PHYSICAL COMPOSITION OF SOILS. 97 



selected for their representative character, in order to deduce 

 therefrom approximate definitions of physical character corre- 

 sponding to popular designations. This table, published in 

 1873 m accordance with results obtained during the two pre- 

 ceding years, does not require any material modification on 

 account of subsequent investigations. It lacks, however, a 

 characteristic representative of the predominant soils of the arid 

 region, viz., the silty soils so prevalent in dry climates, only 

 approximately represented by No. 165 of the table; hence two 

 such, from California, exemplifying respectively the valley 

 deposits of the Sacramento and Colorado rivers, have been 

 added to the list. 



It must not, however, be understood that these typical soils 

 necessarily represent correctly the physical constitution of all 

 soils falling under the same popular designation ; for we are 

 far from being able as yet to predict accurately in every case the 

 tilling qualities of a soil material from its physical composi- 

 tion. To do this it would be necessary not only to know with 

 some degree of precision the several physical coefficients of each 

 of the several grain-sizes, and perhaps of many more inter- 

 mediate ones ; but we would also have to construct a formula 

 according to which each could be given its proper weight when 

 present in varying proportions, and of varying shapes, surface 

 condition, and material. For this our present knowledge is 

 wholly inadequate, if indeed the problem is not beyond the 

 limits of mathematical computation. We must for the pres- 

 ent at least be satisfied with the empirical approximations 

 afforded us by the constantly increasing number of such 

 analyses, correlated with farming experience. 



Since the finest grain-sizes above those classed as " clay " do not 

 tend to "lighten" soils, but even to render them more intractable 

 (" putty soils"), while coarser ones gradually change the dense clay- 

 texture into the " loamy," it is clear that in between there must be a 

 neutral point, some grain si/es which by themselves do not influence 

 soil texture either way. Discussion of numerous physical analyses, and 

 some direct experiments, have led the writer to conclude that this theo- 

 retically neutral grain-size lies at or near the diameter of .025 mm., or 

 .5 mm. hydraulic value. In correlating the results of analysis with the 

 tilling qualities of the soil as to " heaviness and lightness," therefore, 

 that grain-size may usually be left out of consideration. 

 7 



