mois- 
ture in the upper 2 feet of soil, as shown by moisture determinations, 
would never occur during the period of plant development. Plat 2 
was to be irrigated shortly before the depletion of all available 
moisture in the upper 4 feet of soil, and plats 3 and 4 were to receive 
water when it was indicated that the content of available moisture 
in the upper 5 feet was near the point of exhaustion. 2 In irrigating, 
the water was to be measured by means of a Cipoletti weir and 
applied in such quantities that the content of " available " soil 
moisture in the upper 4 feet of soil of plat 1 would be increased to 
approximately 7 per cent, that of plat 2 to approximately 5J per 
cent, and that of plats 3 and 4 to approximately 4 per cent. It had 
been determined by preliminary experiments that a flow of one-fifth 
of a second-foot for four hours w^ould increase the moisture content 
of an acfe of soil approximately 1 per cent to a depth of 4 feet. 
After cultivations were discontinued some difficulty was encountered 
in maintaining close checks between the quantities of wafer applied, 
as measured by the weir, and that shown by moisture determinations 
following the irrigation, it being difficult to secure deep penetration, 
but the quantities determined in the soil samples for the different 
plats were relatively the same. 
To determine whether the supply of plant food might bear some 
relation to the water requirement, plat 4 was given an application 
of cotton-seed meal and 16 per. cent acid phosphate in the drill at 
the rate of 500 pounds per acre by means of a 1-row cotton planter. 
Under the plan of moisture control, irrigations were found to be 
necessary on the dates shown in Table I. 
Table I. — Irrigations of Pima cotton found to be necessary for moisture control. 
Plat. 
First. 
Second. 
Third. 
Fourth. 
Fifth. 
. 
No.l =.... 
June 10 
do 
June 26 
do 
June 26 
July 2S.. ...... 
Aug. 15 
do 
Julv 28 
Aug. 21 
Sept. 2 
do... 
Aug. 21 
Sept.2 


Sept. 2. 
No. 2. . 
No. 3.. 
No. 4.. 


2 The available soil moisture in a field of growing cotton is first exhausted in the super- 
ficial layers of soil and is gradually reduced in ever-increasing depths of soil until 
the roots are limited in their power of penetration or the action is retarded by the appli- 
cation of water to the surface layers. 
