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42FIELD TEXTURES AND PHYSICAL COMPOSITIONDETERMINED BY TWO METHODS OF MECHANICALANALYSIS

CHAS. F. SHAW(University of California R keley, California U.S.A.)The textures of soils are u5ually designated y conventional orcolloquial names, such as sandy loam, loam, clay loam, clay, adobeclay, and .the like, that have been established y general usage.

Definite standards for these textural grades were established y\:Vhitney (7, 8) as a result of the correlation between the textural namesand the laboratory analyses of several thousand samples sent in yLhe ficlt.l men. \\Then the mean composition of each texture had beenestablished, the mechanical analyses were thereafter used to checkdc\iations and to hold men working in the various parts of the countryto relatively close standards of textural designations.

At rhc University of California the analytical ark necessary forg-uidance in the textural naming of soils has followed two methods.The first of these, used from 1913 to Hl30, followed the centrifugemethod originally de\eloped by the Bureau 'Of Soils (2, 3). Ammoniawas used as the dispersing agent, tht soils being shaken for 12 to lRhours, then bv decantation from the shaker bottles the silts and dayswere c ; e p a r a t ~ d from the sands. The clays were separated from thesilts y repeated decanting after accelerated sedimentation in a centrifuge. The upper limit of " clay " as determined y this ammoniacentrifuge method was fhc microns, and " silt " was fifty microns, theseparations being chccl.:cd y microscopic measurements of t l H ~ suspended materials.

The second method, which has been used for selected analyses si11ccHl30, is a mmlilication of the International met 1od (G). The soils arcprctrcntl'd with hydrof,' en pcroxidc and hydrochloric acid, shaken withsodium oxalate as a dispersant, washed through a 300 mesh sieve toseparate the sands, and the iilt and clays determined by pipettesampling-.

The \'icld of clavs b\ the modilicd-lntcrnational method was muchhigher th.an y the 3mn;onia-etntrifuge method, and the former couldnot be used in checking the textural name ' as established by the fieldmen. From time to time thr rcsuhs of analvsl: s by the two methods\vere compared, but there did not 'cem to be a c o n ~ i s t e n t difference inthe ,iclds of the finer fractions .

. In order to dctcrmine the differences in results, and to settle questions regarding the consistence with which textural grades 1vere beingusetl, sixty-four samples of surface soils from the Lodi and Napa soilsurvey areas (unpublished), which had been suveyed y the same twomen, worldng together, were analyzed y both methods. ln order to

, make the comparisons direct, the upper limit of the silt fraction washdd at O microns anrl of the clay at 5 microns in the modified-Inter-national ns "ell as in the a m m o n i ~ c e n t r i f u g c methods.

The results of the analyses were plotted on the triangular diagramof Davis and Bennett (1). Ten of the soils had been classed by thefield men .as .sandy Joams, 14 as learns, 27 as clay learns, and 14 asclays. Tht ammon in-centrifuge analyses showed that all of the sandyloams had a physical composition that classed them properly in thattc:..:turnl grade as established by the Bureau of Chemistry and Soils.

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i~

.

I a

lc

le

CL VLOAM S

I/

....

....

....

lb

~

ld

II

..-\.\

~ . 1 d

\ ~ 'liLY

L ; ',\ \ 4." UlJ

FIG .-Differences in physical composition indicated by two methods ormechanical analysis. 1'he butt of the arrow indicates the composition asdetermined by the ammonia-centrifuge method, while the poinl shows thecomposition of the same soil when analyzed by the modified-lnternationa.lmethod.In-Analyses of 10 soils call d sandy loams.lb-Analvses of 14 soils called loams.lc-AnalYses of 2i soils called clay learns.Id-Analyses of 14 soils called days.le-Average analyses of each of the lour textural grades.If-The triangular diagram shov.-ing the composition areatextural grade. assigned to each

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Of the learns, however, only 6 were properly named learns and all ofthese were very heavy textured, lying close to the day loams, Two ofthe 11 Joams " fell within the sandy clay loam textures, 5 in the clayJoams, and 1 was a light day. Of the 27 clay learns, 14 analyzed asclay loam, 1 as n loam, 3 as sandy clay learns, 2 as silty clay loams, Ias a silty clay, and li as light clays. Of the 4 clays, 13 analyzed asclays and 1 as a silty clay. From these studies it became apparent thatfrom the standpoint of mechanical analysis the field men were inclinedto name the soils of intermediate textures in one grade too coarse., lightclay learns being judged as learns and light clays as clay learns. Noneof the so-ca1led 11 clay learns, however, fell far outside the clay loamportion of the triangular diagram, whereas several of the " learns "were well away from the loam section. The clays and sandy learnswere consistently named. This " drift " in the judgment of texturesappears to be due to a tendency to make the determination comparative. When working in a region of dominantly heavy soils, the sandiersoils are often named in a textural grade that is too light or coarse,whereas in a region of dominantly sandy sqBs, many soils of finer r x ~ture arc often designated by a grade heavier or finer than theircomposition would warrant.

The marked difference jn yield of the fines, the silt and clay, dueto different methods of analysis is shown by the arrows jn the triangulardiagrams in Fig 1, and the average change is shown in Fig Je andin Table 1 Under the more severe treatment of the modified-Inter-

TABLE IAverage composiliqn of texlur;LJ grades by the ammonia-centrifuge and the modifiedInternational methods or anPlys s,

Sandv loams(10)

Loams1 4)

Clay loams(27)

Clays(H)

Cent.1nt.

Cent.Int.

Cent.1nt.

Ce:nt.Int.

Sands Silts Claysn2 i4 228u Hsafi22ii 2835 ] 959- lO

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decrease of 12 5 per cent. of sands a very small amount being- addedto the silt fraction and practically all going into the clay. The claysoils lost 87 per cent. of their sands and 76 per cent. of silt, all ofthe material falling into the clay fraction. This shift of textures fromthe sandy learns to the clays is shown by the open arrows on thetriangular diagrams and in Fig Ie, and emphasizes the difficulty thatwill probably be encountered in any endeavor to intercept the fieldtexture by using the International method of mechanical analysis.Field texture expresses not the actual or ultimate texture of thesoil, but ratl1er a combination of texture and of the structure or degree.of aggregation. The International method in its fundamental purposeaims to break down all structural aggregates and to resolve the soilinto its individual particles. Since the structure of the soil dependsto a considerable extent upon its chemical composition and on its e x ~changeable base status the field texture cannot be expected to be directly reflected by the ultimate texture. Methods of mechanicalanalysis that are not sufficiently drastic to break down all of the aggre-gates but which resolve only the less stable aggregates Into theirultimate particles will give a better measure of the mechanical composition of the field grades. I t is very doubtful if mechanical analysisby the International method will permit an interpretation of texturalgrades on the basis of a triangular diagram or even in a three-dimensiollal figure.

The drift of the analyses, as shown in the diagrams in Fig. 11would also indicate the grave difficulty in endeavouring to adjustmechanical analysis by the International method with the standardsestablished by the ammonia-centrifuge method. Prescott (4) andTommerup (6) have each endeavored to develop such adjustments.Apparently they have assumed that the yield of clay under the methodsin use by the Bureau of Soils prior to 1925 was comparable to theyield of clay by the International method and that therefore a comparison could be made between soils analyzed by the different methods.The marked difference in dispersion due to the use of ammonia in onemethod and the usc of a sodium compound following severe pretreat-ment in the other makes such conversions of results thoroughly unsatisfactory. I f a standardization of field texture is to be accomplished bymeans of the International method, it can only be done by accumulatingseveral thousand analyses of soils whose textural grade has been d e t e r ~mined in tl1e field by men worlting in close co-operation and who havemaintained consistent standards for their grade separations. In viewof the tendency to drift toward.s lighter or heavier designations, asshown by the two men working in California, both of whom had yearsof experience in the field, consistence in textural designations can onlybe maintained by frequently checking textural judgment by comparingfield samplc>fi with soils that have been selected as textural standardsor by means of mechanical analyses that follow the meth9d of dispersion by usc of ammonia without pre1reatment, by which the firsttextural standardizations were established.

LITERATURE1 Dt\\- IS, R. 0 E., and BENNETT, H. H., 1927. U.S. Dept Agr Circ. 419.

~ D R I G G S L J., Mt\RTJS, F. 0. nnd PEARCE, j R., 1904. U.S. Depl. Agr.Bur Soils Rul. 24.Ft.ErCHER, C. C., nnd BRYAN, H., 1912. U..S. Dept. Agr. But Soih ,Bul. 84. PRF.SCOTT, J A., TA\ LOR, J. K., and MARSHALL, T. ]. 1934 P l t y . ~ i q u e rluSol, 1 rn11s. First Comm. Int. Soc Soil Sci. pp. 14J-15S.

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OLMSTEAD, L. B., ALEJW\DER, L. T., and MIDDLETON, H. E., 1930. U.S.Dept. Agr., Tech. Bul 170. .1 TOMMiiRUP, E. C., 1934 Pltylique du Sol, Tr1m.r. First Comm., Int. Soc.Soil Sci. pp. t5o-158.J WHI rNEV, MILTON, tgo6, The Soil Survey Field Book. U.S. Dept.

Agr., Bur. Soils. WHJTNF.Y MILTON, tgn. U.S Dept. Agr. Buro Soils, Bul. 78.

FLOTATION APPLIED TO THE STUDY OF SOILCOLLOIDS

N.K. KRUPSKYa.boratory _of Soil Chemistry Chief A. N. Sokolovsky; Khorkov)

..... - - - - . ; o o l i : : _ 1 ' \ ~ :Purpose of this investigation was to thid the metliods- .dif

ferential study of soil colloids and to e s t a b l i s ~ the contrOl:iof the processes of interaction of soil colloids. It has been fOurld :that:l The behaviour of different fractions of soil 'colloids on thedividing surface separating two liquids, of Which one is more polarthan the other, is not the same. Under corresponding conditions

organic colloids are easily wetted by less polar liquids, -wheri leaving ahydrous medium. Mineral colloids are more easily wetted by morepolar liquids (in this case with water) and always r e m < i ~ n in hydrousphase.

2. At or near the neutral reaction the particles of humus soil arenot wetted by faintly polar liquids: During the process. of acidification, humus rapidly becomes hydrophobic, and below the limit of coagulation, the humus entirely passes into non-hydrous phase. During theprocess of acidification the double electrical layer is progressivelydestroyed, which facilitates the wetting of the surface of humus particleby faintly polar liquids. Wetting with non-polar ljquids and the removal of humus from the hydrous medium may also be brought aboutin neutrnl or alkaline conditions. To do this a third liquid u ~ t beadded to a system of hydrous and non-hydrous phases i this third liquidmixing in any proportions with either liquid composing the system(such as ethyl alcohol, acetone, etc.). By replacing water in hydratedfilm of micella, this liquid facilitates the wetting of the surface of ahumus particle with faintly polar liquid. Highly alkaline humus alsopartly passes into the non-hydrous phase, even without applying anauxiliary dehydrator. Hence it is obvious that humus is a typicalhydrophobic colloid. Most of the surface of the humus particle mustbe of carbon-like properties.

3. The passing of humus into the non-hydrous phase owing to theprocess of acidification can be brought about gradually. At the beginning, at lower concentration of H ions, particles richer in carbon andfree of mineral admixture are isolated, they are followed by the fractionricher in nitrogen and, as a rule, with considerable mineral admixture.No connexion bet\\-'een the sequence of removal and the grade of dispcrsitY of garticles was established.

4 . The addition of neutral electrolytes does not effect completeseparation of humus sol into the non-hydrous phase. As the c o a g u l a ~tion by neutral electrolytes causes a considerable decrease of hydrated