﻿WAteS utilization BY SPIUMG WHEAT. 5 



way, McDole, and Trumbull 5 have recognized the wilting coefficient 

 of a soil as the dividing line between wetness and dryness and have 

 pointed out the possibility of distinguishing by observation whether 

 the moisture content of a soil is appreciably above or below that point. 

 In at least the first few foot sections of a soil the minimum point of 

 exhaustion is considerably low^er than the wilting coefficient, and the 

 soil when at or near the minimum point can be recognized as dry. 



The minimum point has been determined for each foot section of 

 the soil studied by an average of determinations made while the 

 crop was suffering for water, the water content of the soil not being 

 reduced. The point varies with the character of the soil and to some 

 extent with the distance from the surface. 



A soil in which the moisture content is at the minimum point is 

 spoken of in this bulletin as dry. One in which the moisture content 

 is at the field carrying capacity is spoken of as filled with water, and 

 one in which the moisture content is between the minimum point and 

 the field carrying capacity is spoken of as partly filled. All water 

 held aboA^e the minimum point is termed " available water." 



The range between the field carrying capacity and the minimum 

 point of exhaustion in a uniform soil becomes gradually less as the 

 distance from the surface increases. In such a soil the sixth foot 

 section, for example, even when filled with water, contains consider- 

 ably less water available to plants than the first or second foot of soil. 

 The range between the minimum point and the field carrying capac- 

 ity is particularly small in the lower depths of some of the heavier 

 soils. In a few of these it is often difficult to determine whether or 

 not moisture available to the wheat crop is present in the fifth and 

 sixth foot sections. 



MANNER OF STUDY. 



The moisture determinations made on the wheat plats consist of 

 measurements of the water content of each individual foot of soil to 

 a depth great enough to include the zone of feeding of the crop. 

 The depth varies from 2 to 6 feet with the different soils. The 

 moisture content is expressed as a percentage of the weight of water- 

 free soil. 



The records at most stations have been kept long enough to permit 

 the establishment of the field carrying capacity and the minimum 

 point of exhaustion for most foot sections of soil under conditions 

 ordinarily obtaining in these plats in the field. At stations with a 

 shorter record the fewer determinations combined with the known 

 moisture equivalent and wilting coefficient of the soils have per- 

 mitted a close estimation of these factors. With the carrying ca- 

 pacity and minimum point of exhaustion of a soil established, a 

 determination of the water content of any foot section enables one 

 to determine whether available water is present in the soil and 

 whether the water-storage capacity of the soil is being fully utilized. 



5 Alway, F. J., McDole, G. R„ and Trumbull, R. S. Interpretation of field observa- 

 tions on the moistness of the subsoil. Jour. Anier. Soc. Agron., v. 10, No. 7/S, pp. 

 265-278. 1918. 



