INDIRECT DETERMINATION OF THE WILTING COEFFICIENT. 63 



observations of the wilting of the crop. A study of the moisture 

 curve shows that the moisture in the first foot was reduced to the 

 wilting coefficient on June 16; and in the second foot on June 23. 

 At this time the field notes show that the plants were beginning 

 to suffer from lack of moisture. The larger plants were apparently 

 drawing their principal moisture supply from the third foot. The 

 smaller plants with more restricted root systems were confined to 

 soil masses whose water content had been reduced below the wilting 

 coefficient. 



The plants continued to show evidence of lack of sufficient moisture 

 until June 27 ', although wilting did not actually occur. On this date 

 a decided improvement in their condition was noted, due to cool, 

 cloudy weather for three successive days and to the fact that the 

 root systems were beginning to extract moisture from the fourth 

 foot. Hot dry weather followed, and by July 1 the crop was beginning 

 to wilt. It is significant that at this time all of the moisture curves 

 are either practically coincident with or below the line representing 

 the calculated wilting coefficient. In other words, the wilting of 

 the plants occurred simultaneously with the exhaustion of the soil 

 moisture available for growth, as calculated in a wholly independent 

 way from the moisture-equivalent determinations. 



After July 1 the crop never recovered and from this time gradually 

 dried and ripened. However, the curves show that the loss of water 

 from the 4 feet of soil through which the roots had penetrated con- 

 tinued even after the plants had died. 



The data presented show clearly that the wilting coefficient de- 

 termined indirectly in the laboratory is applicable to field conditions 

 and that by the use of the ratio already established it is possible to 

 estimate with sufficient accuracy for field purposes the moisture 

 content at which plants growing under field conditions will become 

 permanently wilted. A study of the moisture curves also shows 

 that it is practically impossible to measure the wilting coefficient by 

 means of field samples. There is no way of determining in an un- 

 confined soil mass the zone from which the plant draws its moisture 

 supply at the time of wilting. The upper soil mass is usually reduced 

 below the wilting coefficient long before wilting actually takes place ; 

 while, if the samples are taken from the extreme limit of the root 

 system, soil masses which have not been penetrated may contain 

 moisture in excess of the wilting coefficient. Moreover, comparisons 

 of field determinations of the wilting coefficient of two plants have 

 no significance whatever unless the relative moisture retentiveness 

 of the two soil samples has been independently determined. This is 

 well illustrated in the chart, the wilting coefficient ranging from 

 13.5 to 7.5 per cent in the different foot-sections. In other words, 



230 



