Results of such analyses.—A tabular presentation of the results of analyses made in accordance with the above plan will give a good idea of the differences between the various grades of soils recognized in farm practice, to any one accustomed to the study of figures. But a much more satisfactory showing is made by placing the several grain-sizes segregated, into small vials or tubes of identical diameter and placing them in parallel series alongside of each other.[26] The curves formed by the surfaces of the several sediment-columns in each series show to the eye very strikingly the relations of the several grades of soils to each other, and suggest at once that while gentle slopes or gently undulating curves belong to soils of intermediate, loamy character, steep grades and zigzags show soils of extreme types. This is exemplified in the subjoined Figures:

Fig. 8.—Illustration of Results of Hydraulic Elutriation,
showing extremes of soil texture,
and intermediate loam.

Fig. 9.—Illustration of Results of Hydraulic Elutriation,
showing Alluvial Silts and Pine-Woods Soil.

Physical composition corresponding to popular designations of Soil quality.—The subjoined table illustrates the physical composition of a number of soils from the State of Mississippi, selected for their representative character, in order to deduce therefrom approximate definitions of physical character corresponding to popular designations. This table, published in 1873 in accordance with results obtained during the two preceding 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 composition. 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 intermediate 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 present 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 sizes 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 theoretically 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.

PHYSICAL ANALYSES OF SOILS AND SUBSOILS.