THE CHEMICAL COMPOSITION OF SOIL COLLOIDS 
21 
The difference in composition between the fresh and decomposed 
rock caused by the above process has received considerable attention 
from Merrill (28), Clarke (8), and others. Merrill found that the 
residues resulting from decomposition of rocks were lower in silica, 
lime, soda, potash, and magnesia and higher in alumina, iron, and 
combined water than the fresh rock. In the decomposition of rocks, 
then, which attends the formation of colloidal matter, more silica 
and monovalent and divalent bases are lost than alumina and iron. 
The relative quantities of constituents lost and the effect of this 
loss in causing a difference in composition between the fresh rock 
and decomposed residue are predicable from other data. Clarke 
gives the average composition of the dissolved matter in the river 
waters of North America at 8.60 per cent Si0 2 , 0.64 per cent A1 2 3 + 
Fe 2 3 , 19.36 per cent CaO, 4.87 per cent MgO, 1.77 per cent K 2 0, 
and 7.46 per cent Na 2 0. The average composition of the lithosphere, 
according to Clarke, is 59.77 per cent Si0 2 , 21.35 per cent Al a 3 + 
Fe 2 O s , 4.86 per cent CaO, 3.74 per cent MgO, 2.98 per cent K 2 0, and 
3.25 per cent Na 2 0. Keducing the Al 2 3 -}-Fe 2 3 in both these com- 
positions to unity in order to compare the proportions of constitu- 
ents in the leachings with the proportions of the constituents in the 
lithosphere we have: 
Si0 2 
Al 2 3 +Fe203 
CaO 
MgO 
K 2 
Na 2 
Lithosphere 
Per cent 
2.81 
13.4 
Per cent 
1.00 
1.00 
Per cent 
0.21 
30.3 
Per cent 
0.18 
7.81 
Per cent 
0.44 
2.72 
Per cent 
0.16 
River water 
11.7 
It can be seen that in a broad general way the effect of leaching is 
to increase the alumina and iron oxide and to decrease the silica, 
lime, soda, magnesia, and potash in the leached residue of the 
lithosphere. 
Soils and residual clays are in an advanced stage of rock decom- 
position. They should, then, show the effect of leaching in their 
compositions. It has already been pointed out that the colloidal 
matter is lower in silica 14 and higher in alumina and iron oxide than 
the whole soil, hence the colloidal matter shows the effect of the 
leaching and decomposition processes to a greater extent than the 
whole soil, which still contains some unaltered minerals. 
The composition of the colloidal matter from any particular soil 
depends of course upon the conditions 15 under which the colloidal 
matter was formed and upon the conditions to which it was subse- 
quently exposed. The inorganic colloidal matter in the soil, although 
a residual product of mineral composition, is subject to further 
alteration through interaction with soil water. It is difficult to 
evaluate all the complex conditions which have determined the com- 
position of any particular soil colloid as it exists to-day. Some of 
these conditions may have been local and may have operated for only 
a relatively sho,rt time. The effects of certain conditions, probably 
14 This difference between the silica content of the whole soil and of the colloidal matter 
may be augmented by the segregation of nearly pure silica in the form of secondary 
quartz in the coarser soil particles, the source of the silica being the silicates which by 
decomposition form the colloids. 
15 " Conditions " is used here in a very broad sense and includes the composition of the 
parent material. 
10875—24- 
