Differentiation of Spodosols and Andepts in a Western Washington Soil Climosequence1

W. D. NETTLETON, A. GOLDIN, AND R. ENGEL2

ABSTRACTFive Spodosols were sampled on mountain slopes about 20-km

southwest of Mount Baker in the Cascade Range of western Wash-ington. The soils have formed in colluvium derived from Darringtonphyllite and an admixture of 1 to 30 dag tephra kg"1. Four of thepedons form a climosequence on steep, south-facing slopes. The fifthis at the foot of the mountain, on an east-facing slope, and is moreweathered than one would have predicted from its low elevation. Thethree pedons sampled above 700 m have horizons that meet thechemical criteria for Spodosols. These three, as well as the fourthof the climosequence, have silt-size pellets and cracked coatings. Thefifth, at low elevation, has a horizon that meets both the chemicaland morphological criteria for Spodosols. All of these pedons haveone or more horizons that have the high NaF pH, high 15-bar toclay and CEC to clay ratios, and the smeary feel characteristic ofhorizons that contain amorphous material. However, unlike Andeptsthese pedons have E horizons that contain crystalline silicate clays,mostly smectite, have low NaF pH, and lack the smeary feel. Exceptfor the fourth pedon of the climosequence, the pedons meet the humicacid color criteria recently proposed for Spodosols. Some of the Bhorizons have >2 dag kg ' of (NH4)2C2O4 extractable Al and >1.5

1 U.S. Dep. of Agriculture, Soil Conservation Service. Received10 Oct. 1985.2 Research Soil Scientist, National Soil Survey Laboratory, Mid-west National Technical Center, USDA-SCS, Federal Building, Lin-coln, NE 68508; Soil Survey Party Leader, SCS, Bellingham, WA;and Soil Correlator, SCS, Spokane, WA.

dag kg ' of 4 M KOH extractable Al, but are not thick enough forthe soils to be placed in the proposed Andisols.

Additional Index Words: Andisols, amorphous material, tephra,Pleistocene, Whatcom County, Darrington phyllite, humic acid color.

Nettleton, W.D., A. Goldin, and R. Engel. 1986. Differentiation ofSpodosols and Andepts in a western Washington soil climosequ-ence. Soil Sci. Soc. Am. J. 50:987-992.

THE SPODOSOL ORDER is the first of the mineralsoils in the key to Soil Taxonomy orders (18)

because it was understood when the key was devisedthat many of these soils have B horizons that containallophane and have some properties in common withthe Andepts (16). The decision not to include the Ehorizon (16) as a criterion for identification of Spo-dosols has made their recognition in the field moredifficult in areas where they occur with Andepts. Morerecent studies (2, 4, 5, 6, 7, 15) have described thenature ofSpodosol allophane and leave little doubt ofits similarity to Andept allophane. Five pedons wereanalyzed in this study to correctly classify them andother soils from the same area. Both Spodosols andAndepts were mapped and soils were placed in eithermedial or mixed mineralogy families.

988 SOIL SCI. SOC. AM. J., VOL. 50, 1986

A Mt. Rainier

WASHINGTON

ISO KMFig. 1. Location of the study area.

MATERIALS AND METHODSThe Transect in Phyllite Colluvium

The five soils formed in Darrington phyllite colluviumabout 20-km southwest of Mount Baker in the Cascade Rangeof western Washington (Fig. 1). In the late Pleistocene,Vashon Drift covered the area (3), but during the last 12 000

yr soil creep has modified the drift. Small amounts of sur-ficial volcanic ash have been incorporated into the collu-vium.

The pedons sampled (Table 1) are within 4 km of eachother. They are at elevations of about 250 to 1250 m, wherethe mean annual precipitation ranges from 1600 to 2400 mm(Table 2). Mean annual soil temperatures range from 8 to5°C (mesic to cryic).

The sites are on timbered, south-facing mountain slopesof 35 to 50% except for the Squires pedon, which is on aneast-facing slope of 25%. Douglas-fir (Pseudotsuga menziesiiMirb. Franco), western hemlock (Tsuga heterophylla Raf.Sarg.), and western redcedar (Thuja plicata Donn) are themore common trees. The undergrowth is mostly westernswordfern (Polystichum muniium Kaulf. K. Presl.), red huc-kleberry (Vacciniuni parvifolium J.E. Smith), salal (Gaulth-eria shallon Pursh), and Oregon-grape (Berberis nervosaPursh).

Depth to bedrock ranges from 0.5 to 1 m. The coldestfour soils, Hinker, Crinker, Springsteen, and Rinker are partof a climosequence on Darrington phyllite colluvium. TheDarrington phyllite is a pre-Jurassic rock with abundantquartz and orthoclase and with a thinly layered appearancedue to microscopic crystals of muscovite. It is hght gray toblack, the latter color resulting from graphite derived fromorganic-rich shale. It commonly contains white quartz lenses

Table 1. Morphology of the pedons.

Color, moist

Depth,Soil Survey no. cm Horizon Hue

Value/ Struc- Moistchroma Texturef turet consistence! Boundary! Other components

S81WA-073-016

S81WA-073-013

S81WA-073-012

S81W4-073-11

S81WA-073-017

12-00-99-14

14-2727-3939-5252-72

15-00-88-15

15-2323-4646-6161-65 +

10-00-13

13-3333-6161-76

OaElE2BhslBhs2BhsSR

OaElE2BhsBslBs2R

OaEBsCR

5-0 Oa0-5 E5-18 Bsl

18-28 Bs2

28-76 C76-102 R

15-00-55-13

13-3030-4545-7575-80

OiEBslBs2BCCR

Hinker; loamy-skeletal, mixed Humic CryorthodsDecomposed organic mat; many very fine, fine, medium, and coarse roots7.5YR7.5YR7.5YR7.5YR7.5YR

6/25/34/44/44/4

1silsil1

sil

mm2vcsbk2vcsbk2vcsbk

frfrfififi

csasciciaw

Unweathered phyllite

40% channers, 5% cobbles40% channers, 5% cobbles, slightly sticky40% channers, weakly smeary35% channers, weakly smeary50% channers, weakly smeary

Crinker; loamy-skeletal, mixed Typic CryorthodsDecomposed needles, leaves, and twigs; many very fine, fine, medium, and coarse roots7.5YR5YR7.5YR10YR10YR

4/25/35/65/65/4

1silsilsilsil

Unweathered phyllite

mmmmIfsbk

frfrvfivfifi

csascscscs

35% channers40% channers50% channers, weakly smeary55% channers, weakly smeary60% channers, weakly smeary

Springsteen; loamy-skeletal, mixed Typic CryorthodsDecomposed needles, leaves and twigs; many very fine, fine, medium, and common coarse roots7.5YR 5/2 1 m fr10YR 4/4 1 2msbk vfr10YR 4/3 1 Ifsbk vfr

Unweathered phyllite, few fine roots in fractures

csaw

35% pebbles, slightly sticky40% pebbles, 10% cobbles, weakly smeary30% pebbles, 20% cobbles, weakly smeary

Rinker; loamy-skeletal, mixed, frigid Typic HaplorthodsDecomposed forest litter and hemlock log; many very fine, fine, medium, and coarse roots7.5YR 5/2 sil m vfr aw7.5YR 4/4 sil 2msbk vfr cw10YR 4/4 sil Ifsbk vfr cw

2.5Y 4/4 1 Ifsbk vfr cwFractured phyllite

Squires; loamy-skeletal, mixed, mesic Typic HaplorthodsUndecomposed needles, leaves, and twigs; many very fine, fine, medium, and coarse roots

25% pebbles, 5% cobbles

30% channers, 10% pebbles40% channers, 5% pebbles, weakly smeary45% channers, 10% pebbles, 10% cobbles,

weakly smeary60% channers, 10% cobbles

7.5YR7.5YR7.5YR10YR2.5Y

5/23/44/44/45/4

silsil11si

ImsbkImsbkImsbkmm

Fractured phyllite

fr cw 25% channers, 10% cobbles, weakly smearyfr cw 35% channers, 15% cobbles, weakly smearyfr cw 30% channers, 15% cobbles, weakly smeary

vfr aw 40% channers, 20% cobbles, weakly smeary

t Symbols used are the same as given in Soi7 Survey Manual (17; p. 139-140) and as follows, csirregular, aw = abrupt wavy, and cw = clear wavy.

clear smooth, as = abrupt smooth, ci = clear

NETTLETON ET AL: DIFFERENTIATION OF SPODOSOLS AND ANDEPTS 989

and deformed veins varying from microscopic dimensionsto many centimeters thick and several meters long. In handspecimen, Darrington phyllite is dark gray with a pearly lus-ter and has the smeary feel of graphite (20).

MethodsAluminum and Si were extracted by 4 M KOH (10), by

(NH^CjO,, (13) as modified by Holmgren and Kimble (10),and by boiling 0.5 M KOH (8). Humic acid color was mea-sured as described by Holmgren and Holzhey (9). Glass con-tent is based on a count of 300 grains of the very fine sandfraction. The glass content includes crystalline particlescoated with glass and partially devitrified glass. The methodsidentified by codes in Table 3 are described in Soil SurveyInvestigations Report no. 1 (19).

RESULTSChemical and Physical Properties

For these soils, dithionite-citrate extracted almosttwice as much Fe as did pyrophosphate, but the

amounts of Al extracted by the two methods are nearlyequal (Table 3). The amounts of Al extracted by di-thionite-citrate also are close to the amounts extractedby the 4 M KOH, (NH4)2C2O4, and boiling 0.5 M KOHmethods. All these data relationships for Al extractedby the different methods (Table 3) have r2 values ofabout 0.9 or better.

Each of the soils has one or more horizons in whichthe sum of pyrophosphate-extractable Fe plus Al ishalf or more of the sum of dithionite-citrate-extract-able Fe plus Al (Table 3). All of the soils have one ormore horizons that have >0.1% dithionite-citrate-ex-tractable Fe, and all except Rinker have one or morehorizons with a value of 0.2 or more for the ratio ofFe plus Al extractable by pyrophosphate at pH 10 topercentage of clay (Tables 3 and 4). All of these pedonsthen, except Rinker, meet the Spodosol chemical cri-teria.

None of these soils contain enough volcanic glassto be in ashy families (Table 4), but all of them have

Table 2. Some site characteristics and pedon totals for the sites.

Pedon totals

Pedon

HinkerCrinkerSpringsteenRinkerSquires

Elevation

m12501000750500250

Precipi-tation

mm24002200200018001600

Mean annualsoil

temperatures

°C56678

Spodicindext

12301190663617667

OrganicC

186140997878

PyrophosphateextractableFe and Al

5654312849

Dithionite-citrate

extractable Al

3233151334

t Spodic index is calculated by subtracting half of the clay percentage from CEC at pH 8.2 and multiplying the remainder by the thickness of the sub-horizons in centimeters. These were then summed for each pedon (18).

Table 3. Selected chemical dataf for the pedons.

Dithionite-citrate Na Pyrophosphateextract extract

—————————————— —————————————— 4MKOJFe Al Fe Al ———————

Horizon (6C2b) (6G7a) (6G7a) (6G10) Al

I extract (NH,)AO. Boiling 0.5 M KOH Humic acid

Si Al Si Al Si L-pcu/g———————————————————————————————————————————————————————————————————————— Aon b-«-l ————————————————————————————————————————————————————————————————————————

ElBhslBhs2R

OaE2BhsBs2

EBsR

EBslBs2R

EBslBCR

0.34.02.0NA

0.42.56.31.4

0.82.4NA

0.92.01.6NA

0.93.81.7NA

0.11.81.9NA

0.30.31.31.1

0.11.2NA

0.20.40.6NA

0.11.80.9NA

0.12.10.6NA

0.21.53.20.5

0.41.1NA

Hinker; loamy skeletal, mixed Humic Cyrothods0.1 0.1 0.0 0.11.7 1.5 0.1 1.81.4 NA NA 2.3NA 0.1 0.0 0.1

Crinker; loamy-skeletal, mixed Typic Cryorthods0.30.30.90.7

0.20.1NA1.0

0.00.0NA0.2

0.30.3NA1.1

Springsteen; loamy-skeletal, mixed Typic Cryorthods0.21.1NA

0.11.00.1

0.00.10.0

0.21.20.1

0.00.20.90.0

0.00.2NA0.2

0.00.20.0

Rinker; loamy-skeletal, mixed, frigid Typic Haplorthods0.51.20.9NA

0.20.40.6NA

0.20.4NA0.0

0.00.0NA0.0

0.30.3NA0.1

0.00.0NA0.0

Squires; loamy-skeletal, mixed, mesic Typic Haplorthods0.51.70.4NA

0.11.50.6NA

0.11.71.20.1

0.00.10.20.0

0.32.41.20.2

0.00.20.20.1

0.12.2NA0.2

0.30.2NA1.2

0.21.20.3

0.20.4NA0.1

0.22.11.60.2

0.10.3NA0.1

0.20.3NA1.2

0.40.50.2

0.50.3NA0.1

0.40.30.40.7

NA646NA

NANANANA

NA21NA

NA86

NA

NA261

NA

tThe codes, i.e., (3A1), refer to methods used. See Soil Survey Investigations Report no. 1 (19).

990 SOIL SCI. SOC. AM. J., VOL. 50, 1986

Fig. 2. Photomicrograph of a thin section of the Bhsl horizon of theHinker pedon in plane light showing the dark silt-size aggregatesand grain coatings.

some horizons having most of the properties of soilsin which amorphous material dominates the exchangecomplex (18). However, only the boiling 0.5 M KOHextracted significant amounts of Si from these soils.All of the horizons have bulk densities >0.85 Mg m~3.The E horizons of all the soils have either smectite orsmectite and vermiculite as the dominant clay min-erals. The Bs horizons are mostly amorphous to x-raydiffraction analysis and have strong low-temperatureendotherms.

Morphological PropertiesThe Bhs and Bs horizons of all the soils studied

have some isptropic plasma coating sand grains andforming silt-size or coarser aggregates. In thin sectionsof oven-dried samples, this plasma may be cracked,particularly in parts of the fabric that contain moresand and less silt. The microfabrics of Hinker (Fig. 2)and Rinker (Fig. 3) are from parts of the Bs horizonsthat are more silty and in which the plasma is mostlyisotropic. We consider that the silt-size pellets and thegrain coatings meet the requirements of criterion 2 forthe spodic horizon (18).

DISCUSSIONSoil Taxonomy (18) did not use values for Al ex-

tracted by the 4 M KOH, (NH4)2C2O4 and boiling 0.5M KOH methods as criteria for either Andepts or Spo-dosols. These kinds of analyses along with humic acidcolor, however, are now being studied as possible cri-teria (9, 10, 11, 12, 14).

The Hinker, Springsteen, and Squires (Table 3) allmeet the proposed humic acid color criteria (11) forSpodosols. The Rinker pedon has a slightly lowerhumic acid color than the 10-L platinum color unitsgram"1 proposed (11) and it also has <7 g kg"1 ofKOH Al.

Of those samples we analyzed, only the Bsl horizonof Squires and the Bhsl horizon of Hinker would reacheither the (NH4)2C2O4 or 4 M KOH extractable Allevels proposed (14) for the Andisols (Table 3) (theorder being proposed for the Andepts). Even if the Bs2of Squires also contains >2 dag kg"1 of (NH4)2C2O4

Table 4. Selected characterization dataf for the pedons.

Horizon

Clay<2 mm

(3A1)

Water1500 kPa

(4B2a)1 L

OrganicC

(6Alc)

CECNH4OAc(5A8b)

Sumbases(5B5a)

Extractable

Al(6G9a)

L.~-I ....„.„

Acidity(6H5a)

pH

CaCl,(8Clb)

NaF(8C2d|

Bulkdensity

3300 kPa(4Ald)

Glasscontentof VFSJ

0.05-0.1 mm

ElBhslR

OaE2BhsBs2

EBsR

EBslBs2R

2.58.4

NA

4.912.311.56.7

8.910.9NA

12.014.812.9NA

Hinker; loamy-skeletal, mixed Humic Cyrothods17.825.31.7

80.212.321.69.6

10.014.62.6

13.512.28.91.5

8.86.9

NA

37.54.37.53.5

4.75.6

NA

43.249.6NA

3.20.6

NA

7.511.2NA

42.158.6NA

Crinker; loamy-skeletal, mixed Typic Cryorthods139.143.060.020.4

20.20.60.40.4

6.414.115.14.2

124.445.061.229.6

Springsteen; loamy-skeletal, mixed Typic Cryorthods48.833.9NA

5.00.7

NA

11.56.7

NA

31.639.5NA

Rinker; loamy-skeletal, mixed, frigid Typic Haplorthods6.93.51.9

NA

43.334.918.1NA

3.40.80.8

NA

13.014.04.3

NA

41.139.122.5NA

3.24.15.1

3.13.33.74.3

3.54.24.5

3.53.64.26.0

6.411.3NA

5.97.08.4

10.6

6.910.3NA

6.98.4

10.1NA

Squires; loamy-skeletal, mixed, mesic Typic Haplorthods

fThe codes, i.e., (3A1), refer to methods used. See Soil Survey Investigations Report no. 1 (19).j VFS = very fine sand.§ The bulk density value for the El horizon is the value reported.

1.00NANA

NA1.17§NANA

1.15NANA

NA1.06NA2.61

164

NA

1431

NA

9116

NA

EBslBCR

12.010.96.9

NA

14.818.67.81.9

4.44.71.60.4

30.431.212.8NA

2.70.90.3

NA

10.44.61.3

NA

32.345.019.9NA

3.64.54.85.3

7.311.311.1NA

NA0.95NANA

9113

NA

NETTLETON ET AL.: DIFFERENTIATION OF SPODOSOLS AND ANDEPTS 991

Fig. 3. Photomicrograph of a thin section of the Bsl horizon of theRinker pedon in plane light showing the dark plasma coating sandgrains and forming a few silt-size aggregates.

extractable Al, the combined horizons are too thin toplace this pedon as an Andept [also in the key to SoilTaxonomy, the Spodosols are the first mineral soils(18)]. The Al and Si extracted by boiling 0.5 M KOHshow that the Bs horizons of the five soils contain onlyabout 2 to 8 dag kg~' amorphous aluminosilicates (1).

Within the climosequence (Hinker through Rinker)there are a number of close relationships between cli-mate (for convenience we use elevation) and a numberof soil properties (Table 2). The accumulation of or-ganic C in each pedon, for example, increases withelevation even though there is a progressive thicken-ing of the profiles at lower elevations (Fig. 4). Withincrease in elevation in the climosequence there is alsoan increase in the spodic index; the accumulation ofpyrophosphate-extractable sesquioxides; and the ac-cumulation of dithionite-citrate-extractable Al, whichwe are using as a weathering index (Fig. 4).

The Squires pedon does not fit within the climo-sequence for any of these properties. It contains moreorganic C, has a thinner profile, larger spodic index,contains more pyrophosphate extractable sesquiox-ides, and is more weathered than its elevation wouldlead one to expect. Presumably, these differences area result of its east-facing aspect and its toeslope po-sition, both of which tend to make it a wetter site thanprecipitation and temperature would indicate.

The five pedons studied are more spodic than andic.Andepts and most andic subgroups of other ordershave A horizons that contain more organic matter andare darker than their B horizons. All of the soils ofthis transect have E horizons as their first mineral ho-rizons and their B horizons have lower color valuesor higher chroma, or both.

The B horizons for the most part have a smearyfeel, a NaF pH that exceeds 9.4, and particles thatdisperse poorly and are partly amorphous. However,bulk densities are slightly higher than the presentlyaccepted values for Andepts or andic subgroups (Ta-ble 4) and all the samples and especially the E hori-zons contain major amounts of crystalline clay min-erals.

w*

••• •

• •.

• •

•'

1 80

1 2 0

60

7 5

65

5 5

wotc<0oz<o<r0

iH0.Ul0_J6CO

• •--

-

• • •• •_

.

• •-

• • •"

• •1 1 1 1 1

1 2 0 0 > cQZ

O800 o

£CO

50 '

i *32 | LU £

tc. < H>- I X0- 0. Ul

40i

Ul <25 1- 4.

- Ul T*• 1— <D0 < °

10 I tt Ht t XQ O HI

2 5 0 5 0 0 7 5 0 1000 1 2 5 OE L E V A T I O N , i n

Fig. 4. Relationship between the accumulation of organic carbon(OC), soil depth, soil spodic index, pyrophosphate extractable Fe+ Al, and dithionite-citrate extractable Fe + Al to elevation.

These pedons, then, seem best classified as Spodo-sols. None of their control sections contain enoughglass or amorphous silicate clay to place in medial orashy families. Also, because of the more crystallinenature of their E horizons, none would seem to fit intoandic subgroups of Spodosols, should these subgroupsbe provided and denned like andeptic subgroups ofother orders.

992 SOIL SCI. SOC. AM. J., VOL. 50, 1986