DITHIONITE. AND OXALATE.EXTRACTABLE FE AND AI ASAIDS IN DIFFERENTIATING VARIOUS CLASSES OF SOILS'

J. A. NlcKrecue eNu J. H. Der:Soil Resedrch lnstitute, Canada Departnzent of Agriculntre, Ottat:sa

Received July 19, 1965

ABSTRACTIron and aluminum u'erc determined in acid anmonium oxalate extracts and indithionite-citrate-bicarbonate extracts of a rvide range of Canadian soils, severaloxide and silicate minerals, and sone amorphous preparations of iron oraluminum and silica. Thc oxalate extraction dissolved much of the iron andaluminum from the amorphous materials but very iittle from crystalline oxides,whereas the dithionite exrraction dissolved a large proportion of the crystallineiron oxides as rvell as much of the amorohous materials. Oxalate-extractableiron and alunrinum gavc a useful indication of Bf horizon developntent in many-soils. even if tl-re oarent naterials rvere hish in iron oxides. In one class ofGleysolic soils, howevcr, the Bfg horizons-were high in dithionite-extractableiron and low'in oxalate-extractibl" irotr. An accimulation of goethite wasfound in the Bfg horizon of some of these soils- In some other Gleysolic soilsiron was deoleted in the A horizon but there was no horizon of iron accumula-tion. Extrattior-r of soils with oxalate as u'ell as with dithionite is useful indiffercntiating certain classes of soils and in identifying horizons of accumula-tion of secondary sesquioxides.

INTRODUCTIONThe National Soil Survey Committee of Canada has defined

^n "f" horizon

tentativelv as, "a horizon enriched'rvith hvdrated iron (Fe). It has a chromaof J or more and is redder than the horizon above or below" (12). A numberof practical problems arise, however, in the identification of such horizons.Some horizons desisnated as Bf on the bases of chroma and position containless free iron (dithlonite) than the C horizon. Furthermore, in some soilsformed from reddish parent materials, such as those in the Atlantic Provinces.B horizons enriched in hvdrated iron sometimes are not as red as the C horizon(13, 1+). In many studies of soil classification and genesis it would be usefulto differentiate between the free oxides formed as products of recent weatheringand those inherited from the parent material. Free iron is determined usuallybv a dithionite method: this does not distinsuish between hydrated iron oxideweathering products and crystalline primary iron oxides (5, 6, 11). Schwert-mann (19) retested acid ammonium oxalate as an extractant of iron oxides andfound that in darkness it dissolved only X-amorphous oxides. Years agoLundblad (7) showed that this reagent was uscfui in differentiating certa'1nclasses of soil.

The main obiect of this studv was to test the usefulness of acid ammoniumoxalate-extractable Fe and Al. in comparison with dithionite-extractable Fe andAl, for distinguishing Bf horizons in J wide range of Canadian soils. Publishedresults (7,17,19) indicated that oxalate should extract amorphous products ofrecent weathering and thus be useful in the identification and definition of Bfnorlzons.

MATERIALS AND METHODSSoils and minersls.-Samples of the major horizons of a wide variety of soilswere obtained either from or with the aid of soil survey personnel from NovaScotia, Nerv Brunswick, Quebec, Ontario, Alberta, British Columbia, thelC*.ibu.ion No. 154.

Cm. |. Soil Sci. Vol. 46 (1966)

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l4 CANADIAN JOURNAL OF SOIL SCIENCE

'I-able 1. Classification of soils anal;'zed (12)

lVol. 46

Great Group Subgroup Soil*

Brorvn Forest

Broq.n \\Iooded

Gralr \\i666"6

Gra-v BrownPodzolic

Acid Brou'n\\'oodcd

AcidBrou'n Forest

ConcretionaryBrown

Podzol

Humic Podzol

Humic Gleyso.l

Glel'561

OrthicGleyed

Orthic

Sub-Arctic

Orthic

BisequaG.lcyed

OrthicMinimalBrunisolicGleyed

OrthicSub-ArcticGle-ved

Orthic

OrthicGleyed

Orthic

GleyedMinimalGleyed MinimalMinimal OrtsteinNlinirnal Sub-ArcticOrtsteinConcretionaryTextural

Orthic

Rego

Orthic

Rego

Orthic

Peaty

Glel'solic soils $'ith strong Bfg

Harkarvay, Dunedin, \\''olford, Grenville ( 15 )\{orrisburg, Nlatilda (15)

Chanpagne, Enterprise, Poplar, Sarristo,Providence, Mackenzie, Harris (21)

Rcindcer soil (4)

Breton, Cooking Lake, Braeburn, Nlaywood,Modeste (1); Morrisey', Caen, Cache,Bratnober (21); Kingsclear (13)

Carolinc, Prentice, Lobley (1)Nampa (1)

Oneida, Huron, Schomberg (15)Saugeen (15)Guelph (15)Elderslie, Haldimand (15)

Gatineau (16); Borvser, Dashu-ood, Shawnigan (2)ILeindeer soil (4)Bamel' catena, fhom catena (14)

\\'-endover (8);Caribou catena (13); BelleRiviere (16); Cadboro, Lazo, Merville,Saanichton (2)

Fairbridge, Alberni, Puntledge, Sproat (2)Gil (2)

Holmesville (13); Richibucto;f formentine,Gibraltar (14); Br6beuf, Ste. Agathe (16);Quinsam (2); Uplands (9)

Johnville, Poitras (13); Rubicon (9)Spence R. (21); Falmouth (14)Mira (14)Sa1,-l'ard (2 )Iteindeer soil (4)Kingsport, Ba1'srvater ( 14)NIemel<ay (2)Barney (14); Queens (13)

'-fuodool<,t Thom (14); Arago (16); Richibuctocatena,t Millar (14)

Parkhill, Simcoe (15); Tolmie (2);Mendenhall (21)

Brookston, Chesle-v, Morley, Jeddo, Crombie (15);Cowichan, Irwin (2); Washburn (13)

Lyons (15);Swede, Dory, Mills Lake,Redknife (21)

Tuodook catena,i Kingsclear catena (13);Masstown, Queens catena (14)

Laplaine (8); Osnabruck (15)

Kings, Nackawick (13); Kingsville, Joggins (14);Lincoln (15); Bearbrool< (8); St. Samuel (9)

+The series name is given when possible,fThese soils have not been described in a published report,

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February 1965] McKEAGUE AND DAy-DIFFERENTIATI\c cLAssES oF soILS 15

Yukon, and Northwest Territories. The soils studied are listed according toGreat Group and Subgroup, and a publication in rvhich each soil is describid is

indicated when possible (Table 1). Samples of the air-dried, 2-mm samples

were ground to 100 mesh. Ntlineral samples r'r,ere obtained from Wards NaturalScience Establishment or from the Mineralosy Section of the Soil ResearchInstitute. The amorpl-rous silicates

",r.r" pt.pri"d bv mixing solutions of sodium

silicate and chloride salts of iron and aluminum. The resulting precipitateswere washed thoroughly and dried.Analytical.-Organic matter was determined bv rvet oxidation. The pH was

measlred in 0.0i M CaCl". The colors reported are for soils at field moisturecontent. Samples ground to 100 mesh were extracted two times by the di-thionite-citrate-bicarbonate procedure of X{ehra and Jackson (10). An aliquotof the extract was digested rvith HNO, and H,SO. to destroY organic matter,and the digest was analyzed colorimetrically for Fe (o-phenanthroline) and Al(aluminon). The general procedure used for the oxalate extraction was as

follows:

1. Ten milliliters of 0.2 M acidified ammonium oxalate were added to thesample (125 or 250 mg) in a 15-ml tube r,vhich \l/as stoppered tightly.

2. The tubes were placed horizontallr. inside a box and the box was closedin order to maintain the samples in darkness. The tubes were shaken

on a reciprocating shaker and centrifuged.

3. An aliquot of the extract was digested r'vith HNO' and H,SO, and Feand Al were determined colorimetricallv as for the dithionite extracts.

RESULTS AND DISCUSSION

Evaluatiott of the Extraction MethodsThe effects of several factors on the extraction of Fe and Al by acid ammoniumoxalate were tested to serve as a basis for the choice of the details of a procedure.

'fable 2. Effect of pH on the extractabilitl' of Fe.and Al ( 7o) from soils b1' 0.2 l[ acid ammo-

Initial pH

Soil* FeAIFeAIFe Final pH

1

234

o

0.39 0.44t.4+ 1.720.39 0.370.58 2.120.34 0 .725.80 5.12

0.36 0.461.4+ 1.750.34 0.330.58 2.260.16 0.665.68 5.06

0.27 0.371.34 r.780.27 0.250.50 2.o20. 10 0.604.80 3.36

+.24.54.34.44.26.2

+Soil 1.)

6.

Bm horizon of Harkaway loam -

Orthic Brown Forest.Bfh horizon of Tormentine sandy loam

- Orthic Podzol,

Bfg horizon of Kingsvitle loam -

Gleysolic soil with prominent Bfg.Bfh horizon of Gatineau loam

- Orthic Acid Brown Wooded.

C horizon of Gatineau loam -

Orthic Acid Brown Wooded.Bfh horizon of Bowser loamy sand

- Orthic Acid Brown Wooded,

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l6 CANADI.\){ JOLRNAI, OF SOII- SCIENCE lVol. 46

pH.-Samples were extracted with 0.2 M ammonium oxalate solutions ofpH 2, 3, and 4.2 for 4 hours. Approximatelv the same amounts of Fe and Alwere extracted by the solutions of pH 2 and 3, but the solution of pH 4.2 ex-tracted somer,vhat less Fe and Al from most of the samples (Table 2). Thesolutions of pH 2 and 3 maintained their initial pH values within 0.1 pH unitbut the solution of pH 4.2 u'as not bulTered stronglv enough to maintain a

constant pH with soils high in extractable Fe and AI. The final pH of thisextract from soil 6 was 6.2, and the Fe and Al values were much lorver thanthose of the other extracts of this soil. Oxalate solutions of pH 2 or 3 appearedto be suitable, but a pH of 3 u,as chosen as there ',vould presumably be less

breakdown of silicate minerals at the hiqher pH. The similar amounts of Feand Al extracted at these two pH valuesl horvever. suggest that complexing ofFe and AI rvith oxalate is involved rather than acid dissolution.

Time of extraction.-A comparison was made of the extraction of Fe andAI from six samples by four successive 1-hour extractions and by single extrac-tions of 2, 4, and 8 hours. Prolonging the extraction time or increasing thenumber of extractions with oxalate vielded greater amounts of Fe and Al(Table 3). While a single treatment of t houi removed much of the oxalate-extractable Fe and Al fiom most samples. a few released significant amountsbevond I hour (for cxample. soil 6 (T;ble l)). Therefore a single extractiontime of 4 hours was selected somer,vhat arbitrarilv for further work.

Contparisott of oxilate ud dithiottite procedLffes.-The oxalate procedureextracted iittle Fe from 1O0-mesh soethite or hematite u'hile the dithionite pro-cedure extrlcted much of rhe iion fronr these minerals lTable 4). Finergrinding resulted in a greater extraction of iron by both methods but twodithionite treatments did not dissolve the minerals comoletelv. Gorbunov etat. (6\ also found that iron oxide minerals were not diisolved completely byNlehra and Jackson's (10) treatment. Results of Mehra and Jactson (to),however. show complete dissolution of hen.ratite and soethite after two or threedithionite treatmenfs. Their sampies nlust have beEn ground extremely fine.

Table 3. Effect of tirne and ":ff3"f".:.,;0"X,ffi:lif,l theextraction of Fe aud Al (%) from

Successivc 1-hr extractions Single extractions (hr)

Soil*

Fe

1

2345o

1

23A

5

0 .27 0. 10 0 .071.31 0.11 0.04o .28 0 .06 0 .070 .49 0 .09 0 .050.12 0.06 0.201.32 1.18 0.63

0.050 .030.040 .030 .080.3s

0 .030.020.020.020 .010 .05

0.421.780.302.160.634.84

0 .451.720.342.120.694.96

0.361 .680.252.O80.53

o.191 .520 .450.660 .466.48

0.531 .840 .342.070.685.O7

0.34 0.39 0.451.40 1.39 1.480.30 0.35 0 .470.54 0.58 0 .700. 15 0. 19 0 .765 .08 5 .92 6.56

0.371 .680.251 .850.604.JJ

0.07 0.060.10 0.040.04 0.030. 14 0.060.04 0.030.32 0. 15

*See Table 2 for identification of soils.

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February 19661 rvrcKnAGUE AND DAv-Dr1-I'ERENTTATTNc cLASSES oF soILS 17

Table 4. Effect of particle-size on dissoh'tion of hematite and goethite by dithionite and b5,oxalate extractl0n

Hematite Goethite

Particle size ExtractaDt Fe extracted (/6)

100 mesh

300 rnesh

DithioniteOxalate

DithioniteOxalate

28.80.08

17 .90.12

39.60.16

28.80.58

Since soil samples for free iron analysis are not commonly ground much finerthan 100 mesh, it is clear that only a pa-rt of the iron oxide minerals occurringas silt or sand-sized particles are dissolved by the usual dithionite treatments.Both the oxalate and the dithionite procedures removed much more Fe and Alfrom the amorphous AI-Si and Fe-Si preparations than from the silicate mineralstested (Table 5). Appreciable amounts of Fe, however, were extracted fromolivine and hornblend-e. l{uch more Al u'as removed bv the oxalate Drocedurethan by the dithionite procedure from the amorphous Ai-Si material.

'

Effects of tredtments on X-rdy diff'raction pttterns.-Both oxalate anddithionite extraction of clavs from the Bfh horizons of Uolands and Rubicon(9) resulted in samples thai gave much more clearly defin6d X-rav diffractionpatterns than the unffeated clays. The X-ray diffraction patterns of somenaturally occurring Al-chloritized clays were affected only slightly by the4-hour oxalate treatment or bv the dithionite treatment. The oxalate treatmentdid, howevet, result in a slight expansion after glycerol treatment of someartificially prepared Al-chloritized bentonite. This indicates some removal ofthe interlayer Al from the artificiallv prepared Al-chloritized clays.

Oxalate and Dithionite Ext'raction of lron and Aluminum from SoilsData for soils representing different subgroups are presented (Table 6) anddiscussed along with unreported data for the other soils listed in Table 1. Inthe followinq discussion. the series names of soils included in Table 6 are initalics.

Table 5. Fe and Al (%) dissolved frour various substauces (100 mesh) by dithir-rnite ald byoxatate

Dithionite Oxalate

Mileral AIFeFe

GibbsiteFeldsparHorublendeMuscoviteOlivineKaoliniteMonttrorilloniteIllite

Amorphous Al-silicatc*Amorphous Fe-silicatet

0.1

o.2

;tr.0.10.1

usI ^,)tr

0.20.2

tr.0.3tr.0-7tr.0.'10.3

wa

tr.tr.0.10.2tr.tr.0.10.1

.r.8r0.6

7.0

*Total values: Al 7.27a, Si 24.8Va.fTotal values: Fe 74.7 7o, Si 24.OVa.

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18 CANADIAN JOURNAI, OF SOIL SCIENCE lVol. 46

Table 6. Color, organic matter, pH, and extractable Fe and Al (c/6) for selected soils

AIFeFeSoil Hor Color

pHO.M. in(%) CaCl:

Dithionite Oxalate ActiveFe

ratio*

Flarkaway (15), AhOrthic Brown BmlForest Blr.2

CK

Huron (15),Orthic GrayBrown Podzolic

Kingsclear (13),O.tL i. (lra rr

Wooded

Caroline ( 1),Bisequa GrayWooded

Gatineau (16), AhOrthic Acid BfhlBrown Wooded Bfh2

C

14.99 6.512.83 6.651.61 7 .100. 19

7 .91 6.881.34 6.881.00 7 .r30 .36

1 .02 3 .99| .02 4. 15o.+7 +.070.19 6.29

7.svp. 6/2 1.52 5. 137.5YR 5/6 | .66 5. 1610YR 6 /2 0. 48 4 .8710YR 4/4 0 .48 5 .262.5Y 5/6 0.48

7.'YR3/2 12.40 4.83sYR 4/8 5.37 4 .737.5YR 4/4 3 .53 4.83loYR 5/4 o .62 5. 11

AhAeBtCK

AeABBt1C

AeBfAEBtC

10YR 2/11OYR 4/31OYR 3/310vPt 7 /+

10vR 3/2I <V q/'10YR 3/31OYR 3/3

sYR 5/32.sYR 1/410R +/42.sYR 4/s

1OYR 2/1loYR 5/310YR 5/.110YR 5/1

sYR 6/2sYR 5/6? qv (/c2.sY 4/2

sYP. 7 /2sYR 4/4sYR 4/41 qvp 4/L

5YR 5/2sYP. 2/1sYR 3/2(v (./t

7.sYI?. 6/2sYR 4/67.sYR s/45YR 4/310R 4/3

loYP. 7 /27.sYR s/42.svP. 4/42.sYR 4/4

2.sY 6/27.sYR 5/62.5YR 4/3sYR 5/4

1.28 0.341.51 0.34r.1+ 0.26o.64 0.11

0.6.5 0. 160.87 0 .210.92 0.230.65 0.16

0.38 0.34 0.300.39 0.36 0.240.32 0.29 0.280.14 0.07 0.22

0.32 0.20 0.490.q 0.18 0.470.28 0.25 0.300.r7 0.1+ 0.26

o.23 0.12 0.270.46 0.20 0.240.15 0.18 0.080.09 0.10 0.04

0.18 0.14 0.540.58 0.86 0.690 .23 0. 10 0.380.34 0.19 0.31o.27 0.17 0.27

0.72 0.76 0.580.58 2.O8 0.62o .22 1 .05 0 .1+0. 19 0.53 0.50

0.861 .88r.922.32

0 .330.840.607.711 .00

1.240.930.500 .38

0.120.200. 180.09

0. 140.490. 110.200. 18

0.95r.441 .040.43

Cadboro (2),Orthic AcidBrown Forest

AhBfhBCC

Holmesville (13), AeOrthic Podzoi Bfhl

Bfh2(-

Falmouth (14), AeMinimal Podzol Bfh

BCC

Richibucto catena, AeHumus Podzol Bhcl

Bhgcg1

Queens (13),TexturalPodzol

Masstown (14),Orthic Gleysol

AeBftBCgCBC

0 .79 0 .66 0 .9+ 0. s31 .50 0.95 3 .26 0 .720.44 0.49 0.68 0.+40.23 0.36 0.31 0.33

18.92 5 .418.74 1.922.28 4.661.2+ +.52

1.88 3.33 0.187 .26 1.29 3 .262.13 4.90 1.220 .22 5. 11 0.56

2 .07 4.50 0 .+71 .96 4.81 1 .500 .29 4.85 1 .20a.29 5.01 1.50

1.75 3.74 0.049.62 3.76 0.142.80 4.17 0. 160.59 4.63 0 .27

nt

.JI

.11

.18

3.86 0.664.15 1.184.08 0.934.06 1 . 164.64 1.16

0.18 0.01a .79 0.04o .46 0.050.26 0.07

0.16 0.310.+2 0.600.19 0.34o.17 0.370. 10 0 .21

0.14 0.250.66 0 .280.42 0.31o.28 0.26

0.16 0.470.43 0.510.18 0.370.18 0.320.09 0.18

0.16 0.05 0.15 0.281.40 2.60 1.38 0.800.79 0.56 1.01 0.460.38 0.18 0.18 0.32

0.13 0.14 0.09 0.300.45 0.52 0.s7 0.350.20 0.38 0.16 0.320.19 0.23 0.12 0.15

0.661.200.320 .360. 14

0. 690.410. 1.1

0.14

Kingsville (14),Gleysolic soilwith Bfg

4.56 0.055.53 0.676.60 1.437 .07 1 .20

AeABgBCC

l-\egrBfgBC

0.55 4.09 0.061.04 4.19 5.560 .46 4.2r 3 .30a.46 1.47 2.56

0.12 0.00 0.12 0.000 .17 0. 13 0.12 0. 190.13 0.11 0.08 0.080.08 0.06 0.06 0.05

0.09 0.00 0.05 0.000.6J 0.35 0.25 0.060.37 0.33 0.20 0.100.30 0.29 0.22 0.11

+Ratio of oxalate- to dithionite-extractable Fe (19)

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February 19661 TMCKEAGUE AND DAy-DTFFERENTTATING cLASSES oF solr-s 19

The Brown Forest soils and Brown Wooded soils with the exception ofHarris, which showed greater differentiation of B and C horizons, had extract-able Fe and Al profiles similar to that of Harkausay. Both oxalate and dithionitegenerally extrJcted somewhat more Fe and Al from horizons of the sola thanfrom the C. Ratios of oxalate- to dithionite-extractable Fe generally decreasedwith depth and were usuallv below 0.3 3 in all horizons. ThJdata indicate someaccumuiation of Fe and Ai weathering products in the sola of these soils butlittle or no translocation of Fe or Al.

Most of the Gray Brown Podzolic and Gray \,Vooded soils resembledHuron in that they had weakly differentiated Fe and Al profiles and usually a

marked decrease with deoth in the ratio of oxalate- to dithionite-extractableFe. The Bt horizons of these soils were usually highest in dithionite-extractableFe. In the Orthic Gray Wooded soils. the Ae horizon was consistently thelowest in extractable Fe'and Al. Kingsclear, a reddish soil. differed from theother Grav Wooded soils in that the dithionite-extractable Fe increased withdepth whereas oxalate-extractable Fe and extractable Al were greatest in theAB horizon. The data for Caroline are reDresentative of the"Bisecua GravWooded soilsl thev had Bf horizons with distinct maxima of oxalate-eitractabieFe and Al.

Many of the Acid Brown Wooded, Acid Brown Forest, and ConcretionaryBrown soils had more strongly expressed extractable Fe profiles than Gatineauor Cadboro. The ratio of oxalate- to dithionite-extractable Fe for the upperhorizons commonly was greater than 0.5. The Acid Brorvn Wooded memberof the Caribou ."tena, iVendover, Dashrvood, and Fairbridge, however, hadrveakly expressed extractable Fe and Al profiles and they resembled in thisrespect some of the Brown Wooded and Brown Forest soils.

The data for Holrnesaille are reasonably representative of extractable Feand Al in many of the Podzols. and of the Orthic Humic Podzols studied. TheB horizons were markedly higher in extractable Fe and Al than the A or Chorizons. Oxalate-extraciable"Al frequentlv exceeded dithionite-extractable Aland the ratio of oxalate to dithionite-extractable Fe commonly was high for Bhorizons. Since oxalate extracts Fe and Al organic complexes from sloil (tS;,it is concluded that some of the Fe and Al extracted bv oxilate from B horizonsof Podzols eristed as metal-organic complexes. The soils classified as variouskinds of Minimal Podzols, llke Falwoutli, had relatively weakly expressed ex-tractable Fe and Al profiles except for the Reindeer soil which was viry high inextractable Fe (4), and Memekay, a Concretionary Podzol with a'thii Aehorizon, which had a well-expreised Fe and AI piofile characteristic of theOrthic Podzols. Pojtras, a Cleyed Podzol. had a^weakly expressed Fe and astrongly expressed Al profile. Both Humus Podzols, lik'e the member of theRichibucto cdtend, had very low amounts of extractable Fe but moderate ac-cumulations of extractable-AI in their B horizons. Dithionite-extractable Fewas about the same in the B and C horizons of Queens, a soil formed fromreddish material, but there were maxima of oxalate-extractable Fe and of ex-tractable AI in the B horizon. The other Textural Podzol studied. Barnev. hada strongly expressed extractable Fe and Al profile.

Mrly oj the Humic Clevsols and CIevsgls had weakly expressed profiles ofextractable Fe and Al; they were similar to Harka,tuay or Huion in tliis respect.

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20 CANADIAN JOURNAL OF SOIL SCIENCD [VoI. 46

The Ae horizon of Masstowl, howevet, was depleted of extractable Fe and ithad no horizon of marked Fe accumulation. Tne z\ horizons of Mills Lakeand of a Gleysol of the Queens catena were also low in dithionite-extractableFe relative to the lower horizons. The Ah horizons of some of the HumicGleysols, Chesley, Washburn, Irwin, were highest in oxalate-exffactable Fe

and Al.Kingsville was the most stril<ing example of the group of unclassified

Gleysoil soils with strongly develolped Bfg horizons. *

Th^ese soils all hadstro'ngly expressed dithionii6lextractadle Fe piofiles, but much less pronouncedoxalaie-extriactable Fe and AI profiles. The X-ray diffraction pattern of an

untreate,l sample of rhc Bfg hoiizon of Kingsville showed a moderatelv stronggoethite peak",vhich disappeared 'ivhen the-sample was heated to 300oC. Asihe C hoiizon material of-this soil gave only a weak goethite pattern, it appears

that goethite was formed in the Efg horizon. The B horizons of two othersoils 5f this class, Bearbrook and Kinfs, also contained goethite. k is postulatedthat Fe is accumulated in some Glevsolic soils as a result of oxidation of Fe.' andthat the resulting hydrated ferric oxide crystallizes much more rapidly than theorganic-complexed Fe in Podzol B horizons.

CONCLUSIONSBoth the oxalate-extractable Fe and Al values and the dithionite-extractable Fevalues aid in the distinction of Podzols from Glevsolic soils with pronouncedhorizons of accumulation of dithionite-exrract;ble Fe ( 8, 9).

- Podzol B

horizons senerallv have hieh ratios of oxalate- to dithionite-extractable Fewhereas tf,'ose for'Gleysolic"soils are lolv. Furthermore, all Podzol B horizonsstudied had distinct aicumulations of oxalrte-extractable Al; this was not tlueof any of the Gleysolic soils. Gleysolic soils with Bfg horizons can probablybe distinguished from Gleyed Podzols by their morphology. The B horizonsof Podzols commonly increase in color value with depth; this was not the case

r,vith the Glevsolic soils in question. Furthermore. the Bfs' horizons of these

Gleysolic soiis were mottled^stronglv. It appears that a neri' class of Gleysolicsoili is necessarv as such soils with prominent Bfg horizons do not fit thedefinitions of any subgroup of Gleysolic soils (12). It seems undesirable toinclude soils having sirongly developed horizonation with Orthic Gleysols;thus a new Great Group may be requlred. It could be defined on the basis ofthe properties of the Bfg horizon.

The oxalate- and dithionite-extractable Fe and Al values for the soils listedin Table 1 were used in developing a tentative definition of a Bf horizon. Themain guiding principle in the choice of limits was that the more strongly de-veloped soils such as Podzols, Acid Brown Wooded, and Acid Brown Forestshould have Bf horizons, and that most of the weakly weathered soils such as

Brown Wooded and Brown Forest should not have Bf horizons. It was evidentthat the definition should be based mainlv on the oxalate values as dithionitevalues frequently indicated no Bf horizon development in soils formed fromreddish materials. It is proposed tentatively that horizons in which the per-centage of oxalate-extractable Fe plus AI exceeds that of the C horizons by0.8 % or more, or horizons in r.vhich dithionite-extractable Fe exceeds that in theC horizon by l% or mo.re, be designated as Bf horizons. According to thisdefinition, most of the soils classified as Acid Brown Wooded, Acid Brown

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Forest, Concretionary Brown, and Podzol, and many of those classified asBisequa Grav Wooded, would have Bf horizons on the basis of oxalate-extract-able Fe and Al (Table 7). The B horizons of most Brown Wooded, BrownForest, Gray Wooded, Grav Brown Podzolic, and Gleysolic soils were too lowin oxalate-extractable Fe and Al to meet this requiremenr for a Bf horizon.Oxalate-extractable Fe and Al is not a suitable critirion of Bf horizon develop-ment for the Gleysolic soils having horizons of accumulation of secondarv.crystalline, iron oiides. For such sloils, e.g. Kingsville, the suggesred criteri6nof a Bf horizon is based upon dithionite-extractable Fe.

Table 7. Average differences of oxalate-extractable Fe and Al betu,'eerr the upper B a1d Chorizons of various classes of soil, and relative number of soils u.ith Bf horizons based on

oxalate values

No. ofFefAl (B-C) soils

(%) arrall'zedClass of soilFe (B-C)

(%)Al (B-C)

(%)

No. ofsoils

rvith Bf$

Brown ForestBrown WoodedGray Wooded*Bisequa Gray \\'oodedGray Brown Podzolic-{.cid Brown \\roodedAcid Brown ForestConcretionary Brou-uPodzoltHumus PodzolGleysolic soils{

0.350.230.120.380.200.930.470.581 .130 .000.21

0.230.100.100 .460. 150.80l-2.11.271 .130.210.12

0.580 .330.220.840.35l. /.)r.701 .852.260.2+0.36

68

1137875

252

30

1

1

020665

2201

xExcluding Bisequa Gray Wooded.tlncluding Orthic llumic Podzols.lSix of these soils had Bfg horizons based upon dithionite Fe values.

$L-elAl (B-C) is 0.8/e or more.

Chromas of 3 or more \r-ould be indicative of Bf horizons in manv soils.but such horizons are not necessarily redder than the horizon below 63, l+).The inclusion of extractable Al as vell as Fe in the definition of a Bf horizonseems reasonable as the weathering of manv primary minerals in soils must yieldAI as well as Fe. The "f" \\'ould thus indicate a ho-rizon enriched with freeFe and Al.

Both oxalate- and dithionite-extractable Fe and AI values are useful instudies of soil genesis and classification. The oxalate values give an approxima-tion of the degree- of .accumulati-on of amorphous products of recent weatheringin the horizons of soils formed from materials varyinq widelv in texture. coloi.pH, organic matter, and total iron oxides. The'ditfrionite Fe values approxi-mate the combined content of amorphous forms of Fe and of crystallin6 ironoxides. The oxalate values are superior to the dithionite vaiues'in providingan indication of the degree of Bf horizon development especially in soils formedfrom reddish materials. Furthermore, oxalate is the more efFective exrracranrof amorphous forms of AI (Table 5). The oxalate-extractable material has amajor influence on some of the properties of soils (11, 20). High amounts ofoxalate-extractable Fe and AI are associated with horizons having high pH-dependent charge (3) and high P-firing capacity (17).

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22 [\rol. 16CA\ADIA\ JOLR\,{I- OF SOIL SCIE\CE

ACKNOWLEDGMENTS

The authors gratefully acknorvledge rhe cooperation_ gf T:T9..r of the soil

survey units #ho ptonid"d samples." Thev also thank N. N{. tr{iles, who did the

X-raf analysis, rt-tb R. Guertin,^u'ho assisted with the chemical analyses.

REFERENCES

1. Ar"senre Sorr- SunvBv. Repts. Nos. 19, 20, and 21.

2. Bnrrrsn Cor-ur'rera Sorr- Sunvev. Rept' No. 6'

3. cr-anx, J. S. and Hrlr, R. G. 1964. T'he pH-per cent base saturation relationships ofsoils. Soil Sci. Soc. Am. Proc. 28, 490492.

.1. D,lr, J. H. ancl Rrce, H. trI. 1961. charactcristics of sonrc perntafrost soils in the

r\'{ackcnzic \ralley, N.W.T. Arctic, 17, 223-236'

5. Des. B. C. 1949. The estimation of frec iron oxides in soils and cla-vs and theirremovai. J. Soil Sci. 1,212 220.

6. GoncuNov. N. I.. DzvenEvrclr, G. S., antl TUsrx, B. Il. 1461. \lethods of determining" r".-rif i."i. -"-oipnor.-r'nJ

crl.staltin" scsquioxides in soils arrd clays. Soi ict Soil Sci.

No. 11. pp. 1252-1259.

7. Luxonrer, K. 1934. Studies on Podzols and Brorvn Forest soils. I' SoiI Sci. 37'137-15 5 .

8. McKracuq J. A. 1965. Relationship of $ater table and Eh to properties of thrce cla.v

soils in the Ottav'a Valley' Can. J. SoiI Sci.45' 49-62.

9. N,IcKoecut, J. A. 1965. Properties and genesis of three membcrs of the Uplands catena.

Can. J. Soil Sci.45,63-77.10. ,\{aune, o. P. and Jecr<sox,,\1. L. 1?g0. Ilon oxide rentoval frcrm soils and-clays^by a

dithionite-citr"." ,i'r*nr buffercd s-ith sodiunr bicarbonate. 7tl-r Natl. Conf. on Clal-s

and Clay r\Iincrals. pp. 317-327.11. Mrrculr-1., B. D., FrnMen, V.C., and l{cHennr, w.J. 196'1. Anrorpl-rous inorganic

nraterials in soils. Adr'. Agron. 16,327-383.12. Nerroslr. Sorr- SunvEr: Cot.rr'rt'rrp.e (Cex-.rol). 1963. Rept. 5th NatI. i\'leet., Winnipeg.

11. NBw BnuNss.rcr< Sotr. Sunvrv. Rcpts. Nos. 1, 5, and 6.

14. Nova Scotre Sorr- SunvEv. Repts' Nos. 2, 4, end 12.

15. ONrenro SoIr, Sunvori" Repts. Nos. l+, 17,34, and 38.

16. Qusuoc Sotr Sunvoy. Reports-Gatineau and Pontiac Couuties; Argenteuil, Ts'oMountains and Terrebonne Counties.

17. SeuNorns, W. l'{. H. 1965. Phosphate rerention by.Nerv 7*aland soils and its relation-- ;hlt.;i;" r".q"io"ia"., organic mattcr and other soil propcrries. Nes' Zealand J. Ast'Res. B, 30'57.

I8. ScHrrrzen, M. and Srrxxen. S. I. ll. .196,+. Organo-metallic intcrac'tions in soils:3.-- Fioo.iii.i'of iron- and aluminunr-organic mattcr c-omplexes. preparcd in the laboratori-and-extracted from a soil. Soil Sci. 98, 197-203.

19. Scnrvnnruess, W. 1964. The differentiatior-r of iron oxide in-soils b{ a.ph-otochemi^cal

extraction rvith acicl ammoniunr oxalate. Z. Pflanzenernahr. Dung. Bodcnkunde, 105'194-201.

20. SHonuax, G. D., Marsusare, Y., ft<eu'e, FI., and lJeirena, G.. .196+. -The rolc of theamorphous fraction in the properties of tropical soils. Agrochim. 7' 146-163'

21. YuroN eNn Non11rwrsr TBnnrronrBs Sorr. SunvEv. Reports-Takhini and Dezadeash'-' v;ii"yr'--ir1i: ^;;t-O"uo*r."G

High*"v and Upper^ tr'Iackenzie Ri'er Vallev' Inpreparatlon.

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