T H E 
NEW PHYTOIiOGIST. 
Vol. XII, Nos. 4 & 5. 
April & May, 191 
[Published May 29th, 1913]. 
THE COEFF1CENT OF HUMIDITY: A NEW METHOD 
OF EXPRESSING THE SOIL MOISTURE. 
By W. B. Crump, M.A., 
Heath Grammar School, Halifax. 
[With One Figure in the Text.] 
I.—Introduction. 
HE most obvious distinction between plant habitats is that 
1 based upon the quantity of water present and the permanence 
or fluctuation of the water supply. Such common descriptive 
terms as marsh, swamp, desert, or wet, boggy, dry, express this and 
serve to differentiate habitats in a general way. Proceeding 
further by deduction, field botanists are accustomed to estimate the 
comparative wetness of an area by the vegetation it supports, or by 
the presence of specific plants. But the development of ecology 
demands the replacement of these indefinite characters by more 
exact knowledge based upon quantitative analysis of the edaphic 
factors; knowledge of the magnitude of each that produces the 
static equilibrium revealed as a stable plant-association, and of the 
limiting values that determine the dynamic change from one 
association to another, or even the replacement of one formation by 
another. At present the nature of the master-factors that determine 
the existence of a formation is often a matter of supposition. Until 
a body of data has been accumulated it is impossible to know which 
of several possible factors controls the formation, or what is the 
magnitude of a given factor when it reaches its limiting value. 
The importance of quantitative determinations of water- 
contents has been insisted on by F. E. Clements * 1 and in laying 
stress on the water-content, and particularly the available water- 
content or chresard, as of more importance than the soil structure 
1 “ Research Methods in Ecology.” Lincoln (Nebraska), 1905. 
