tj marshall, mechanical composition of soil in relation to field descriptions of texture

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Council for' Scientific and Industrial Research

BULLETIN No. 224

Mechanical Composition of Soilm Relation to Field Descriptionsof Texture

ByT. J. MARSHALL. M.AGR.Sc . PH,D,

MELBOURNE. 1947

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CON rENTSSummary

1. INTRODUCTION

2. RgtdmANGEMENT or e.S,LR. DIAGRAM (INTERNATIONAL FRACTIONS)

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3. COMI'AltTSON or Ngw TnIANGULI',[t D't.GRAM CINTICRNATtON,\[.FItACI'TONS) WITi[ S'J'ANDAIIOS D ~ V E L o r E D IN TilE UNITED STATES 13 '0Ij, A TEXTURE Dtt.GlIAM n"SF;D ON CLI\Y CONTENT AND AVE,",AG*'.'Z,.E,m' N O N ~ C L A Y FltAC'l'ION - "n. M " ' N T I ~ N " ' N C F ! 01- TP.XTURE STANDARDS

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SUMMARYA triangular texture diagram has been drawn for t!:J Internationalfraction sizes (wilh limiting diamelers of 21" 2:1", and 2 mm.respectively for the clay, silt, and sand fracll ons). This is based onscatter diagrams showing the relation between mC!chanical compositionand field descriptions oI texture made by the DivIsion of Soils between1939 and 1945. No serIous departures have been made from standardsalready established in 1934 on the International basis.It has been found from field descriptions that the relative amountsof sand and silt affect the apparent II claylness II of a sample. Evidenceis presented to show that limiting lines for the clay content of thetexture classes should slope upwards towards the zero sand line ina triangular diagram. A theoretical basis Is discussed for this departurefrom the practice followed in existing diagrams.Rearrangement 011 the basis of sloping lines has brought out amore logical relationship between classes than was apparent in the1934 diagram.The new triangular diagram Is compared with texture standardsof the United States Department of Agriculture based on limitingdiameters of 2 and 50p. respectively for the clay Bnd silt fractions.Serious differences in the limiting percentages of clay for sandy loalns,sanely clay loams, and sanely clays are noted and are discussed in thelight of the sloping lines.An approximate adaptation is maele of the neW diagram (Inter-national fractions) to the fraction sizes of the United States Department of Agriculture. 'A new type of texture diagram Is presented In which the median-size of the non-clay fraction is plotted against the clay cnntent. Thediagram is subdivided horizontally into categories of II fineness II of thenon-clay fraction and vertically into categories of II claylness." Thediagram is unaffected by the size chosen for dividing the silt andsand fractions as is the case in the triangular representation ofmechanical composition. Further it allows differences due to therelative amounts of coarse and fine sand to be brought out. Thesetwo fractions have to be combined as total sand in the usual triangulardiagram. Possible applications of this type of dlc.gram are discussed.Symbols combining the II fineness II and II clayiness " categories areused to name the classes. These symbols avoid the confusion bet weenfraction and texture names and the prejudice involved in the use ofsuch terms as II 10am.11 Conventional texture names are also appliedto the diagram.Descriptive standards have in general been maintained since theoriginal examination was made In 1934. The chief departures In fieldpractice are discussed. Methods are suggested for checking personalfactors and for allowing for the effect of physical properties otherthan mechanical composition.The upper limiting size of the silt fraction has been reviewed andit was concluded that l In Australian field practice", particles between20 and 50p. in diameter are considered to be sand rather than silt.

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Mechanical Composition of Soil in Relation to FieldDescriptions of Texture*

1. Introduction.Texture descriptions made in the field by soil surveyors are usuallyrelated primarily to the mechani cal composition of the soil. Otherproperties such as the nature of the clay minerals and of the exchangeable cations l and the presence of large amounts of organic matter, mayin some soils mask this relation . However, it is possible to express fieldtexture descriptions in terms of mechanical composition with SUfficientreliability to set standards against which trends with time anddifferences between individual workers may be chEcked. Since textureterms are the most important of the soil descriptions made In thefield! It is important that they should be based on a quantitlve standard.

It is further desirable that there should be international agreementon some standard.The first diagrammntic relation between field descriptions of textureand mechanical compOSition of soil was drawn up in 1911 by Whitney(11) who used a right-angled triangle on which the percentages ofclay and silt were represented. In 1927 an eqUilateral triang le wasused by Davis and Bennett (3) on which the percentages of clay,silt and sand were plotted. This triangle has been widely used bysoil worIters in agriculture and engineering throughout the UnitedStates. The areas In both of these diagrams for each texture classwere based on limiting diameters of 5/-L' 50p.1 and 2 mm. for the clay,silt, and sand fractions and were not applicable to results of analysesexpressed on the International fraction basis with limiting diametersof 2fL, 20,lLI and 2mm. respectively. Furthermore the analyses on whichthese diagrams were based were carried out by a dispersion procedurewhich was less complete than that followed in the International method.Because no suitable diagram was available in countries whichadopted the International procedure and fraction sizes, one was drawnup by Prescott, Taylor, and Marshall (8) In 1934 based on field descriptions and mechanical analyses made ,lor soil surveys undertaken bythe Division of Soils. T he present position regardin g textur e standardsin Australia was reviewed in 1945 by a representative group of Australian soil workers before whom a number of the scatter diagramsshown in the present paper were placed.The Vnited States Department of Agriculture has since 1930 adopteda procedure of mechanical analysis (6) which is not greatly differentfrom the International procedure j and in 1938 an upper limit 01 2p.was also adopted Ior the clay Iraction (5). The division between thesilt and sand fractions was allowed to remain at 50ft although measurements were also to be made corresponding to 20 ,1 to enable theconversion of data to the Internat ional basis if desired. These changeshave made necessary a revision of standards in the Untted Statesand, in 1945 1 a triangular diagram based on the new analytical procedure and new particle sizes was drawn up tentativel y. This diagram

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B(Fig. 4a) Is more easily compared with one based on the Internationalprocedure and fraction sizes than was its predecessor, since procedured l f f e r ~ n c e s w 1 ) ~ not now matter greatly on the . average: soil and, theupper Hmlt of the clay fraction is now cominon to both sets of data.A copy of this diagram was made available to the writer throughthe courtesy of officers of the United States Department of Agrlculture* and in return notes were sent dealing with Australian standardsand their probable relation to the proposals of the United Statescommittee dealing with this subject. It was found that althoughrnechEmical composition limits Were essentially similar for most textureclasses, the limiting clay contents for sandy loam, sandy clay loam,and sandy clay were seriously different. Since it is highly desirablethat there should be international agreement on the meaning of te.xturedescriptions, and since the opportunity for the comparison of Australian with United' States data is now 50 much greater, the causesof this disagreement have been investigated in some detail. In thecourse of this inquiry a logical rearrange.ment of the original C.S.I.R.diagram has been worked out and an alternative method of expressingtextural relations has also been developed.2. Rc-armng -cment of C.S.I.R. Diagram (Internl1tionnl F r l l c t l o n ~ ) .

An examination has been made of an samples fo r which fieldtexture descriptions and mechanical analyses have been undertakenbetween 1939 and 1945 In the course of the soil survey activities ofthe Division of Soils. The samples have been grouped according totexture classes and plotted according to mechanical composition (Fig.1).** All the mechanical analyses were undertaken using the pipettetechnique described by Piper (7) involving pretreatment of the samplewith hydrogen peroxide and hydrochloric 'acid and dispersion withsodium hydroxlde.t No A- or B-horizon samples were excluded fromthe scatter diagrams except those containing much calcium c a r b o n a t e : ~ and those described In the field as II organic." The number of samplesand the mean 'clay content for each texture class are given in Table 1.It" was noted from these diagrams that the scatter of points fora given texture class tended to slope upward from left to right. Thistrend is demonstrated in Fig. 2 where lines Bre drawn representing

the approximate slope of some of the classes. Re-examination of thedata on which the 1934 C.S.I.R. diagram was based shows that thetrend is present there also. I t is less marked however and passed unnoticed because of the poor representation of soils wIth moderate orhigh slit content at that time. The writer Is Indebted to Professor G. B. Bodman of the University ofCalifornia for drawing his attention to the proposals, and to Mr. J. K.Ableltel' nncl Dr. L. T. Alexander of the United Stntes Department of Agrl-,culture for details regarding the tllagrnm.I I : ~ The writer wishes to thnn\{ Mr. P. D. Hooper and Mr. A. V. Blackmore[or drnwlng the' dlagrnms. Mr. Blackmore also assisted In the tabulationof data.t Ammonia was used as' the dispersing agent prior to 19,14. Used Inconjunction with the present sevel'e pretreatment and mechanical dispersion,ammonia gave clay percentages which were low only on unusual salls.Salls having more than 3 per cent. II loss on acid trentment" were

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I)-! 9FIG. I.-Scntter diagrams showing, the relation of field descrlptlonsof texture to mechanlcal 'composltion (International fractions).Lines for the texture classes ',\re those gIven In Fig. 3. Thosebounding the loamy sand class do not actually apply to thescntter because thM class was n'ot recognized when the sampleswere described. \

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12The lines of Fig. 2 are in effect lines of equal II c1ayiness. 1J Accordingto the centre line for fine sandy clay loam given in Fig. 2, soil c o n ~ talnlng per cent. of clay with 11 per cent. silt feels to the soilsurveyor to be as clayey as one with 28 per cent. of clay and 40 percent. silt. Similarly a soil wlth 20 per cent. clay is described as asandy clay loam if it contains 5 per cent. silt and as a fine sandy loam

if it contains 25 per cent. sUt. Two factor s probably contribu te to thereduction in the effectiveness of the clay fraction with increasingamounts of sUt.(I ) The same weight percentage of non-clay material will " dilute ..the clay less if it Is present as a few large particles than if it is madeup of finely divided particles.(li) Fo r any given percentage of clay, there wlll usually be morenon-colloidal particles present In the clay fraction when the contentof silt Is high than when it Is low.

TABLE I . -TrIm IvlElAN CLAY PmRCENTAOE 2 /1 ) AND 'fHE NUMBElR OFSAMPLElS CORnElSPONDING '1'0 EACH TRXTUIl8 CLASS.

Descllptlon. Numbel' of Snmples. Menn Cloy Content.%

Sond 15\ 52Sondy lonm, . 71 130Loum GO 201Silty lonm 8 220Sundy cloy loom 34 215Cloy 100m '13 311Silty cloy 100m 12 300Sonely cloy 24 307Silty clay 18 300Clay-"llght" clay 24 100

"medium" clay .. 3D 500"henvy" clay 165 571clny (unqualified) 143 531nil cloys 371 538

I t appears then'that contrary to the practice In all existing diagrams,the lines defining the limiting percentage of clay for the textureclasses shOUld slope upwards towards the sand base line. With thisprinciple as a basis, texture classes have been ie-defined, maintainingat the same time the essentials of the 1934 C.S.LR. diagram. Nogeneral change in standards is involved, but the' diagram is rearrangedon what is now seen to be a more logical basis. Details are givenIn Fig. 3. Two minor changes have been Introduced which affect thesandy soils. Following recent field practice (as indicated In Fig. 1a)the area allotted to the sand class has been reduced. Further, as theresult of suggestions made at the 1945 meeting of Australian workersin soil science, the class II loamy, sand" ,has been included; This classwould Include a number of the samples plotted In the scatter diagramsas sandy loams and sands if such soils were to be described again.Subdivision of the clays Into light. medium. and heavy clays has been

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dropped since it is clear from the scatter d i a g r ~ m s that . his isimpracticable on the basis of mechanical compositlon alone. Burvlll(2) has shown that fluctuations in the water content of the soil cansufficiently influence the estimate of texture to make such a subdivisionimpracticable.

I t can be seen from the scatter diagrams that In general a betterfit is obtained with the sloping lines for defining the limiting percentageof clay than with the old limits shown In Fig. 2 or with any systeminvolving the use of horizontal lines. The logical sequence betweenrelated classes of equal n c1ayiness "- for example sandy clay loam,clay loam. silty clay loam-Is also brought out bett er than in the 1934diagram. A connection between these and between other groups ofclasses has always been a feature of the triangular diagrams developedby the United State s Department, of Agriculture in which upper claylimits of sandy loam, loam, silty loam and of sandy clay loam, clayloam, and silty clay loam respectively are shown as tWCl continuouslines.

FIC. 3.-Trlangulor texture diagram based on Internatlonnlfractions with efCectlve dlnmeters of 0.002, 002, and 2 mm .COL' the upper limits of the clay, silt, onel sand fractionsrespectively.

3. Compnt'lson of New Trinng-uInr Dlngram (Internntiom'Li Fractions)with Stn.nrlll..rds Developetl In the Unitel! states.. In the early years of soil surveys by the Council for Scientificand Industrial Ftesearch, efforts were made to adopt similar standardsih texture descrIptions to those whIch had been In use In the United

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States for many years before the Australian work commenced in 1928.This has been checked through visits to fieid parties in the UnitedStates and by a comparison of texture diagrams. The comparisonmade in 1934 (8) was not completely satisfactory, as noted by Shaw(9). because of the obstacle of dispersion differences. The position isgreatly simplified now by the changed procedure and fraction sizesof the United States Departmen t of Agriculture. The most substan tialdifference between this and the International proced\-lre is that of thelimiting size dividing the silt and sand fractions.'In order to find the relation of the new C.S.I.R. diagram (International basis) shown in Fig. 3 to the diagram adopted by the UnitedStates Department of Agriculture In 1945 (Fig. 4a ), a conversion ofthe latter was attempted on to the International fraction basis. It wasfound from data for representative salls available on the two fractionsizes (8) that the following approximate relation could be used forconverting the percentage of silt in the U.S.D.A. fraction to that corresponding to the International fraction-

~ p : - e r ; : ; c : : : e : : n ; : ; t a " , g ; ; e : - - ; b i : : e : , ; t : ; w ; : : e o : e n ~ 2 f l ' : . . a ~ . n ; ; : d r i 2 i i I ' ' ' - O d ; ; . i a : : : m : : : ; : . e t ; : ; e : : : r 0.5 + 0.13percentage between 2p. and 5UfI- diameter -The rough conversion shown in Fig. 4b has been made using thIsfactor for adjusting the silt percentage and allowin-g a complementarychange in the sand percentage. No change was made In the clay

percentage since the limiting diameter Is the same under both systems.It will be seen from a comparIson of FIgs. 3 and 4b, that for classeswith moderate or high amounts. of sl1t the relation between the twosets of standards is satisfactory. On the other hand there is \ViMdIsagreement on the position of lines representing the limiting claypercentage of classes with low amounts of silt. For example, fromthe scatter in Fig. lc, it can be calculated that 80 per cent. of sol1sdescribed as sandy clay loam by Australian workers con taln less claythan the lower limit of 27 per cent. in the U.S.D.A. diagram.This position arises because of the fact that the two lines definingthe upper and lower clay percentages of the sandy clay loam, clayloam, and silty clay loam classes are placed horizontally (at 40 percent. and 27 per cent. respectively) in the U.S.D.A. diagram whilesloping lines are used in the C.S.I.R. diagram. From the data dIscussedabove It was concluded that the slopIng trend noted within classes

should also exist between such related classes as sandy loam, loam,silty loam. However this inter-class trend may be a peculiarity ofAustralian descriptive methods resulting from the original arrangement of classes shown in Fig. 2. It is equally possible neverthelessthat further investigation of low silt classes now being undertaken inthe United States may reveai a similar trend. I f this proves to bethe case, closer agreement shOUld result and texture standards maybe found to be so little dIfferent as to be easily adjusted to a commonbasIs. Some objections to the high clay limlts for these classes Inthe U .S.D.A. diagram have in lact been raised in certaIn quarters Inthe United States}. The present U.S.D.A. procedllre differs also In the omission of acidtreatment and the use of sodium metnphosphnte as the dispersing agent.t Personal commllnleallon Cram Dr. L. ' T. Ale:


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16An approximate conversion of the new C.S.LR. diagram from theInternational to the United States ir:action basis Was also made usingas a guIde the inverse process to that already. described. In the resultingdiagram (Fig, 5) the slope of the limiting clay lines is considerabiyreduced, . .

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,-JFlO. 5.-The new It'lnngulnr diagram trnnsfened to the fractionsizes of the United Slales Depnrtment of Agriculture witheHective diameters of 0002, 005, and 2 mm. for the Upper ',i,,'limits of the clay, silt, and !mnd respecti vely. _

4. A Texture Din,v;rnm llllsed on Cln.y Content anti Avcrur;e Size ofNon-clay Fraction.

There are two disadvantages to the triangular diagram for r e p r e ~ senting mechanical composition.(1) I t Is not possible to make a direct comparison between texture

based on International and United States size systems because of thedifferent arbitrary bases for defining t ~ e silt and sand fractions.(Ii) Coarse sand and fine sand ar e grouped together and observabledifferences In field texture due to the relative proportions of thesefractions cannot be r e p r e s ~ n t e d . This applies particularly to salls oflow clay content, bu t it is also probable that errOrs in representingthe texture of other soils can be reduced somewhat if the relativeproportion of coarse and fine sand Is taken into conslderatlon (8).These disadvantages can be avoided in a different type of diagramin \vhlch the average size of the non-clay fractlon is plotted againstlhe clay content. Areas representing the texture classes have beenassigned tq . this diagram (Fig, 6) by reference to scatter diagramssimilar to those prepared in drawing up the triangular d i a g r ~ m . '

nr>-- region: V,S. Dept., Agrlc., Blil ' . Soils Bull. 78 (1911l.

tj marshall, mechanical composition of soil in relation to field descriptions of texture

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