THE AMERICAN NATURALIST [Vol. XLVI 



pected that this point will exhibit some definite relation 

 to the heat of wetting (as this property has been devel- 

 oped by Mitscherlich), and perhaps also to the commonly 

 determined water capacity or water-retaining power of 

 the soil. The last named is a property which, as I have 

 previously pointed out, seems especially worthy of inves- 

 tigation by ecologists who are seeking some easily de- 

 termined soil characteristic for use in studies on plant 

 distribution. 



In this connection it is well to call attention to the 

 apparent futility of the method of mechanical analysis, 

 which is resorted to so extensively — and so expensively 

 — in attempts physically to describe the solid portion of 

 the soil. I think I do not exaggerate when I say that the 

 physical analysis has shown itself to be practically worth- 

 less for any physiological purpose. It assuredly does 

 furnish a means of describing a given soil sample with 

 considerable accuracy, and if two samples could ever be 

 found to exhibit exactly the same proportions of the 

 different sized particles, it might plausibly be supposed 

 that, ceteris paribus, these should possess the same rela- 

 tions toward water and toward plant roots, but the con- 

 verse of this statement is not at all true. This method 

 furnishes a mass of data from which no one has yet been 

 able to derive any single comprehensive summation that 

 will express anything definitely as to the possibilities of 

 the given soil as a substratum for plants. Undoubtedly 

 the size of the component soil particles plays a large part 

 in determining how the water conductivity varies with 

 different conditions of soil moisture, etc., but we need to 

 seek some feature which may be much more readily meas- 

 ured for the soil as a whole than merely the proportions 

 of various-sized particles. 



Should we be able to find out the relations which obtain 

 between the maximum rate of water delivery and the 

 other soil characters that I have mentioned, it might at 

 length become possible physically to assay a given soil 

 by the determination of one or more of the latter, sub- 



