Differentiation of a Spodic Horizon from a Buried A HorizonToyoaki Ito, Sadao Shoji,* Yasuhito Shirato, and Eri Ono

ABSTRACTAndisols with a thin ash layer or C horizon are morphologically

so similar to Spodosols that there is a danger of confusing the ashlayer for an albic horizon or vice versa. In order to solve this problem,it is necessary to differentiate spodic horizons from buried A horizonsunderlying the ash layer or C horizon. The amounts of organic C(OC), pyrophosphate-extractable C (Cp), and fulvic acid C (Cf) weredetermined in 39 tephra-derived pedons and six nontephra-derivedpedons from the USA, New Zealand, Chile, Ecuador, and Japan(117 soil samples). The data indicated that the combination of Cp/OC ratio >0.50 and Cf/Cp ratio >0.50 is a reliable criterion todifferentiate the spodic horizons from the buried A horizons not onlyin tephra-derived soils, but also in nontephra-derived soils.

TEPHRA-DERIVED SPODOSOLS occurring in the vol-canic zones commonly have a multisequum pro-

file, reflecting intermittent tephra deposition, andsome of them have a young C horizon or light-coloredash layer in the upper soil profiles (Fig. 1). Therefore,there is a danger that the C horizon or the light-coloredash layer can be easily confused with an albic horizon.

As described in Fig. 1, if the upper C horizon of anAndisol showing a horizon sequence such as A-C-2Ab-2Bwb-C could be misidentified as an albic horizon,the 2Ab, buried humus horizon would be regarded asa spodic horizon. Since the buried humus horizon ismore weathered and more enriched with Al and Fehumus complexes, compared with the overlying C ho-rizon, it will meet the spodic chemical criteria such as(pyrophosphate-extractable Fe + Al)/(dithionite-ex-tractable Fe + Al) > 0.5 and (pyrophosphate-extract-able Fe + Al)/(clay content) > 0.2 (15). As a

*Cor-Faculty of Agriculture, Tohoku Univ., 1-1, Tsutsumidori-Amami-yamachi, Aobaku, Sendai, 981 Japan. Received 23 Jan. 1990. *rw-responding author.

Published in Soil Sci. Soc. Am. J. 55:438-442 (1991).

consequence of misidentification of a young C horizonor light-colored ash layer, the Andisol would be re-garded as a Spodosol.

In order to solve this problem, differences in theproperties of organic matter between the two horizonsthat reflect the different soil-forming processes werestudied and a useful method for the differentiation wasproposed. For this purpose, we adopted a method de-veloped by McKeague (6), which was used for testingonly nonvolcanic soils. The method is based on theunique properties of organic matter in spodic hori-zons: organic matter is rich in fluvic acid and is largelycomplexed with Al and Fe (3) and is preferentiallyextracted by the pyrophosphate reagent (1).

MATERIALS AND METHODSAs shown in Table 1, 22 pedons are Andisols, 17 pedons

are tephra-derived Spodosols, and 6 pedons are nontephra-derived Spodosols from the USA, New Zealand, Chile, Ec-uador, and Japan.

Pyrophosphate-extractable organic C and Cf were deter-mined using a procedure (Fig. 2) similar to that employedby McKeague (6). Soil OC was determined by the dry com-

Spodosol Andisol

2C

Fig. 1. Morphology of a Spodosol and an Andisol with a thin layeror C horizon.

ITO ET AL.: DIFFERENTIATION OF SPODIC FROM BURIED A HORIZONS 439

bustion method using a Yanagimoto CN Coder (Model MT500, Yanagimoto Manufacturing Co., Kyoto, Japan), whichmeasures evolved CO2 by a thermal conductivity detector.An aliquot (5-20 mL) of pyrophosphate-extractable solutionand an aliquot (10-30 mL) of fulvic acid solution were evap-orated to almost 1 mL at 100 °C for a few hours. Then theconcentrated solutions were used for determining OC ac-cording to the dichromate-oxidation (wet combustion) meth-od (4, p. 400-404).

RESULTS AND DISCUSSIONThe pyrophosphate reagent was very effective in ex-

tracting the OC in spodic horizons of tephra-derivedSpodosols as well as those of npntephra-derived Spo-dosols, and the combined criterion of Cp/OC and Cf/Cp ratios is useful in differentiating spodic horizonsfrom both surface and buried A horizons (Fig. 3, Table1).

Table 1. Analytical data of pyrophosphate-extractable C (Cp), organic C (OC), and fulvic acid C (Cf) in soil samples.

No. Pedon name Classificationt Reference Horizon OC16-h-ex tract

Cp/OC Cf/Cp%

Surface A horizonsAlaska. USA

1Oregon, USA

23

New Zealand45678

Chile9

101112

Ecuador131415

Japan16171819202122232425

Alaska.USA262728

Oregon, USA2930

Chile313233343536

Ecuador373839404142

235

MurtipQuillayute

EgmontMamakuTirauTihoiTaupo

PelchuquinChanleufuPuerto FonckFrutillar

Ec-1Ec-6Ec-9

TsutanumaYunodaiNijibetsuKawakitaChasikotsuOhnodaiMeotozakaMukaiyamaShinjouSasaganaru

235

239

MurtipQuillayute

PelchuquinChanleufuPuerto Fonck

Frutillar

Ec-1

Ec-6

Ec-9

Typic Fluvicryand

Alic HapludandAlic Pachic Melanudand

Typic HapludandTypic HaplohumodTypic HapludandTypic HaplohumodTypic Udivitrand

Typic FluvudandAcrudoxic FluvudandHydric Pachic HapludandDuric Histic Placaquand

Typic UdivitrandVitric HapludandTypic Udivitrand

Acrudoxic FluvudandAcrudoxic Vitric MelanudandThaptic UdivitrandPachic MelanudandPachic MelanudandAlic MelanudandAlic Pachic MelanudandAlic Pachic MelanudandAlic MelanudandAlic Thaptic Melanudand

Buried A

Typic Fluvicryand

Humic Cryorthod

Alic HapludandAlic Pachic Melanudand

Typic FluvudandAcrudoxic FluvudandHydric Pachic Hapludand

Duric Histic Placaquand

Typic Udivitrand

Vitric Hapludand

Typic Udivitrand

13

1111

1414141414

14141414

141414

1212777

1010101010

horizons

13

8

1111

141414

14

14

14

14

Al

AlAp

ApApAAAp

AplAlApAp

ApApAp

AlAlAlAlAlAlAlAlAlAl

A2A3Oab/Ab

A2A3

Ap2A2A2ABAlA2

AlA2A2A3AlA2

9.5

8.216.8

7.611.88.6

14.15.8

11.08.97.8

24.0

2.75.64.6

12.410.77.17.87.7

16.212.914.014.521.2

8.86.39.9

5.89.8

9.45.85.27.57.65.4

2.32.24.84.63.53.4

0.45

0.330.36

0.260.230.300.240.35

0.300.260.400.30

0.250.380.32

0.260.310.350.400.420.430.370.370.290.46

0.530.520.41

0.330.44

0.310.370.260.280.230.23

0.320.350.370.370.350.33

0.54

0.760.47

0.650.570.430.430.51

0.660.750.640.68

0.580.500.47

0.650.470.370.370.310.280.420.420.500.40

0.650.670.67

0.800.49

0.690.780.560.470.900.83

0.490.430.450.430.460.42

t Classified according to Soil Survey Staff (1990).

440

Table 1. (cont.)

SOIL SCI. SOC. AM. J., VOL. 55, MARCH-APRIL 1991

No.

Japan4344454647484950515253575854555659606162636465

Pedon name

YabitsuAkanumaTsutanuma

Yunodai

Nijibetsu

Kawakita

Chasikotsu

OhnodaiMetozakaMukaiyama

ShinjouSasaganaru

Classification!

PlacorthodPlacorthodAcrudoxie Fulvudand

Acrudoxic Vitric Melanudand

Thaptic Udivitrand

Pachic Melanudand

Pachic Melanudand

Alic MelanudandAlic Pachic MelanudandAlic Pachic Melanudand

Alic MelanudandAlic Thaptic Melanudand

Reference

121212

12

7

7

7

101010

1010

Horizon

2Ab2Ab2Abl2Ab22Abl2Ab22Ab4Ab7Ab9AblOAb4Ab5Ab2Ab3AbSAbA23AblA23Ab2AbA22Abl

OC%

17.69.74.94.56.56.56.04.15.13.63.38.8

11.810.710.68.8

16.312.59.1

13.69.4

16.518.1

16-h-ex tract

Cp/OC

0.500.390.380.330.380.410.480.460.500.420.390.540.570.450.560.500.520.580.450.560.310.490.48

Cf/Cp

0.450.550.860.840.510.560.350.360.390.530.460.380.270.310.470.530.250.170.450.190.490.480.24

Tephra-derived spodic horizonsAlaska, USA

66676869707172737475767778798081828384858687888990919293

Washington, USA94959697

New Zealand9899

100101

Japan102103104105

230

232

234

237

239

240

241

243

244

245

252

Findley 1

Findley 2

Mamaku

Tihoi

Yabitsu

Akanuma

Typic Cryohumod

Typic Cryohumod

Typic Cryohumod

Typic Cryohumod

Humic Cryorthod

Sideric Cryaquod

Humic Cryorthod

Humic Cryorthod

Humic Cryorthod

Placic Haplaquod

Humic Cryorthod

Humic Cryorthod

, Humic Cryorthod

Typic Haplohumod

Typic Haplohumod

Placorthod

Placorthod

13

13

13

8

8

8

8

8

8

8

8

2

2

14

14

12

12

Bhs2BhsblBhs2BhsbBhs2Bhsb2BsblBhs2BhsbBhsbBsblBsb22Bhsb2BsbBhsbBsb2BsbBhBhslBslBs2BhBhslBhBhsBhsBslBsl

BslBs2BhsBs

2Bhsl2Bhs2BhsBs

Bsm2BsmbBsm2Bsmb

8.49.28.6

11.26.3

12.710.312.714.312.58.26.5

12.22.5

14.311.09.1

13.211.39.07.1

20.115.820.112.515.110.210.7

7.46.79.37.7

3.96.74.54.3

3.46.92.93.0

0.570.660.590.620.610.630.690.520.500.150.590.550.760.730.880.750.750.900.720.820.780.890.830.830.790.620.710.76

0.590.440.580.60

0.510.650.650.64

0.640.530.530.48

0.680.620.750.770.750.800.810.740.840.790.950.960.780.830.710.860.940.800.830.930.900.760.920.590.890.840.920.90

0.900.900.770.91

0.810.910.660.74

0.780.910.780.9$

t Classified according to Soil Survey Staff (1990).

ITO ET AL.: DIFFERENTIATION OF SPODIC FROM BURIED A HORIZONS 441

Table 1. (cont.)

No.

Canada106

Maine, USA

Pedon name

Reece

107 Dixfield108

Massachusetts, USA109110

New York. USA111112113114115116117

Becket

Becket

Mundel

Worden

Classificationt Reference

Nontephra-derived spodic horizons

Typic Haplorthod 16

Duric Haplorthod 16

Typic Haplorthod 16

Aquic Haplorthod 16

Typic Haplohumod 16

Typic Haplohumod 16

Horizon

Bsl

BhBsl

BhsBsl

BhsBslBhlBh2BhlBh2Bhs

OC%

2.4

3.93.7

4.73.3

6.65.82.52.37.68.55.9

16-h-extract

Cp/OC

0.55

0.550.57

0.740.67

0.560.530.580.520.670.600.60

Cf/Cp

0.95

0.870.91

0.760.88

0.760.810.810.860.850.860.87

t Classified according to Soil Survey Staff (1990).

Add 200 ml of pyrophosphate solution (0.1 M, pH 10.0)to 2 g of air-dried soil (<2 mm)

Ishake for 16 h (or 4 h)

Add 0.2 ml of 0.4 % superfloe solution

ICentrifuge for 30 min at 20000g

IAn aliquot of supernatant

ISupernatant 40 ml

Concentrated by heating Add 0.4 ml of cone. H5SO.at 100°C ^ *

Determine organic C bya wet combustion method

(Cp)

Stand for 2 h

Centrifuge for 15 minat 7000g

An aliquot of supernatant(fulvic acid)

Concentrated by heatingat 100°C

Determine organic C bya wet combustion method

(Cf)Fig. 2. Procedure to determine pyrophosphate-extractable organic

C (Cp) and fulvic acid C (Cf).

The A horizon samples (65 samples), including bur-ied horizons, showed an average Cp/OC ratio of 0.39with a range of 0.23 to 0.58 and an average Cf/Cp ratioof 0.51 with a range of 0.17 to 0.90. The tephra-derivedspodic horizon soils (40 samples) exhibited an averageCp/OC ratio of 0.66 with a range of 0.44 to 0.90 andan average Cf/Cp ratio of 0.85 with a range of 0.59 to0.98. The nontephra-derived spodic horizons had sim-ilar ratios as those of tephra-derived Spodosols.

Though there are considerable differences in the tworatios between the A horizon and spodic horizon soils,it is impossible to differentiate the two horizons usinga single ratio. Therefore, the usefulness of combiningthe two ratios was examined. All but two of the spodichorizon samples (50 samples) satisfied the combinedCp/OC ratio > 0.50 and Cf/Cp ratio 2: 0.50 (Fig. 3).In contrast, all but two of A horizons (63 samples) didnot meet the combined ratios. Therefore, we conclud-ed that the combined Cp/OC ratio > 0.50 and Cf/Cpratio > 0.50 is a very useful parameter to differentiatespodic horizons from buried and surface A horizonsnot only in tephra-derived soils, but also in nontephra-derived soils.

The statistical validity of our model was verified bythe discriminant analysis according to the Mahalan-obis' generalized distance (5, 9). A dotted line givenon Fig. 3 shows the discriminant function as follows:

1.90*2 - 2.01 Y2 + 1.58JST7 - X + 3.717- 2.10 = 0

where X = Cp/OC, and Y = Cf/Cp.

The line differentiates spodic horizons from both sur-face and buried A horizons (Fig. 3).

Though the criterion described is reliable, the pro-cedure uses a 16-h pyrophosphate extraction (Fig. 2).Therefore, the time for pyrophosphate extraction wasstudied. A 4-h extraction is more convenient in a rou-tine procedure than 16-h extraction. The validity ofthis extraction was checked according to the two cor-relation equations as follows:

Op/OC (4-h extraction) = 0.99X Cp/OC (16-h extraction), r 0.993***

Cf/Cp (4-h extraction) = 0.99X Cf/Cp (16-h extraction), r = 0.986***

442 SOIL SCI. SOC. AM. J., VOL. 55, MARCH-APRIL 1991

0.5to

0.8

a. 0.6U\<y W

0.2

U

•*

O *, •o oV.

' » g-\

*

* <&°'9fiSo ° ^

a

Lt» . .rVi?^.-•- •^ •

o V *Q OdS: °^ Spodic horizons

n ° Q, • Tephra-<g> 0 * Nontephra-

°° A horizons^> a Surface

0 Buried

0.5

0.2 0.4 0.6 0.8

Cp/OC

1.0

Fig. 3. Differentiation of spodic horizons from buried A horizonsaccording to pyrophosphate-extractable C/organic C (Cp/OC) andfulvic acid C (Cf )/Cp ratios. Dotted line is discriminant functionto separate spodic horizons from surface and buried A horizons.

The two equations for the regression coefficients arealmost unity and the correlation coefficients are veryhigh. Therefore, the 4-h extraction is satisfactory.

CONCLUSIONSThe combined Cp/OC ratio > 0.50 and Cf/Cp ratio

> 0.50 is a useful criterion for differentiating spodichorizons from both surface and buried A horizons. A4-h pyrophosphate extraction can be employed insteadof the 16-h pyrophosphate extraction.