RESEARCH METHODS IN STUDY OF FOEEST ENVIRONMENT. 79 



The figures resulting from the above computations for the mois- 

 ture of the soil in soil wells may be tabulated on the " Soil Moisture '" 

 form in the columns headed " Sample, Tl." The ratio which has 

 been determined between the moisture of the soil in the well and that 

 of the native soil at same depth, and the computed moisture of the 

 surrounding soil, or the moisture figure read directly from curves, 

 may also be entered for each date. If the native soil moisture is 

 directly determined by sampling, only the third column under each 

 depth will be used. Space is also provided on the " Soil Moisture " 

 form for any computations which it is desired to make, either cur- 

 rently or after obtaining the monthly means; such as, for example, 

 the percentage of available moisture, the availability, or the various 

 percentages on a volume basis. Appropriate headings may be sup- 

 plied. 



Determination of Nonavailable Moisture. 



The method of soil wells does not attempt to standardize soils for 

 different localities, which could only be done thoroughly by using 

 soil from one source in all soil wells. Nor is it desirable that soils of 

 different localities should be compared on the same physical basis, 

 since this physical basis of itself determines quite largely the mean 

 water content of the soil and its attraction for a given species. It is, 

 however, necessary before different sites and localities may be satis- 

 factorily compared as to their soil moisture that it should be known, 

 at least approximately, at what points they become physiologically 

 dry. either for plants in general or for plants of a given species. 

 Briggs and Shantz (114), it is true, after an exhaustive study of this 

 subject which has cleared the way for many other investigations, 

 summarize in part as follows : 



The results of this investigation have led us to conclude that the differences 

 exhibited by plants in this respect are much less than have heretofore been 

 supposed, and are so small as to be of little practical utility from the standpoint 

 of drought resistance. As compared with the great range in the wilting coeffi- 

 cient duo to soil texture, the small differences arising from the use of different 

 species of plants in determining the wilting coefficient become almost insig- 

 nificant. 



Expressing this difference numerically, it is said: 



Taking 100 to represent the average wilting coefficient, the different species 

 tested (except Colocasia and Tsoetes) give on extreme range from 92 for Japan 

 rice to 106 for a variety of corn. 



From these experiments and conclusions the impression has grown 

 up that all plants are capable of extracting (he moisture of (he soil 

 to essentially the same basic point. Shantz may he quoted as say- 

 ing that there was no intent to convey this impression, ami experi- 

 ment* to be described later will show that as hot ween tree species 



