174 The Plaxt World. 



still furnish a means of soil classification and of the coi relation 

 of physical nature and field properties, provided only that the 

 water equivalent of any given soil type be characteristic of that 

 tvpe and constantly gi^■cn by it. Empirical as it is, such a 

 "crnstant" is better than mechanical analysis and if nothing 

 better can be de\"ised it is doubtless destined to a large useful- 

 ness in future soil investigation. As yet, howe\er, it has been 

 \ery little used or studied. 



The Critical Moisture Contcni. The foregoing include all 

 the ''constants'' so far used or prominently suggested. There 

 remains, hoA\ ever, one other possibility, as }et nearly unnoted but 

 perha; s the best of all. In the first and second papers of this 

 series there were discussed the relations of the film water to 

 soil structure and to water mo\ement in the soil, and there was 

 nc ted the discovery by Cameron and Gallagher'" of the"critical 

 moisture content'' as a percentage of contained water, definite 

 and constant for each soil, at which contert all the physical 

 properties of the soil are either at a maximum or a minimum. 



It will be recalled that this critical moisture content is 

 believed to correspond to the point of maximum efficiency of 

 the forces resident on the surfaces of the water films between 

 the soil particles. If this be true here is a soil constant worth 

 Avhile. It is closely (and nearly exclusively) related to the 

 phvsical character of the sjil (since the water-film-system is so 

 related) ; its variations with this physical character are wide 

 enough to be easily measurable; it is expressed simply and in 

 a single unit; and, best of all, its relation to the physical character 

 and to other physical properties is real and rational. It is a 

 real property of the soil — ^a constant which means something. 

 It may le determined in a dozen different ways and neither the 

 accidents of the determination, the history of the sam])le, nor 

 the personality of the experimentor will afi"ect its value. Un- 

 fortunately it has one very serious disadvantage — its determina- 

 tion is exceedingly tedious, laborious and costly. '^' So much 

 is this the case that the critical moisture content, as at present 

 determined, is entirely out of the question as a generally useful 

 constant and can find utilitv onlv in the most extensive scientific 



♦Bulletin SO Hun.au of Soils, U. S. Department .if .\griciihurc. I'iO?. 

 ♦For the actual methods the reader is rtferred to Bulletin 50, Bureau of Soils, U. S. 

 Department of Agriculture, above quoted 



