THE CHEMICAL COMPOSITION OF SOIL COLLOIDS 39 
loidal matter first dispersed differs considerably from the small part 
that is obtained toward the end of the extraction process. Xo data 
are given on the composition of imextracted colloidal matter. 
The extent of the leaching to which the colloids have been ex- 
posed is probably the most important cause of the variations in 
their compositions. Other factors, such as composition of the parent 
material, character of the leaching, and enrichment, probably pro- 
duce variations also. The present rainfall is not always a reliable 
indication of the extent to which leaching has taken place; though, 
in general, soil colloids developed in humid climates show the effects 
of leaching to the greatest extent. Red or yellow soil colloids show 
the effect of more profound leaching than gray or black colloids. 
The colloidal matter of soils is chiefly made up of the products 
of the chemical weathering of soil-forming minerals. It may, how- 
ever, contain small amounts of fine mineral fragments, and some 
soil colloids may contain more of these mineral fragments than 
others. 
The colloidal matter behaves as a very intimate mixture and 
strongly resists separation into fractions of different composition. 
It is probable that separate particles of different composition do not 
exist in the colloidal matter of soils. 
The lime, soda, potash, magnesia, and silica are extracted more 
easily from soil colloids by water than the alumina and iron, though 
the colloidal matter holds all the constituents with great tenacity. 
Lime and soda are extracted somewhat more easily than potash and 
magnesia. Digestion with dilute acids removes the red or yellow 
colors from soil colloids. There are indications of two forms of iron 
in soil colloids — one a red or yellow hydrous iron oxide and the 
other some colorless combination, probably a silicate. 
Stoichiometrical calculations show that all the silica, alumina, iron, 
and water in the colloidal matter are not present in the proportion 
to form the commoner hydrated silicates of alumina and iron, such 
as kaolinite and nontronite, There may, however, be some or a 
considerable part of the above constituents present in compounds 
of the composition of such minerals as kaolinite, nontronite, halloy- 
site. or pyrophyllite. Some suggestive relations between the color 
and the calculated excesses of free iron oxide were brought out; 
but, on the whole, the calculations were not convincing enough to 
disprove or to establish the presence of the compounds assumed in 
the calculations. 
LITERATURE CITED 
(1) Anderson, M. S., Fry, W. H.. Gtle. P. L., Middleton, H. E., and Robin- 
sou. W. O. 
1922. Absorption by colloidal and noncolloidal soil constituents. U. 
S. Dept. Agi\, Bui. 1122, 20 p. 
(2) Atterberg, A. 
1913. Die Plasticitat unci Bindigkeit liefernden Bestandteile der 
Tone. In Intern. Mitt. f. Bodenkunde., v. 3, p. 291-330. 2 fig. 
(3) Bemmelen, J. M. VAN 
1904. Beitrage zur Kenntnis der Verwitterungsprodukte der Silicate 
in Ton — Vulkanischen und Laterit-Boden. In Zeit. Anorg. 
Allegem. Chem., v. 42, p 265-314. 
(4) • 
1910. Die Absorption. 54S p., illus-., Dresden. 
