WATER-STRESS BEHAVIOR OF PIMA COTTON. 3 



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 5^ 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 would increase the moisture content 

 of an acre 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 water 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 



July28 



do 



Julv 28 



Aug. 21 



Sept. 2 



Aug. 21 



Sept. 2 



Sept. 2. 



No.2 



No. 3 



June 26 



do 





No.4 



.do 











! 





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. 



