22 BULLETIN 1311, U. S. DEPARTMENT OF AGRICULTURE 
having an influence on the composition of the colloidal matter, are 
known in a general way. 
The general factors influencing the composition of the colloid or 
of the whole soil may be summarized under the heads of parent 
material, topography, climate, and vegetation. The numerous effects 
of any factor can hardly be isolated from the effects of other factors, 
although in the case of any one colloid the effect of a certain factor 
might predominate. The effect of parent material would, as a rule, 
be more pronounced in the case of immature soils. Topography, 
with its control of drainage and movement of underground waters, 
would be important in special cases. The effects of vegetation on 
the composition of colloidal matter ma}' be properly included under 
climate, since climate largely controls vegetation. The effect of cli- 
matic conditions would be superimposed upon the effects of parent 
material, and should, as a rule, be more evident in the older colloidal 
material. 
The different colloidal materials, the compositions of which are 
given in Table 3, are from the more important agricultural types 
and are consequently from well-drained or fairly well-drained areas, 
so there should be little influence of topography on the composition 
oi these soil colloids. It is possible, however, that in a few cases 
the colloidal matter has been enriched by exposure to underground 
waters carrying material derived from other areas. The composi- 
tion of the parent material from whieh the colloidal material is 
derived is known in only a few cases, and then only approximately, 
since there is no assurance that the decomposed layers were of the 
same composition as the undecomposed layers now available for 
analysis. The present climate of the locations where the soils were 
obtained, however, is known, and it would seem that the composition 
of the different colloids should show some correlation with the 
climate. Rainfall and temperature, with the attendant control of 
vegetation, are the two most important climatic factors. 
It has already been pointed out that in the different soil colloids 
silica and alumina generally vary inversely, and that the sum of 
the lime, soda, potash, and magnesia are usually high when the 
silica is high and low when the silica is low. These characteristics of 
the composition of different soil colloids make it possible to express 
the variability in chemical composition by index figures which are 
useful for correlation. 
The molecular ratio of silica to the sum of the alumina and iron 
shows the relation between the major acidic and basic elements of the 
colloidal matter and is a useful figure to express the general com- 
position of the various soil colloids. This ratio, given in the third 
column of Table 6, determines the order of the different colloids in 
the table. Column 4 shows the molecular ratio of the sum of the 
lime and soda to the alumina and iron. This ratio shows the rela- 
tion of the more soluble or more readily replaceable monovalent and 
divalent bases to the more insoluble bases. The rainfall and tem- 
perature data for the locations where the soils were obtained, or for 
the nearest locality with available data, were furnished by the 
"Weather Bureau. The colors were determined by T. D. Rice, of this 
bureau, by comparing the samples of colloidal matter with the soil- 
color standards used in the Soil Survey. 
