tTSE OF WATER BY SPRING WHEAT ON GREAT PLAINS. 5 



duction of the soil-moisture chart for wheat plat C in the continuous- 

 cropping series at Belle Fourche in 1916. Determinations on that 

 plat were made to the depth of 4 feet that year. Its water content 

 at the several dates of sampling is consequently shown by the points 

 of four separate curves. Immediate reduction of these to a single 

 curve by averaging the four would result in an error of a magnitude 

 dependent upon the differences in the volume weight of the several 

 units of soil. Soils of the several stations also differ in volume 

 weight, and consequently any given percentage of water content may 

 not indicate the same actual quantities of water in different soils or 

 soil units. Further, it is not possible directly to combine percentages 

 of water in the soil with inches of water added to it by rainfall 

 during any period. Consequently it has been necessary to convert 

 the percentage of water in each foot section of soil to terms of inches 

 of water. 



Prerequisite to such conversion is the determination of the volume 

 weight of each soil unit under study. This has been determined by 

 calculation from the weight of the cores taken by the soil tube. For 

 this determination periods were selected when the soil tubes were 

 known to be new and in good condition with a sharp cutting edge 

 of a circle of the known diameter of 20 millimeters. As large a 

 number of weights as possible, not less than 50 and preferably more 

 than 100, were averaged for each unit, and the weight per cubic foot 

 was calculated from the weight of a known volume. Weights of 

 obviously defective cores were arbitrarily rejected. It is believed 

 that the volume weights determined by this method are as accurate 

 as those that might be determined by any other method based upon 

 a smaller number of cores of larger volume. The volume weights 

 so determined and used as bases of conversion have been carefully 

 considered in the light of all knowledge and information on the 

 several soils and accepted as fair bases of comparison. 



This study is concerned with the amount of change in the water 

 content of the soil during a period rather than with the actual 

 water content. The process, therefore, has been to determine the 

 percentage of change and then to convert this into inches of water, 

 instead of reducing the entire water content to inches and then 

 determining the change by subtraction. 



In determining the water loss from the soil for any period the 

 first step is to subtract the water content of each foot section at the 

 end of the period from the water content of the same unit at the 

 beginning of the period. At this time the water content is expressed 

 as a percentage of the dry weight of the soil. The next step is to 

 convert these expressions to terms of inches of water by calculation 

 from the determined weights of the soil of each unit. An increase 

 in water content is recorded as a minus loss. The algebraic sum of 

 the losses from all the units (foot sections) involved is the loss from 

 the soil of the plat under determination. 



The total water loss or the total water used by the crop in any 

 period is determined by adding the precipitation for the period to 

 the water lost from the soil. It is the algebraic sum of the two 

 quantities. The relative importance of the two quantities will be 

 considered later, and the necessity of combining the two into a 

 single quantity will be shown. 



