THE AUXOTAXIC CURVE AS A MEANS OF CLASSIFY¬ 
ING SOILS AND STUDYING THEIR COLLOIDAL 
PROPERTIES 1 
By A. E. Vinson and C. N. Catlin, Division of Agricultural Chemistry , University of 
Arizona 
In a recent paper the writers 2 described a method of determining 
the swelling coefficient of dry soils when wetted. Attention was called 
to the existence of great differences in the rate at which swelling took 
place, a subject which had not been studied at that time for want of 
suitable equipment. Since then a MacDougal auxograph has been 
equipped with a rapidly moving drum, and important information on the 
rate of swelling is being gathered. Although the lack of time has neces¬ 
sarily limited the number of soils tested, the writers believe that every 
dry soil on swelling in distilled water at a given temperature will pro¬ 
duce a characteristic auxotaxic curve that can be duplicated repeatedly. 
This curve appears to integrate at least four properties of the soil: Tex¬ 
ture, colloidal organic matter, colloidal inorganic matter, and soluble 
salts, and, indirectly, specific gravity, the original thickness of the io gm. 
disk being determined by this factor. In addition to these properties of 
the soil itself, the curve integrates temperature, viscosity, and the pres¬ 
ence of electrolytes and colloids in the medium in which the swelling 
occurs. 
The accompanying auxograph charts (figs, i to 4) drawn by a few south¬ 
western soils show the great variety in form of curve obtained under 
standard conditions. Unfortunately, the ordinate used in figures 2, 
3, and 4 has twice the value of that used in figure 1. This is necessitated 
by the wide range in the swelling coefficient. A magnification of ten 
times the movement of the disk is most satisfactory, but many soils throw 
the pen off the chart with this magnification. Before any systematic 
study of the recognized soil types is undertaken, .charts double the 
width of those shown in the accompanying figures should be provided 
and suitable drums constructed to carry them. It is believed that if a 
large number of auxotaxic curves drawn on the same coordinates and 
representing the recognized soil types were recorded, they could be used 
in the classification of soils to the greatest advantage along with other 
methods now in use, since the curves visualize the combined effect of many 
soil properties, especially those depending on colloids. It is also not un¬ 
likely that by a comparison of the auxotaxic curves of new soils with 
those of old ones of known behavior under cultivation the probable agri¬ 
cultural value of the new soil could be predicted. Such determinations 
might be of the greatest value in considering the desirability of installing 
new irrigation projects. 
1 Accepted for publication June 25,1923. 
* Vinson, A. E., and Catlin, C. N. Determination op the swelling coefficient of dry soils 
when wetted. In Jour. Amer. Soc. Agron., v. 14, p. 302-307. 1922. 
Journal of Agricultural Research, 
Washington, D. C. 
agu 
Vol. XXVI, No. 1 
Oct. 6, 1923 
Key No. Arir. -1 
(«) 
