THE CHEMICAL COMPOSITION OK SOIL COLLOIDS 17 
Although there is a wide variation in the composition of colloids 
from different soils, the colloidal matters extracted from any one soil 
and the corresponding subsoil are very similar in chemical composi- 
tion. Most of the constituents present show but little variation. 
For instance, the average variation of certain constituents in both 
soil and subsoil colloids are as follows: Si0 2 , 1.88 per cent; Al o 3 , 
0.97 per cent; Fe 2 0,. 1.12 per cent; CaO. 0.27 per cent; MgO. 0.22 
per cent; and-K 2 0, 0.19 per cent. 
While the colloids from the soil and corresponding subsoil are 
much alike in composition, there are certain differences which ap- 
pear to be consistent throughout the series. The organic matter, 
nitrogen, sulphur, manganese, and phosphorus are, with very few 
exceptions, higher in the surface soil colloids, and the differences 
between the quantities of these constituents in the surface soil and 
subsoil colloids are comparatively large. Lime is just as consistently 
higher in the surface soil colloid, though the differences are not so 
large as in the case of the above-mentioned constituents. 
Silica, alumina, and iron appear to concentrate slightly in the 
subsoil colloid. These small differences in concentration may be 
considered caused by the dilution of the mineral matter by a greater 
amount of organic matter in the surface soil colloids; for, if the 
constituents of the colloids are calculated on an inorganic basis, the 
figures for silica, alumina, and generally iron in the colloids of the 
soil and corresponding subsoil agree almost within the limits of 
analytical error. 
DIFFERENCE IN COMPOSITION OF COLLOIDAL MATTER. WHOLE SOIL, AND 
COARSER PARTICLES 
The general difference between the composition of the colloidal 
matter and the composition of the minerals making up the noncol- 
loidal part of soils is shown in Table 4. The composition of the 
colloidal matter given in the first line is the average of 45 colloids 
given in Table 3. The average composition of the coarser mineral 
particles given in the second line was calculated for 35 of the soils 
from which colloidal matter had been extracted. In this calcula- 
tion the following data were used : The composition of the whole soil 
(Table 2), the composition of the colloidal matter, and the quantity 
of the colloid in the soil, 8 as determined by the water-absorption 
method 9 . Another estimation of the average chemical composition 
of the mineral particles in soils common in the United States is given 
in the third line of the table. This was based on petrographic de- 
terminations 10 of the minerals in the fine sands and silt of 26 dif- 
ferent soils and subsoils from the humid part of the United States. 
8 In this calculation it is assumed that the chemical composition of the colloidal matter 
which could not he extracted is the same as the composition of the extracted colloid. 
9 The absorption data are given in a former publication (17). While the water-absorp- 
tion method is not an absolute determination of the colloidal matter in all cases, it is 
apparently fairly accurate. 
10 These determinations show that the fine sands and silt average 64 per cent quartz, 
3.8 per cent orthoclase. 4.2 per cent muscovite, and 28 per cent other minerals. For 
calculating the chemical composition, the 28 per cent of other minerals was assumed to 
consist of equal parts of hornblende, biotite, plngioclase, and microcline (3)). 
10875—24 3 
