26 BOTANICAL GAZETTE [JANUARY 
by the soils employed, the ratio of the moisture equivalent to the 
wilting coefficient appears to be constant within the limits of 
experimental error. In other words, two determinations of the 
moisture retentiveness of these soils, one physical and the other 
physiological, show a linear relationship which is independent 
of the texture of the soil. The relationship is expressed by the 
following formula: 
Moisture equivalent _ wilting jstcene 
1.840.013 
In order to compare the available moisture content of one soil 
with that of another, we must know or be able to estimate accurately 
the wilting coefficient of each soil. The minimum limit of moisture 
available for growth is the datum line from which all comparisons 
should be made. This datum can be established directly by wilting 
coefficient measurements, or it can now be calculated by means 
of the ratio just established. The latter method for field work 
is far simpler and more expedient. The soil sample taken in the 
field for soil moisture determination, although ample for duplicate 
measurements of the moisture equivalent, is usually not large 
enough for a single wilting coefficient determination. Moreover, 
the period of time required for wilting coefficient determinations, 
combined with the uncertainty which accompanies all physiological 
work when duplication is impossible, makes this determination less 
expedient and the results in such cases less reliable than those 
derived from the moisture equivalent by the use of the ratio here 
established. 
The relationship established between the wilting coefficient 
and the moisture equivalent led us to believe that a similar rela- 
tionship might be found for some of the other physical measure- 
ments of soil moisture retentivity. We have accordingly made 
similar comparisons of the wilting coefficient with the hygroscopic 
coefficient, the moisture holding capacity, and the soil texture, as 
expressed by mechanical analysis. The last mentioned determina- 
tion does not measure moisture retentivity, but it does measure 
certain properties of the soil which determine the moisture reten- 
tivity to a large extent. We will now consider the results of these 
comparisons. 
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