36 BOTANICAL GAZETTE [JANUARY 
SUBSIDIARY FORMULAE 
We have also included the subsidiary formulae which follow 
as the result of the interrelationships established. The probable 
error has been omitted, since its determination from the formulae 
would always include the experimental errors of the wilting coeffi- 
cient determination, due to the fact that the physical measure- 
ments are not directly compared. 
For the determination of moisture equivalent 
Moisture equivalent = wilting coefficient X 1.84 
Moisture equivalent=hygroscopic coefficient X 2.71 
Moisture equivalent = (moisture holding capacity—21) X 0.635 
Moisture equivalent=o.o02 sand + o. 22 silt + 1.05 clay 
For the determination of the hygroscopic coefficient 
Hygroscopic coefficient = wilting coefficient X 0.68 
Hygroscopic coefficient = moisture equivalent X 0.37 
Hygroscopic coefficient = (moisture holding capacity— 21) X 0.234 
Hygroscopic coefficient =0.007 sand + 0.082 silt + 0.39 clay 
For the determination of the moisture holding capacity 
Moisture holding capacity = (wilting coefficient & 2.9) + 21 
Moisture holding capacity = (moisture equivalent X 1.57) + 21 
Moisture holding capacity = (hygroscopic coefficient X 4.26) + 21 
Moisture holding capacity = (0.03 sand + 0.35 silt + 1.65 clay) + 21 
These formulae establish for the first time a relationship between 
the various physical and physiological measurements of moisture 
retentivity, and while the coefficients may be modified as a result 
of further investigation, it is believed that the equations will prove 
of practical value in the study of the relationship of the plant to 
soil moisture, both in the field and laboratory. 
For the determinations of the maximum available moisture 
The maximum moisture available for growth in any soil is 
represented by the difference between the moisture holding capacity 
and the wilting coefficient. It is possible, therefore, to express the 
maximum amount of available moisture that a soil is capable of 
holding in terms of the relationships given above. It should be 
recalled that the moisture-holding capacity determinations, upon 
24 These equations refer to moisture equivalent determinations made with a 
centrifugal force equal to 1tooo grams and should not be confused with the equation 
given by Briccs and McLane (/.c.) in which a force of 3000 grams was employed. 
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