FURTHER WORK ON THE DETERMINATION OF THEMINERAL CONTENT OF SOIL COLLOIDS

EMIL TRUOGUniversity of Wisconsin

In a report last year (Am. Soil Sur-vey Assoc. Bul. 16 (1935) 136), a. tenta-tive procedure was given for the deter-mination of the mineral content of soilcolloids. This procedure included an out-line of a new method for the separationand determination of the free iron oxidesof soil colloids involving the use of hy-drogen sulfide. This method has beenused rather extensively during the pastyear, and details of it have been givenelsewhere (see Jour. Amer. Soc. Agron.,27 (1935) 312).

It should be added that with certainsoils containing large amounts of freeiron oxides, especially in the form ofhematite, it is necessary to prolong theH2S treatment for a longer period thanone-half hour in order to effect completeremoval of the free iron oxide. Themethod is far superior to any other pre-viously tried, since, with most soils, itmakes possible complete removal of thefree iron oxides without material changeof other constituents. The method is es-pecially effective in cleaning mineralgrains, preparatory to petrographic analy-sis.

In soil studies for various purposesconsiderable emphasis is placed on theSiO2-R2O3 ratio of the colloid fraction.In most cases the Al203 of the colloidfraction is largely combined with silica,

but much of the Fe2O3 exist as thefree oxide. The use of the H2S methodfor the determination of the free ironoxides makes lt possible to presentSiO2-R2O3 ratios which Involve only thecomoined iron. Expression of ratios onthis basis may add to the value or sig-nificance of these ratios. It is hopedthat other investigators may use themethod in this connection.

A point which urgently needs theattention of soils investigators is thatpertaining to the nomenclature used inspeaking of the various fractions (frac-tions based on size of particle) of soilcolloids. There is also the question asto what "size of particle should be con-sidered as representing the upper limitof particle size of soil colloids. Someassume the upper limit to be a diameterof 1.0 u; others a diameter of 0.1 u.Still another question has to do with therelation of the colloid fraction to whatis commonly called the clay fraction.Since the characteristic properties ofthe clay fraction are due to the colloidspresent, would it not be simpler and morelogical to merge the two so that clayfraction and colloid fraction would besynonymous? It is hoped that these ques-tions may be discussed in detail at thenext meeting of the Association.

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