Soil-landscape Relationships in the Occidental Plateau of Sao Paulo State, Brazil: II. SoilMorphology, Genesis, and Classification1

I. F. LEPSCH, S. W. BUOL, AND R. B. DANIELS"ABSTRACT mined from cores. Soil pH was measured potentiometrically in

water and IN KC1 in 1:2.5 soil/liquid ratio. Total organic carbonNine profiles representing the major soils in the Occidental Plateau was determined by oxidation with acid dichromate using external

of Sao Paulo State, Brazil, were characterized after detailed geomor- heat (Allison, 1965). Exchangeable bases were extracted by soilphic and soil mapping were completed on a 70.8 km2 area. The stable percolation with 0.057V HNO3 (Paiva Neto et al., 1961). Calciumupland geomorphic surface had Oxisols, surrounded by younger ero- and magnesium were determined by atomic absorption spec-sional surfaces with Ultisols, Alfisols, and Inceptisols. Mollisols trophotometry; K was determined with a flame photometer.formed where the erosional surfaces exposed calcareous sand- Exchangeable acidity was determined by extraction with IN KC1,stone. The argillic horizons of the Ultisols and Alfisols in these posi- and NaOH titration with bromothymol. Extractable hydrogen wastions apparently have formed in material that would qualify as an oxic determined by leaching the soil with \N sodium acetate buffered athorizon before it was exposed. Laterally moving water at these sloping pH 7; titrating to a phenolphtalein end point with 0. IN NaOHsites is believed to initiate free Fe removal and lessivagc. Lateral water (Paiva Neto et al., 1961) and subtracting the exchangeable Al.movement at the contact between the surficial deposits of oxic com- Base saturation was calculated from the sum of exchangeableposition and the underlying calcareous sandstones appears to supply bases Plus exchangeable Al and exchangeable hydrogen. Free Fethe bases for the Alfisols developed in the oxic material. was extracted by the citrate dithionite method (Kittnck and Hope,

Kaolinite dominates the clay mineralogy of the soils formed in un- 19£3> an.d determmed by atormc absorption spectrometry.i-j i j j M u * ii i •. j . • .u The clay (<2 am) and very fine sand (100-50 um) fractionsconsolidated depos,ts, but attapulgite and smechte are present m the ^ ^ £^ th/procedure of Kittrick and Hope

calcareous sandstone. In the sods shallow to calcareous sandstone the (, %3) Oriented d imens for x diffraction study wereattapulgite apparently has weathered to smectite withm a few centime- prepared Qn g,ass s]ides The samples were Mg saturated and

ters of the rock. glyco, solvated; K saturated and heated to 350°C and 500°C (Jack-Additional Index Words: Oxisol, Mollisol, Alfisol, Ultisol, tropical son 1956) c,ay mineral contents were estimated from character-

soils, soil age, geomorphic surface. jstjc |,asai plane peak intensities and differential thermal analysis__________________ of the Mg-saturated clays in a flowing N2 atmosphere with a tem-

perature range from 25°C to 650°C. The amorphous material con-

THIS STUDY is LOCATED in a 70.8 km2 area of the Oc- tent was determined according to Hashimoto and Jackson (1960).cidental Plateau of Sao Paulo State, Brazil The approxi- Samples of the very fine sand fraction were mounted on a glass

mate coordinates are 22° 28' latitude south and 50° 15' west slide under a cover slip with 1.54 index of refraction oil for petro-i •» j /i • u T^U u i »• c graphic examination. Small undisturbed soil blocks, 2-3 cm in di-Zg f™ uW ' ^ T™"8 elevat'°ns Va7m8 fr°m ameter, were oven dried and impregnated under vacuum with a490 to 690 m above sea level. The average annual tempera- mixture of castolite resin, methyl meacrilate solvent, and benzoilture is 20.3°C and annual rainfall is 1,350 mm. Natural veg- peroxide (Cent and Brewer, 1971). The impregnated samples wereelation is tropical evergreen forest and edaphic savannah mounted on glass slides and ground to a thickness of about 30 jumthat is locally called cerrado. Detailed descriptions of the and described according to the terminology developed by Brewersoil parent material, soil mapping units, and the geomorphic (1964).units are given in a previous paper (Lepsch et al., 1977).This paper reports the composition of typifying pedons RESULTS AND DISCUSSIONselected during the soil and geomorphic mapping. „. . . , . , , , , , ,The location of the nine pedons sampled and the slope

MATERIALS AND METHODS dasses for the Study arca is shown in Fig" l' The cross sec-The pedogenic horizons of nine pedons were sampled and ipaper. No. 4974 of the Journal Series of the North Carolina Agric. Exp.

described according to the Soil Survey Manual (Soil Survey Staff, Stn., Raleigh, NC 27607. This work was supported in part by the Fun-1951) although colors were interpreted between Munsell chips. dagao de Amparo a Pesquisa do Estado de Sao Paulo, Brazil and by Con-Soil samples were air dried and processed to pass a 2-mm sieve. <mct ̂ l^-1236 withthe U'S,' £gTCy for,International Development.Particle-size distribution was determined by the NaOH-pipette *^ffi ̂ ̂ ^^Tc^s, Braz.l, andmethod (Krlmer and Alexander, 1949). Water dispersible clay was FAPESP scholar at North Carolina State Univ., Professor of Soil Sciencemeasured according to Vettori (1969). Bulk density was deter- and Soil Scientist, USDA-SCS, respectively.

110 SOIL SCI. SOC. AM. J . , VOL. 41, 1977

Percent slope

|=̂ 0 - 2 4+»- gullies

Illllllll 2 - 5 T typifying pedon

| |̂ 10 - 20

2kmscale

Fig. 1—Slope classes and location of typifying pedons in the studyarea.

tion (Fig. 2) shows the relative position of the pedons on thelandscape and their relations to the soil mapping units andgeomorphic surfaces. The geomorphic surfaces are labelledwith roman numerals that are sequentially arranged accord-ing to the relative surface ages; surface I is the oldest andsurface V is the youngest. (Lepsch et al., 1977).

Morphological descriptions of the pedons are in Table 1.Pedon 1 represents soil mapping unit A, which correspondsin its major part to geomorphic surface I, located on the top

of a small but conspicuous plateau. Pedon 2 represents aninclusion in soil mapping unit A. Soils like pedon 2 werealways found in the transitional zone between soil mappingunit A and soil mapping unit B. Pedon 3 is typical of soilmapping unit B. Although this pedon was sampled onlyabout 300 m from pedon 2, it has a more abrupt transitionbetween the A and B horizons, a stronger grade of structureand more clay skins in the B horizon. It also has a thickerand lighter colored A2 horizon.

Pedons 4 and 5 represent soils from mapping unit C.Pedon 4 typifies the shallow soils over calcareous sandstonefrom the steepest slopes. Pedon 5 typifies the deepest soilson the footslopes of the scarp. Both pedons 4 and 5 haveclear evidence of lithological discontinuities.

Pedon 6 represents soils from mapping unit D. Thispedon is similar to pedon 3 but the transition between the Aand B horizons is not as abrupt as in Pedon 3 and it containsless clay than in the B horizon. Pedons 3 and 6 correspondto Podzolized soils, variation Marilia, as described byLemos and Bennema (1960) and Comissao de Solos (1960).

Pedon 7 represents soils from mapping unit E. This soil issimilar to Pedon 1, with gradual to diffuse boundaries be-tween horizons and a B horizon with a porous, massive ap-pearance. Pedon 7 is less friable than Pedon 1 even thoughit has less clay. Both Pedons 1 and 7 correspond to the DarkRed Latosol, sandy phase (Comissa de Solos, 1960).

Pedon 8 represents soil mapping unit F. This soil has amorphology similar to pedon 2. Both have a sandy clayloam, dark red B2 horizon over a very friable, almost mas-sive, B3 horizon. Pedon 9 typifies one of the soils in thecomplex mapping unit G. This soil contains textural lamel-lae in the subsoil.

Micromorphological DescriptionsPlasmic structures, related fabric distribution, and

amount of oriented clay bodies for the pedons sampled are

700SURFACE I H _rU V SURFACE m

E

LLlQ

600 -

10 km

SURFACE ffl IV V,__[V SURFACE

600-

LJO13

500 -

SOIL MAPPING UNITS

A (OXISOLS)B (ULTISOLS AND

ALFISOLS)C (MOLLISOLS)

D (ULTISOLS)

E (OXISOLS)

F (ULTISOLS)

G (ULTISOLS)km

Fig. 2—North-south and east-west cross sections of the study area showing soil geomorphic surface relations and position of the typifying pedons.

LEPSCH ET AL.: SOIL-LANDSCAPE IN THE OCCIDENTAL PLATEAU OF SAO PAULO STATE, BRAZIL: II. I l l

Table 1—Field morphological descriptions and micromorphology data.

Field morphology^ &J Ctrllotlll-o-t-

Horizon

AlA3BlB21B22

ApA3B21tB22tB31B32

ApA12A2B21tB22tB31

AllIIA12IIA3IIIR

AllA12A2B21tB22tB31B32

AllA12A21A32B21tB22tB23tB31

AlA3BlB21B22

ApA21A22B2HB22tB3

AllA12A21

B21?B3?

Depth

cm0-17

17-4646-7070-106

106-148+

0-1818-3737-7070-110

110-150150-180

0-1616-2727-6060-9292-130

130-170+

0-2020-4242-5050-55+

0-2828-4242-7272-9494-122

122-150150-190

0-1717-3232-6060-7676-9292-127

127-160160-260

0-2020-4242-7575-110

110-180

0-2020-3737-5353-9292-135

135-180

0-1616-3838-7373-135

135-143143-190

Boundary^

csgsdsdsds

cwgsgsdsds

cwcwawgsgs

awcwaw

Color (moist) and consistence §

Pedon No. 1— (on surficial deposits)5YR3/4 2, m, gr; so4YR4/4 2, m, sb; so4YR4/4 l ,m, sb; so2.5YR3/6 2, vf, gr; so2.5YR4/6 2, vf, gr; so

Pedon No. 2— (on surficial deposits)2.5YR3/4 1, m, gr; so2.5YR3/5 l ,m, sb;sh2.5YR3/6 2, m, sb; h2.5YR3/6 1, c, sb; h2.5YR3/6 1, c, sb; sh2.5YR3/6 l,c,sb;so

Pedon No. 3— (on surficial deposits)7.5YR3/2 2, m, gr; so5YR3/4 2, m, gr; so5YR4/4 1, m, gr; so3.5YR3/6 2, m,sb;h3.5YR4/6 2, c, sb; h3.5YR4/7 1, c, sb; sh

Pedon No. 4— (on calcareous sandstone)10YR3/2 2, m, gr; so10YR3/1 l ,m,sb;sh10YR4/2 1, m, sb; h

Micromorphology HPlasmic

structure

und.und.iso-und.iso-und.iso-und.

in.in.in.in.und.und.

und.in.in.und.+as.und.+as.in.+vos

in.in.+skel.as.+skel.

Related fabricdistribution

m. por.m. por.m. por.m. por.m. por.

m. por.por. & agg.m. por.m. por.m. por.m. por.

m. agg.m. agg.m. agg.m. inter.m. inter.m. por.

agg.agg.agg.

Orientedclay bodies

%00000

trtr3tr00

0trtr

1030

000

Consolidated sandstone

cwcwawcscscwaw

cscscsawcsgsgsds

gsdsdsds

awgwgwdsds

cwgsawawcw

Pedon No. 5 — (on colluvium and calcareous sandstone)10YR3/1.5 2,m,gr;so10YR3/3 l ,f ,gr;so9YR4/4 sg; so5YR4/4 2, m, sb; v5YR4/6 2, c, sb; vh as.5YR4/7 2, c, sb; vh as.Gravel bed

Pedon No. 6— (on colluvium)5YR3/3 2, f, gr; so5YR3/4 1, f, gr; so5YR4/4 sg; so5YR4/6 l,m,gr;sh2.5YR4/6 2, m, sb; sh2.5YR4/6 2, m, sb; h2.5YR4/6 l ,m, sb; sh2.5YR4/6 l,c, sb;so

Pedon No. 7— (on surficial deposits)2.5YR3/4 2, f, gr; so2.5YR3/5 l,m, sb; so2.5YR3/6 1, m, sb; so2.5YR3/6 1, m, sb; so2.5YR3/6 1, vf, gr; so

Pedon No. 8 — (on surficial deposits)5YR3/4 2, f, gr; so3.5YR3/5 l ,m,gr;so3.5YR3/5 l ,f ,sb;so3.5YR3/6 2, m, sb; h3YR3/6 2, c, sb; h3.5YR3.5/6 1, c, sb; sh

Pedon No. 9— (on colluvium)7YR3/2 2, m, gr; lo5YR5/6 l , f ,gr ; lo5YR4/6 1, f, gr; loeleven lamella4YR4/6 l ,m, sb; so4YR4/6 1, m, sb; so

in.in.in .-as.

as.+skel

in.skel.skel.skel.ins.ins.ins.in.

in.in.iso-und.iso-und.in.

isoiso.und.in.in.und.

in.

in.in.in.in.

agg.agg.agg.

inter.inter.agg.

agg.agg.agg.agg.agg.inter.inter.agg. & por.

agg. & por.por.por.por.agg. & por.

agg.+por.agg.+por.agg.+por.inter.inter.por.+agg.

agg.

agg.agg.agg.agg.

000

74

tr00trtr

10tr0

00000

00trtr2tr

0

trtr-5tr3

c = clear; s = smooth; w = wavy; g = gradual; d = diffuse; a = abrupt.2 = moderate; 1 - weak, vf - very fine; f - fine; m - medium; c - coarse; gr - granular; sb - subangular blocky; sg = single grained.(dry) so - weakly coherent; sh - slightly hard; h = hard; vh - very hard.m = mostly; und = undulic; as = asepic; iso. = isotic; in = inudulic; vos = vosepic; skel = skelsepic; agg = agglomeroplasmic; por = porphyroskelic; inter = intertexic.

presented in Table 1. The optically oriented clay bodies arereferred to as illuviation argillans, papules, quasicutans,and neocutans (Brewer, 1964). Illuviation argillans wereidentified in all horizons named Bt, thus confirming the

illuvial nature of these horizons. In Pedons 1 and 7, noilluviation argillans or other evidences of illuviated clay arepresent and the horizons meet the criteria for oxic horizons.Pedon 4 has a cambic horizon.

112 SOIL SCI. SOC. AM. J., VOL. 41, 1977

Table 2—Selected chemical and physical properties.

Horizon Depthcm

AlA3BlB21B22B-C?

ApA3B21tB22tB31B32B-C?

ApA12A2B2UB22tB31B32B-C?

AllIIA12HAS

AllA12A2B22tB23tB31tB32IIC1

AllA12A21A22B21tB22tB23tB31

AlA3BlB21B22B23B-C?

ApA21A22B21tB22tB3B-C?

AllA12A2B2?B3?B-C?

0-1717-4646-7070-106

106-148570-620

0-1818-3737-7070-110

110-150150-180530-580

0-1616-2727-6060-9292-130

130-170170-220420-470

0-2020-4242-50

0-2828-4242-7272-9494-122

122-150150-170170-190

0-1717-3232-6060-7676-9292-127

127-160160-260

0-2020-4242-7575-110

110-150150-180580-630

0-2020-3737-5353-9292-135

135-180430-480

0-1616-3838-73

135-143143-190240-290

Clay%

182022222628

16203232293035

9138

3737303028

149

10

1174

3129272521

81078

24302721

17161718212222

9111223232322

710

72213

7

Silt(20-2 /urn)

%

124315

2342231

54323228

433

33444465

22223212

1222223

3122110

11126

11

Fine sand/coarse sand

ratio

0.30.40.50.50.50.7

0.50.60.60.70.70.80.9

0.70.60.81.00.90.91.11.3

1.91.71.7

2.12.32.42.42.42.42.53.2

1.41.21.11.11.31.31.41.5

1.31.41.51.31.52.21.6

0.70.80.81.00.90.91.4

1.21.11.71.52.02.1

H20disp.clay%

1294

tr-

34

19650-

3336

1060

1.40.83.3

342

191211169

4232

1613

42

273311

152593-

151556

Bulkdensityg/cm3

1.61.41.31.41.4

1.51.31.41.41.31.3

1.41.61.51.41.51.6..-

1.51.41.4

1.51.51.61.61.71.9-.-

1.61.41.31.71.51.61.51.3

1.51.41.51.51.41.5

1.61.61.51.61.61.5

1.41.51.6-1.8-

pHH2O KC1

Pedon No. 15.5 4.95.3 4.44.7 4.24.7 4.14.8 4.25.6 4.0Pedon No. 24.9 3.94.6 4.44.9 4.44.9 4.54.9 4.34.9 3.95.1 4.0Pedon No. 34.7 4.34.9 4.65.0 4.75.2 4.95.3 4.54.8 4.95.3 4.95.7 5.4Pedon No. 45.1 4.85.2 5.05.4 5.1Pedon No. 55.0 4.95.0 4.95.3 4.94.9 4.65.3 4.85.5 5.15.3 5.15.8 4.9Pedon No. 65.3 5.45.5 5.15.5 5.05.5 4.35.4 4.35.7 4.15.5 4.15.2 4.1Pedon No. 74.5 3.94.5 4.04.6 3.94.5 4.04.3 4.04.6 4.14.6 3.9Pedon No. 85.3 5.15.4 5.15.3 4.95.3 4.83.9 4.04.6 4.04.7 4.3Pedon No. 94.8 4.44.9 4.35.0 4.45.3 4.45.0 4.34.6 3.8

OrganicC%

0.80.50.40.30.2

<0.2

0.50.50.40.30.30.2

<0.2

0.80.90.40.30.3

<0.2<0.2<0.2

1.91.20.5

0.90.30.20.40.30.30.2

<0.2

0.60.4

<0.2<0.2

0.30.30.2

<0.2

0.80.50.30.30.20.20.2

0.90.40.20.3

<0.2<0.2<0.2

0.60.3

<0.2<0.2<0.2<0.2

Exchangeableions

Ca

0.30.10.10.10.10.1

1.01.92.11.70.90.30.1

1.42.41.42.52.21.61.21.3

7.48.14.7

3.82.11.45.54.54.03.73.4

1.21.40.80.91.91.00.60.2

0.40.10.20.10.10.10.1

2.20.90.71.10.30.20.1

1.21.10.71.41.10.2

Mg

0.40.1trftrtrtr

0.10.20.20.40.30.1tr

0.30.20.10.40.50.30.20.3

0.90.80.7

0.50.20.21.21.11.21.11.4

0.40.30.20.20.80.91.00.5

0.20.10.1trtrtr0.2

0.80.20.20.50.20.1tr

0.40.10.10.20.20.1

K Almeq/100 g

0.20.1trtrtrtr

0.10.1tr0.10.10.10.1

0.20.3tr0.10.10.10.1tr

0.1trtr

0.1trtr0.20.20.20.20.1

0.20.1tr0.10.20.30.20.1

0.1trtrtrtrtr0.1

0.30.40.20.20.10.1tr

0.1trtrtrtrtr

0.41.11.21.31.00.4

0.40.10.20.10.51.20.9

0.10.20.10.10.10.10.10.1

0.100

0.10.20.10.10.10.10.20.1

0.10.10.10.10.21.51.42.1

1,51.51.51.61.71.52.2

0.10.10.10.11.11.41.1

0.10.10.10.20.41.4

Extr.H

2.52.22.11.11.50.9

1.52.01.51.61.61.30.9

1.82.00.51.51.10.40.80.4

2.81.30.8

2.21.30.71.91.71.41.21.1

1.51.20.40.51.51.81.20.7

2.81.62.82.50.90.90.8

1.21.41.31.81.71.20.9

1.31.40.81.31.20.8

Basesat.%

2285467

36505855381610

5057716570796376

748787

6566557776787880

5257676663394120

1378433

12

7149444716105

54434652459

"free"Fe203

%

3.03.84.34.54.85.9

3.23.95.25.05.24.85.5

2.53.22.55.45.75.05.06.1

1.00.50.5

1.61.31.14.13.93.93.93.0

1.41.81.42.03.84.24.13.2

2.73.03.23.23.83.93.3

2.02.32.74.54.84.74.8

0.81.31.11.81.4-

Fe203/clay ratio

0.170.190.200.200.180.23

0.200.200.160.160.180.160.16

0.280.250.310.140.150.170.170.22

0.130.100.09

0.150.180.270.130.140.150.160.14

0.180.180.200.250.160.140.150.15

0.160.190.190.180.180.180.15

0.220.210.220.190.210.200.22

0.120.130.150.100.18-

f t r is <0.1 meq/100 g.

LEPSCH ET AL.: SOIL-LANDSCAPE IN THE OCCIDENTAL PLATEAU OF SAG PAULO STATE, BRAZIL: II. 113

In Pedon 9, all the lamellae have illuviation argillans.The argillans were more abundant in the deepest layers.Illuviation argillans in the coarser and finer textured lamel-lae suggest that they are a succession of thin alternatingcoarser and finer textured sedimentary layers with morerecent lessivage processes introducing the argillans.

Most pedons have an almost completely isotropicplasma, which was described as undulic and isotic. InPedons 2, 3, and 8 the only isotropic bodies, besides theskeleton grains, are the plasma concentrations related toclay illuviation. Pedons 4, 5, 6, and 9 have, in most hori-zons, a plasma with many insepic, skelsepic, or asepicanisotropic domains. The isotropic character of the S-matrixis related to strong weathering of parent material (Stoops,1968). This seems to be confirmed in this study since soilswith almost isotropic plasma are formed in highly weath-ered, sandy loam, surficial deposits.

In the B horizon of Pedons 1 and 7, the plasma is a densegroundmass partially or completely coating the skeletongrains. The skeleton grains are grouped into subroundedand mammilated peds of 0.5 to 1.0 mm diameter that arethemselves only partially accomodated, leaving large poresbetween peds. These features show that these soils are notmassive or structureless as they are often described uponfield observation with the naked eye, but they have very finemoderate or strong granular structure. This arrangement ofthe soil matrix also explains the great porosity and friabilityof these soils.

Mechanical Analysis and Chemical DataPedons 1,4, and 7 have no clear evidence of clay translo-

cation (Table 2). In pedons 1 and 7 the absence of an illuvialhorizon, great soil thickness, diffuse boundaries betweenhorizons, and low apparent clay CEC values (Lepsch et al.,1977) are properties of oxic horizons. High clay CEC val-ues and texture finer than loamy very fine sand in the A3 ho-rizon suggest a cambic horizon in Pedon 4.

Clay contents sharply increase in the Bt horizons ofPedons 2, 3, 5, 6, and 9 (Table 2). The clay increase of >20% and the argillans make these argillic horizons (SoilSurvey Staff, 1973).

The variation of the fine sand/coarse sand ratio (Table 2)with depth is what one would expect in stratified sedimen-tary deposits. The discontinuities in the sand ratiosfrequently do not coincide with an increase in clay contentwith depth.

Water dispersible clay content is always less than the claydetermined using dispersants (Table 2). Highest amounts ofwater dispersible clay were found in the illuvial horizonsidentified as B2t or argillic horizons. The higher contents ofwater dispersible clay in the B2t horizons suggest that inthese soils this characteristic is associated with the presenceof illuviated clay. Oxic horizons have very low contents ofwater dispersible clay (Pedons 1 and 7).

Pedons 2, 3, 5, 6, and 8 have free Fe2O3/clay ratio max-ima in the surface Al and A2 horizons (Table 2). Thesehigher ratios seem to suggest a relative concentration of Fein these eluvial horizons by preferential removal of partiallydeferrated clay.

Base saturation values (Table 2) contrast markedly be-tween some of the pedons. Pedon 1 and 7 have very low

Table 3—Quantitative estimation of minerals in the clay and sandfractions.

Clay (< 2/mi) Sand (50-150 pirn) §

Kaol.f Arn.f Othersft Q" Pel Mi H. & Op.

AllB23

ApB22

ApB21

A12A3R

AllB22t

AllB21t

AlB23

ApB21t

AllB2?

1tr

1t

tr1

00

trtr

0tr

11

00

00

7365

6667

5765

162

3037

3853

6965

7572

5453

106

„6

76

..95

118

118

87

810

..10

——

Pedon 1ItItQt

Pedon 2ItVtlt

Pedon 3Rtl tMtVt

Pedon 4AISIRIMtAISIRtMtA2S1

Pedon 5R2M1SIS1M1R1

Pedon 6RIMtStRIMtVtSt

Pedon 7ItIt

Pedon 8ItIt

Pedon 9VtStQtVtStQt

9897

9797

9796

9694

9690

9697

9799

9991

9999

00

tr0

0tr

32

38

2tr

00

06

trtr

00

00

00

..11

10

00

00

01

00

23

33

33

11

01

13

31

12

11

t Gib - gibbsite; Kaol - kaolinite; Am - amorphous; tr - traces (<1%); -• - notdetermined.

$ I - Intergradient chlorite-vermiculite; M = mica; S - smectite; A - attapulgite;V - vermiculite; Q - quartz; R - random interstratified 2:1 minerals; t - traces(<10%); 1 - small (10-25%); 2 = medium (25-50%).

§ Qu - quartz; Fel - feldspar; Mi - mica; H & Op - heavies and opaques.

base saturation values that are constant with depth. Valuesare high in the surface horizons of Pedons 2, 6, and 8 butdecrease with depth. In Pedons 3, 4, and 5, high values areeither constant or increase slightly with depth. Pedon 4, theshallowest, is also the one with highest base status. Pedon 5has hard sandstone at 470 cm, but in all the other pedonscalcareous sandstone was not found within 500 cm. Theserelations indicate that the high base status of pedons 3, 4,and 5 is due to the influence of the carbonate cementedsandstone.

Sand and Clay MineralogyQuartz makes up 90 to 99% of the 50 to 150 ju,m fractions

of all samples. Feldspars are present in a few soils and, inmost cases are closely associated with the presence of sand-stone rock. The feldspars apparently are inherited from thesandstone and disappear as weathering progresses.

Quantitative estimation of clay minerals from sandstoneand soil control section samples are given in Table 3. At-tapulgite was found in the control section of Pedon 4, acambic horizon close to the sandstone rock. The cambic ho-rizon had less attapulgite than the underlying sandstone.This suggests that attapulgite is inherited from the parentmaterial and under the present environmental conditions it

114 SOIL SCI. SOC. AM. J . , VOL. 41, 1977

Table 4—Pedon classification.

Pedon

123

4567

89

Subgroup

Typic HaplorthoxRhodic PaleudultArenic Hapludalf

Lithic HapludollOxic ArgiudollArenic PaleudultQuartzipsammentic

HaplorthoxArenic Rhodic PaleudultPsammentic Hapludult

Family

Fine loamy, siliceous, hyperthermicFine loamy, siliceous, hyperthermicSandy over clayey, siliceous,

hyperthermicCoarse loamy, siliceous, hyperthermicFine loamy, siliceous, hyperthermicFine loamy, siliceous, hyperthermic

Fine loamy, siliceous, hyperthermicFine loamy, siliceous, hyperthermicCoarse loamy, siliceous, hyperthermic

weathers to other minerals, primarily smectite. Smectitewas present in rock samples and in various soil horizonslocated on the two youngest geomorphic surfaces. The datasuggest that this mineral may be both a residual productfrom the parent material as well as a possible weatheringproduct of attapulgite. Other identified minerals in the clayfraction were mica, random interstratified 2:1 minerals, ver-miculite, chlorite-vermiculite intergradient, kaolinite, andgibbsite.

Kaolinite is the clay mineral most commonly found in theclay fraction of the soils studied. More kaolinite is found insoils of the older geomorphic surfaces. Because kaolinite isthe most resistant mineral commonly present in the studyarea, its quantities may be useful for estimating the relativeweathering stage of soils.

Gibbsite was detected only by the D.T.A. endotherm be-tween 300 and 350°C. Amounts were very small and only afew samples contain this mineral. We would expect the for-mation and accumulation of gibbsite under the climatic con-ditions of the study area. Gibbsite apparently does not formin these soils because the activities of the ions in the soilsolution are within the stability field of kaolinite, the appar-ent stable mineral of all the soils studied.

Soil ClassificationThe soils were classified at the family level according to

the Soil Taxonomy (Soil Survey Staff, 1973). The soil mois-ture regime is udic and temperature regime is hyperthermicbased on data reported by Oliveira et al. (1975). The clas-sification in Table 4 takes into consideration that base satu-ration values of 49%, determined by sum of cations and ex-tractable acidity at pH 7, correspond to 35% by sum ofcations and extractable acidity at pH 8.2 (Oliveira, 1974).Later unpublished data compiled by S. W. Buol and J. R.Parades show that 50% BS at pH 8.2 equals 62% BS atpH7.

Soil Landscape RelationsThe cross section in Fig. 2, shows that although in most

instances there is a general relationship between geomor-phic surface and soils, the soil boundaries seldom coincidewith the geomorphic boundaries. The geomorphic surfaceboundaries, in most cases, occur at higher altitudes than soilboundaries. According to Daniels et al. (1971) this is to beexpected on an erosional landscape where an erosional sur-face cuts sedimentary beds and soils. Erosion exposes thedeeper weathering zones of the soils under the original sur-face and thus little difference in soil properties might be ex-

pected on the upper part of the erosional landscape. Thisseems to be true for the area studied since two of theerosional surfaces (surfaces II and IV) have cut into Oxisolsthat have little horizon differentiation.

Figure 2 also illustrates that a geomorphic surface com-monly can have more than one kind of soil; i.e., surface IIhas an Ultisol (Pedon 2) and an Alfisol (Pedon 3), surfaceIV has soils of both soil unit D (Arenic Paleudult) and soilunit F (Arenic Rhodic Paleudult), and surface V has Molli-sols where it cuts into sandstone and Ultisols where it cutsinto surficial deposits.

The Mollisols are related both to soil moisture regimeproduced by a shallow depth to bedrock with horizontalmovement above the rock and the high amounts ofexchangeable Ca from the underlying calcareous sandstone(Lepsch et al., 1977). The highly active smectite clay of theMollisols may help the formation of mollic epipedons by re-taining more organic matter than kaolinite (Allison et al.,1949).

Most of the Oxisol pedons are on the older, more levelsurfaces. These data confirm most observations that Oxisolstend to be present on gentle slopes on surfaces of great age(Soil Survey Staff, 1973; Bennema et al., 1962). The datasuggests that Oxisols are not always older than Ultisols orAlfisols since geomorphic surface II has both Ultisols andAlfisols and it may be older than surface HI that is domi-nated by Oxisols. The Ultisols apparently are related tolocal moisture regimes since they have formed in the weath-ered sediments underlying the Oxisols. Oxisols also arefound in minor proportions on the uppermost part of surfaceIV where they have formed in the truncated weatheringzones of the Oxisols on surface III.

Ultisols, on surfaces truncating the Oxisols, apparentlyform in the same highly weathered surficial deposits as Ox-isols. In this case relief is the only soil forming factor con-sistently identified with the presence of an argillic horizon.

The Oxisols are on the most level surfaces and soils withargillic horizons are on the mid to lower portion of the ad-jacent slopes. The explanation for the formation of argillichorizons on steeper slopes adjacent to Oxisols may berelated to lateral movement of water in the surface horizonsand short term reduction of hydrated iron oxides. On theslopes, water can move laterally above the B horizon duringperiods when rainfall exceeds the hydraulic conductivity ofthe B horizons. Water moving downslope above the B hori-zon may become impoverished in oxygen from high bio-logic activity (Daniels et al., 1973) and iron oxides in thesurface horizons could then be reduced and removed freeingthe clay for lessivage. This process is suggested by compar-ingfree iron contents in the A and B horizons of Pedons 1,2, and 3 and Pedons 7, 8, and 9. Although the free Fe con-tents of the pedons with argillic horizons do not greatly dif-fer from the Oxisol pedons, Fe depletion above the argillichorizons appears slightly greater than above the oxic hori-zons. Once depleted of Fe, the individual clay particles maybe detached from previous Fe cemented clusters and mi-grate downward to be accumulated as argillans, forming theargillic horizons. This hypothesis remains unproven but thehigher values of Fe/clay ratios for the A2 horizons (Table 2)when compared with underlying argillic horizons seem tosupport it.

Most soil variations within the study area were found tobe logically related to differences in parent material, surfaceage, slope and landscape position. Differences in parentmaterial, however, can be related to geomorphic history ofthe site. Thus, the geomorphic work was found to be ofgreat utility in improving the conclusions about soil mor-phology and genesis. Also, an understanding of geomorphicrelationships should prove useful in the operations of furtherdetailed soil surveys in similar areas. Since significant soildifferences are present in some instances on the same geo-morphic surface, careful soil observation in the field mustaccompany soil mapping by photointerpretation techniquesin order to prepare soil maps of a quality needed for accu-rate interpretations at these scales.