Jtme 9, 1923 
Microscopic Estimation of Soil Colloids 
883 
per cent is an exceptional soil. Usually the quantity of colloid present 
is much lower. Basing the calculations on the colloidal matter alone, it 
is seen from the table that 34.5 to 60.2 per cent of the total colloids in 
the soils were not extracted by the methods used. Although this method 
of repeated rubbing and washing for the extraction of colloids from soils 
was carried out as carefully and thoroughly as possible, and although 
we are confident that no more than traces of colloidal material could 
have been extracted by further washing and rubbing, it by no means 
follows that other more efficient methods might not be devised. 
As practiced in this bureau, microscopic estimation of the amount of 
colloidal aggregates in the fine and coarse residue is, at best, a long and 
tedious process. In order to insure fair sampling of the sample under 
study several mounts must be examined, and several areal counts should 
be made on each mount. Experience showed that a total of 10 to 12 
counts on about four slides gave averages very comparable with those 
obtained by triple that number of counts on as many more mounts. 
Nevertheless when only 12 counts are made, the method is too long for 
routine application. 
Unfortunately, the preliminary washing and rubbing are essential in 
order to remove coatings of the adhering extractable colloid. These 
coatings are rarely thick enough to interfere with the determination of 
the mineral; but the microscopic estimation of the quantity of colloid 
in the coatings would be extremely difficult and inexact. This necessity 
for washing fixe soils adds tremendously to the time consumed. Never¬ 
theless, the method is of value in special studies. 
SUMMARY 
(1) A microscopic method for estimating the quantity of colloidal 
soil aggregates in soil separates is given. 
(2) This method was applied to the residues left after extracting, by 
repeated rubbing and washing, all colloid possible from eight samples 
of soil. The results showed that from 34.5 to 60.2 per cent of the total 
colloidal material in the soil was in the form of colloidal aggregates not 
extractable by washing and rubbing. 
ADDITIONAI, COPIES 
OF "rms PUBUCATION MAY BE PROCURED FROM 
THE SUPERINTENDENT OF DOCUMENTS 
GOVERNMENT PRINTING OFFICE 
WASHINGTON, D. C. 
AT 
10 CENTS PER COPY 
Subscription Price, $4.00 Per Year 
PURCHASER AGREES NOT TO RESELL OR DISTRIBUTE THIS 
copy for PROFIT.—PUB. RES. 67, APPROVED MAY 11, 1922 
