166 The Plant World. 



constant must be measurable with fair ease and accuracy. 

 If the constant be numerically simple and if its relations to other 

 physical properties and to the physical character be rational 

 ones, so much the better. If, however, numerical complexity 

 cannot be avoided and if the needed correlations must neces- 

 sarily be obtained empirically, we must needs make the best of 

 it, and the fault will not be fatal. 



vSuch a constant would be of incalulable value not only in 

 purely scientific soil investigation, but also in practical agri- 

 culture. It would place the classification and comparison of 

 soils on a far better basis than any now available, and it is not 

 too much to hope that correlations might be obtained with 

 fertility itself, at least in so far as this very complex quantity is 

 related to the physical nature of the soil and to its physical 

 properties. This need has not gone unappreciated. There have 

 been many and various suggestions of possible physical constants, 

 and in particular the mechanical analysis has been much used 

 and has been long enjoyed (wrongly, as I believe) some reputa- 

 tion for adequacy. 



Mechanical Analysis. This mechanical analysis, and the 

 mechanical composition which it appropriately expresses, have 

 been discussed in the first paper of this series and need not 

 now be reviewed. Nor need anything be said concerning the 

 various methods used or suggested for the separation of the 

 soil particles into groups of the various sizes or concerning the 

 difi"erent systems of limits between these groups. * For de- 

 tails on these matters the reader is referred to the textbooks 

 of agricultural analysis. The merits and demerits of the mechan- 

 ical analysis as a soil constant depend upon inherent character- 

 istics which have little or nothing to do with the details of ma- 

 nipulation or the form of the results. And, as I expect to be 

 critical, let me say first that mechanical analysis is by no 

 means useless nor to be belittled as a means of soil investiga- 

 tion. It has been much used and, on the whole, pretty success- 

 fully, and we now are in possession of man}' thousand analyses 

 and of at least some beginnings toward correlations with the phy- 

 sical properties observed in the field. But it can hardly be con- 

 sidered as fully satisfying the requirements which we have seen 



♦Studies in Soil Physics I, Plant World, 14: 31. 1911. 



