soil-landscape relationships in the occidental plateau of são paulo state, brazil: i. geomorphic...

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Soil-landscape Relationships in the Occidental Plateau of Sao Paulo State, Brazil: I. Geomorphic Surfaces and Soil Mapping Units 1 I. F. LEPSCH, S. W. BUOL AND R. B. Daniels 2 ABSTRACT This study was conducted in a 70.8 km 2 area that includes most of the soils and landscapes present in the Occidental Plateau region of Sao Paulo State, Brazil. Surficial deposits of unknown derivation and age and Cretaceous age carbonate cemented sandstone are the soil parent materials. Both geomorphic surfaces and soils were mapped. Surface and subsurface soil chemical and physical characteristics of 103 sites were summarized to evaluate the soil variation of geomorphic surfaces and soil mapping units. Soil properties that are expressions of weathering indices are related to geomorphic surface, but clay illuviation, base saturation, and car- bon content do not regularly increase or decrease from the oldest to the youngest surface. More than one soil mapping unit can be found on one geomorphic surface. Most soil physical and chemical properties have smaller coef- ficients of variation when grouped by soil mapping unit than when grouped by geomorphic surface. Additional Index Words: tropical soils, soil age, soil landscape rela- tions, soil geomorphic surface relations T HE OCCIDENTAL PLATEAU is an area of 100,000 km 2 with a subtropical humid climate located in the western part of Sao Paulo State, Brazil. Although geologic maps show a single geologic formation and the area has uniform climate, reconnaissance surveys delineate soils that have contrasting characteristics. These conditions are very favor- able for soil-geomorphic studies designed to understand the reasons for the development of contrasting soils. Once the reasons for contrasting soils are understood, current and fu- ture soil surveys can use this information to increase the speed and accuracy of the surveys. The purpose of this study was to characterize the soils, identify the geomorphic surfaces, and to illustrate the relationship between soil properties and geomorphic surfaces. DESCRIPTION OF THE STUDY AREA The study was carried out in a 70.8 km 2 area located in the southern part of the Occidental Plateau, Sao Paulo State, Brazil (Fig. 1). The area was selected to include the major soils identified in a reconnaissance Soil Survey (Comissao de Solos, 1960) and to include a range of geomorphic surfaces present on the Plateau. The present climate is characterized as subtropical high land with a dry winter, Cwa according to Koppen (Critchfield, 1960). Bigarella and Andrade (1965) have suggested that during the Qua- ternary the climate changed from humid tropical during the in- terglacial phases to arid or semiarid during the glacial phases. Two main forms of natural vegetation, semideciduous tropical broadleaf forest and an edaphic savannah locally named cerrado, are present in the area. Most of the area that was originally forested is presently used for pasture and crops. Among the main 'Paper no. 4972 of the Journal Series of the North Carolina Agric. Exp. Stn., Raleigh, NC 27607. This work was supported in part by the Fun- dacao de Amparo a Pesquisa do Estado de Sao Paulo, Brazil and by Con- tract AID/ta-c-1236 with the U.S. Agency for International Development. Received 6 May 1976. Approved 24 Aug. 1976. 2 Soil Scientist with Institute Agronomico de Campinas, Brazil and FAPESP scholar at N.C. State University, Professor of Soil Science and Soil Scientist, USDA-SCS, respectively. crops are coffee (Coffea arabica), upland rice (Oryza sativa), corn (Zea mays L.), oranges (Citrus sinensis), soybeans (Glycine soja), cassava (Manihot dulcis), and peanuts (Arachis hypogaea). Most of the area of cerrado is used as natural pastures. According to the state geologic map (Instituto Geografico e Geologico, 1974) the study area is located in the outcrop area of the Bauru Formation of upper Cretaceous age. The Bauru Forma- tion is a subarkosic sandstone, often cemented with carbonate, that may vary both laterally and vertically to siltstone, shale, and con- glomerate (Freitas, 1964). The formation may be 300 m thick and is exposed at the surface in a major part of the Sao Paulo Occiden- tal Plateau. MATERIAL AND METHODS Field work consisted of geomorphic surface mapping (Daniels et al., 1971), soil mapping at a semidetailed level (Soil Survey Staff, 1951), soil sampling, and generalized observations about the stratigraphy. A total of 103 random sites were sampled using an auger. Both the A horizon (0-20 cm depth) and the B horizon (60 to 80 cm depth or to just above hard rock when shallower than 80 cm) were sampled at each site. Nine complete profiles repre- senting the major soils in the area were also sampled for character- ization and will be reported elsewhere. (Lepsch et al., 1977) The soil samples were air dried and processed to pass through a 2-mm sieve. Particle size was determined by the pipette method (Kilmer and Alexander, 1949) using NaOH as a dispersant. Par- ticle-size fractionation followed the international classification suggested by Atterberg (1912). The organic carbon was deter- mined by oxidation with acid dichromate using external heat (Allison, 1965). Exchangeable bases (Ca, Mg, and K) were ex- tracted with O.OSyV HNO 3 (Paiva Neto et al., 1961). Exchangeable acidity was determined by leaching with IN KC1 and titration with NaOH (Paiva Neto etal., 1961). The soil cation exchange capacity (CEC) was calculated by the sum of exchangeable bases, Al, and H extracted at pH 7. The apparent cation exchange capacity of the clay (CEC/100 g of clay) was calculated by dividing the soil cation capacity by the percentage of clay and multiplying the result by 100. Base saturation was calculated as percentage of bases re- tained by the cation exchange complex at pH 7. The pH values were determined potentiometrically in 1:2.5 solutions of H 2 O and IN KC1. RESULTS AND DISCUSSION Stratigraphy and Geomorphology The field work indicated that in the study area there are at least three different kinds of sediments (Fig. 2): (i) Subarko- sic sandstone with a calcareous cement of the Bauru Forma- tion, that crops out on a scarp and on a creek valley floor; (ii) Two noncalcareous red, massive but friable sandy loam surficial deposits, 2-20 m thick, that overlie the Bauru For- mation. One outcrop is on top of a small plateau and the other forms a group of undulating hills below the plateau. These sediments are post-Cretaceous, post-Bauru, and probably derived in part from rocks of the Bauru Formation. They can range in age from Cretaceous to Pleistocene; (iii) Yellowish sandy loam colluvial deposits with a texture and consistency similar to the sandy loam surficial deposits that occur mostly on the footslopes of the scarp. These colluvial deposits by their landscape position probably are relatively young, or Pleistocene to Holocene. 104

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Soil-landscape Relationships in the Occidental Plateau of Sao Paulo State, Brazil: I. GeomorphicSurfaces and Soil Mapping Units1

I. F. LEPSCH, S. W. BUOL AND R. B. Daniels2

ABSTRACT

This study was conducted in a 70.8 km2 area that includes most ofthe soils and landscapes present in the Occidental Plateau region ofSao Paulo State, Brazil. Surficial deposits of unknown derivation andage and Cretaceous age carbonate cemented sandstone are the soilparent materials. Both geomorphic surfaces and soils were mapped.Surface and subsurface soil chemical and physical characteristics of103 sites were summarized to evaluate the soil variation of geomorphicsurfaces and soil mapping units.

Soil properties that are expressions of weathering indices are relatedto geomorphic surface, but clay illuviation, base saturation, and car-bon content do not regularly increase or decrease from the oldest tothe youngest surface.

More than one soil mapping unit can be found on one geomorphicsurface. Most soil physical and chemical properties have smaller coef-ficients of variation when grouped by soil mapping unit than whengrouped by geomorphic surface.

Additional Index Words: tropical soils, soil age, soil landscape rela-tions, soil geomorphic surface relations

THE OCCIDENTAL PLATEAU is an area of 100,000 km2

with a subtropical humid climate located in the westernpart of Sao Paulo State, Brazil. Although geologic mapsshow a single geologic formation and the area has uniformclimate, reconnaissance surveys delineate soils that havecontrasting characteristics. These conditions are very favor-able for soil-geomorphic studies designed to understand thereasons for the development of contrasting soils. Once thereasons for contrasting soils are understood, current and fu-ture soil surveys can use this information to increase thespeed and accuracy of the surveys. The purpose of thisstudy was to characterize the soils, identify the geomorphicsurfaces, and to illustrate the relationship between soilproperties and geomorphic surfaces.

DESCRIPTION OF THE STUDY AREA

The study was carried out in a 70.8 km2 area located in thesouthern part of the Occidental Plateau, Sao Paulo State, Brazil(Fig. 1). The area was selected to include the major soils identifiedin a reconnaissance Soil Survey (Comissao de Solos, 1960) and toinclude a range of geomorphic surfaces present on the Plateau.

The present climate is characterized as subtropical high landwith a dry winter, Cwa according to Koppen (Critchfield, 1960).Bigarella and Andrade (1965) have suggested that during the Qua-ternary the climate changed from humid tropical during the in-terglacial phases to arid or semiarid during the glacial phases.

Two main forms of natural vegetation, semideciduous tropicalbroadleaf forest and an edaphic savannah locally named cerrado,are present in the area. Most of the area that was originallyforested is presently used for pasture and crops. Among the main

'Paper no. 4972 of the Journal Series of the North Carolina Agric. Exp.Stn., Raleigh, NC 27607. This work was supported in part by the Fun-dacao de Amparo a Pesquisa do Estado de Sao Paulo, Brazil and by Con-tract AID/ta-c-1236 with the U.S. Agency for International Development.Received 6 May 1976. Approved 24 Aug. 1976.2Soil Scientist with Institute Agronomico de Campinas, Brazil andFAPESP scholar at N.C. State University, Professor of Soil Science andSoil Scientist, USDA-SCS, respectively.

crops are coffee (Coffea arabica), upland rice (Oryza sativa), corn(Zea mays L.), oranges (Citrus sinensis), soybeans (Glycine soja),cassava (Manihot dulcis), and peanuts (Arachis hypogaea). Mostof the area of cerrado is used as natural pastures.

According to the state geologic map (Instituto Geografico eGeologico, 1974) the study area is located in the outcrop area ofthe Bauru Formation of upper Cretaceous age. The Bauru Forma-tion is a subarkosic sandstone, often cemented with carbonate, thatmay vary both laterally and vertically to siltstone, shale, and con-glomerate (Freitas, 1964). The formation may be 300 m thick andis exposed at the surface in a major part of the Sao Paulo Occiden-tal Plateau.

MATERIAL AND METHODS

Field work consisted of geomorphic surface mapping (Danielset al., 1971), soil mapping at a semidetailed level (Soil SurveyStaff, 1951), soil sampling, and generalized observations aboutthe stratigraphy. A total of 103 random sites were sampled usingan auger. Both the A horizon (0-20 cm depth) and the B horizon(60 to 80 cm depth or to just above hard rock when shallower than80 cm) were sampled at each site. Nine complete profiles repre-senting the major soils in the area were also sampled for character-ization and will be reported elsewhere. (Lepsch et al., 1977)

The soil samples were air dried and processed to pass through a2-mm sieve. Particle size was determined by the pipette method(Kilmer and Alexander, 1949) using NaOH as a dispersant. Par-ticle-size fractionation followed the international classificationsuggested by Atterberg (1912). The organic carbon was deter-mined by oxidation with acid dichromate using external heat(Allison, 1965). Exchangeable bases (Ca, Mg, and K) were ex-tracted with O.OSyV HNO3 (Paiva Neto et al., 1961). Exchangeableacidity was determined by leaching with IN KC1 and titration withNaOH (Paiva Neto etal. , 1961). The soil cation exchange capacity(CEC) was calculated by the sum of exchangeable bases, Al, andH extracted at pH 7. The apparent cation exchange capacity of theclay (CEC/100 g of clay) was calculated by dividing the soil cationcapacity by the percentage of clay and multiplying the result by100. Base saturation was calculated as percentage of bases re-tained by the cation exchange complex at pH 7. The pH valueswere determined potentiometrically in 1:2.5 solutions of H2O andIN KC1.

RESULTS AND DISCUSSION

Stratigraphy and Geomorphology

The field work indicated that in the study area there are atleast three different kinds of sediments (Fig. 2): (i) Subarko-sic sandstone with a calcareous cement of the Bauru Forma-tion, that crops out on a scarp and on a creek valley floor;(ii) Two noncalcareous red, massive but friable sandy loamsurficial deposits, 2-20 m thick, that overlie the Bauru For-mation. One outcrop is on top of a small plateau and theother forms a group of undulating hills below the plateau.These sediments are post-Cretaceous, post-Bauru, andprobably derived in part from rocks of the Bauru Formation.They can range in age from Cretaceous to Pleistocene; (iii)Yellowish sandy loam colluvial deposits with a texture andconsistency similar to the sandy loam surficial deposits thatoccur mostly on the footslopes of the scarp. These colluvialdeposits by their landscape position probably are relativelyyoung, or Pleistocene to Holocene.

104

LEPSCH ET AL.: SOIL-LANDSCAPE IN THE OCCIDENTAL PLATEAU OF SAG PAULO STATE, BRAZIL: I. 105

SCALE

Fig. 1—Location of the Occidental Plateau and study area in Sao Paulo State, Brazil.

NE

SW

BAURU FORMATION (CRETACEOUS) RECENT COLLUVIAL DEPOSITS (HOLOCENE)

SANDY-LOAM SURFICIAL DEPOSITS (POST-CRETACEOUS) 0000° GRAVEL BED

Fig. 2—Schematic block diagram showing the general physiography and the geology of the study area.

The area has a small plateau, 50 to 3,000 m wide, sur-rounded by a scarp and by gently undulating hills (Fig. 2).This small plateau, where the town of Echapora is located,extends east and west outside the study area.

Five geomorphic surfaces were identified and mapped inthe study area. Their areal distribution is shown in Fig. 3.Surface I is on the central part of the small plateau. It has avery smooth relief and no well-defined drainage ways. Alti-tudes range from about 680 to 690 m. The thick and poroussoil mantle immediately under this surface, the almost leveltopography, and the original forest cover suggest that this isa depositional surface. This surface occupies a plateau thatis protected by the hard sandstones of the Bauru Formation.The altitude and relation of the plateau to other sedimentssuggest, but are not proof of, a Tertiary age for this surface.Surface II is on the rims of the small plateau. The slopes

range from 2 to about 8%. In most instances it was possibleto clearly distinguish both a footslope and a backslope. Thissurface is erosional because it is cut into the sedimentsunder surface I. Surface III is on the higher parts of thegentle undulating hills southwest of the small plateau.Slopes range from 1 to 5% and no drainage ways were ob-served except for a few gullies. Surface IV is located on thegentle slopes cut below surface III and on the top of a fewsmall hills closer to the plateau scarp. Slopes are about 5%and grade to a drainage system considerably above thepresent system. Surface V is on the scarp and around thevalley sides that grade to the present drainage system.Slopes on this surface are the steepest in the area and mayrange from 10% to almost vertical cliffs in some gullies.This surface is mostly erosive, but some depositional siteswere observed at the foot of the scarp.

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

GEOMORPHIC SURFACE

Table 1—Area and number of sample sites in the soil mapping units.

Soil mapping unit A B C D B F G

Fig. 3—Areal distribution of the geomorphic surfaces.

SOIL MAPPING UNITSi

Fig. 4—Areal distribution of the soil mapping units.

By the law of superposition, surface I is the oldest in thearea. Surface II is younger than surface I because it trun-cates surface I. Like surface II, surface IV is younger thansurface II because it truncates surface III. Surface V is theyoungest in the area since it cuts both surfaces II and IV aridit grades to the modern streams. There is some doubt aboutthe relative age relationship between surface II and the othersurfaces. It is possible that surface II is bedrock controlled

Area (ha)No. of sites sampled

474.4 101.6 396.8 446.5 3405.0 1397.6 889.314 7 10 11 27 26 8

t Averages for soil mapping units indicated with lettered dots.J LSD bars refer to geomorphic units only.

and is relatively young or related to surfaces IV or V. Butthe continuity and areal extent of surface II on the edges ofthe plateau suggest that it is the same age as or older thansurface III. For purposes of discussion, we will assume thatsurface II is older than surface III. The ages of these sur-faces are unknown because the underlying sediments havenot been dated. Surfaces I, II, and III can range from Ter-tiary to Pleistocene although a Pleistocene age for surface Ican be questioned because 20 to 50 m of sandy surficial sed-iment has been deposited on a younger erosion surface andsubsequent erosion has produced 200 m of relief since itwas formed. Surface IV and V are reasonably late in the de-velopment of the landscape, and probably are Pleistocene toHolocene with the Holocene age most likely for surface V.

SoilsSeven soil mapping units were identified and delineated.

Their areal distribution is shown in Fig. 4. The computedareas for each unit, as well as the number of sites sampledfor laboratory analysis, is given in Table 1.

Soils in unit A and unit E are somewhat excessivelydrained Oxisols, > 2 m thick. Slopes range from 0 to 5%.Boundaries between horizons are diffuse and there is nofield evidence of an argillic horizon. They correlate as DarkRed Latosols, sandy phase (Comissao de Solos, 1960). Soilmapping unit B is a complex of well-drained Alfisols andUltisols on 5 to 10% slopes. Soil mapping units D and F arewell-drained Alfisols and Ultisols, respectively. The pedonshave clear or abrupt boundaries between an A2 horizon andan argillic horizon. Slope range from 5 to 20%. They corre-late as "Podzolized Soils, variation Lins and Marilia," asdescribed by Lemos and Bennema (1960). The soils of unitC were mapped as a Mollisol and soil complex whose maincomponents are either moderately well-drained shallowsoils (< 50 cm thick) or thicker soils with argillic horizons;in both cases calcareous cemented sandstone underlies thesoil and a thick (30 to 40 cm) dark-colored epipedon ispresent. Slopes range from 10 to 45%. Soils of mappingunit G were mapped as an Ultisol soil complex. The maindifferences among pedons were the thickness of the loamysand A horizon and the continuity of the B horizon. Slopesrange from 5 to 20%.

Table 2 contains some soil chemical and physical proper-ties for samples taken in each mapping unit. The data areaverages from the 103 randomly sampled sites. Detaileddescriptions and laboratory analyses from typifying pedonsof these mapping units are presented in another paper(Lepschet al., 1977).

The apparent clay CEC has been commonly used as aweathering index (Soil Survey Staff, 1973). Comparisonsof apparent clay CEC values for the 60-80 cm layer (Table2) indicate that there may be considerable difference in claymineralogy between some soil units. Units A, B, E, and Fwith similar values of apparent clay CEC seem to be in a

LEPSCH ET AL.: SOIL-LANDSCAPE IN THE OCCIDENTAL PLATEAU OF SAG PAULO STATE, BRAZIL: I. 107

Table 2—Mean values of selected soil properties for the A (0-20 cm) and B (60-80 cm) horizons by soil mapping unit.Soil mapping unit

Soil property

Clay,%

Textural ratioOrganic carbon, %

Exchangeable Cat

Base saturationf

Clay CECfpH (H20)

ApH

Horizon*

AB

A/BABABABBABAB

A

18 d26 b0.70 b0.6 b0.5 c0.4 ab0.4 a

14.6 a12.3 ab15.0 a4.6 a4.4 a

-0.7 a-0.5 a

B

l ib33 c0.33 a0.6 ab0.5 c1.2 c1.7 b

43.2 c47.8 d14.3 a5.1 b5.1 c

-0.6 a-0.8 b

C

10 be15 a0.67 b1.1 c0.3 ab4.4 d3.4 d

65.6 e73.5 e57. 5 d

5.5 d5.5 d

-0.5 a-0.7 ab

D

6a23 b0.25 a0.6 b0.4 be1.8 c2.8 c

55.6 d59.9 d28.5 be5.3c5.5 d

-0.6 a-0.9 b

E

12 c16 a0.76 b0.6 b0.3 ab0.2 a0.1 a7.9 a4.6 a

18.5 a4.6 a4.7 b

-0.7 a-0.7 b

F

10 b17 a0.58 ab0.5 ab0.3 ab0.4 ab0.4 a

23.6 b14.7 b19.7 ab

4.8 b4.8 be

-0.6 a-0.8 b

G

6a12 a0.5 ab0.4 a0.2 a0.9 b0.7 a

42.7 c33.2 c32.8 c5.1 b5.0 c

-0.7 a-0.8b

* Means for each line not followed by same letter are significantly different at the 0.05 probability level,t meq/100 g.

more advanced stage of weathering than units D and G.Unit C with an average value of 57.5 meq/100 g of clay hasthe least weathered soils.

The fine sand-coarse sand ratio is useful in testing parentmaterial homogeneity both between mapping units and be-tween horizons within profiles (Barshad, 1964). The sam-ples from the lower gently rolling hills south of the Echa-pora Plateau (soil mapping unit E) have intermediate sandratios when compared with samples from the top of the pla-teau and with samples from the scarp footslopes (Lepsch etal., 1977, Table 2). This suggests that these soils developedfrom sediments that are derived from a mixture of bothBauru sandstone and surficial deposits similar to the oneunder the Echapora Plateau. In soil unit C, considerable dif-ference between the 0-20 cm layer and 60-80 cm layer finesand coarse sand ratio suggests lithological discontinuitiesthat indeed were observed in the field.

The clay (0-20 cm) divided by clay (60-80 cm) (Table 2),used here as a illuviation index, indicates that units A and Ehave small textural gradient. This minimal increase in claywith depth, plus the diffuse horizon boundaries, and lowclay CEC, suggests that the mapping units are Oxisols (SoilSurvey Staff, 1975).

The ApH value (pH in KC1 - pH in H2O) is an indicator ofthe net charge of soil colloids, and highly weathered soilswith high Fe oxide and Al oxide content normally havesmall negative or positive ApH values (Mekaru and Uehara,1972). The average ApH values are negative and there islittle difference between soil mapping units (Table 2). TheApH values for the 60-80 cm layer are small and are unre-lated to clay CEC values (Table 2). This points out that ApHvalues are not directly related to the weathering stage as in-dicated by apparent CEC values of the clay.

Carbon contents of the 0-20 cm layers were similar for allsoil mapping units except unit C whose average is consider-ably higher than any other soil unit. These high carbon con-tents are allied with high base saturation, friable consis-tence, and to soil color chromas and values < 3. Thesecharacteristics are diagnostic for the mollic epipedon (SoilSurvey Staff, 1973).

These mollic epipedons on steep slopes may have formedas a result of high base status and somewhat restricted

Table 3—Percentage of soil mapping unit in each geomorphic surface.

unit 1

ABCDEFG

89.8(11)000000

Geomorphic surfacefII

10.2 (3)100.0 (7)

00000

III

0000

85.7 (23)5.3(2)

0

IV

00

0.5 (0)60.7 (7)13.7 (4)68.0 (17)

0

V

00

99.5 (10)39.3 (4)

0.6 (0)26.7 (7)

100.0 (8)

t Number of sites sampled on each geomorphic surface is given in parentheses.

drainage produced by shallow depth to rock. Soils of unit Chave considerable exchangeable Ca that probably is derivedfrom the calcareous cement of the underlying Bauru sand-stone. Water perches above the Bauru sandstone during wetperiods, although the soil morphology suggests that reduc-ing conditions are rare. The large amounts of Ca in thesesoils probably helps produce Ca humates that form waterstable complexes with clays (Kohnke, 1969).

Base saturation is indicative of soil fertility status. Thelarge variation in this characteristic among the soil mappingunits (Table 2) points out that generalized statements aboutsoil fertility and macroclimatic relationships are subject todrastic alteration by local conditions of parent material,relief, and surface age.

Soil Geomorphic Surface RelationsThe study area is composed of five geomorphic surfaces

and seven soil mapping units. Geomorphic surfaces I, II,and III cover most of the area of soil mapping units A, B,and E respectively (Table 3). Geomorphic surface IV en-compasses most of both soil mapping units D and F andgeomorphic surface V encompasses almost all of the areamapped as soil units C and G.

Table 4 lists the average coefficient of variation for a fewsoil properties when the random soil samples were groupedboth by the seven soil mapping units and by the five geo-morphic units. In most cases, the variation of soil character-istics for both 0-20 cm and 60-80 cm soil depths was higherwhen the samples were grouped by geomorphic surface.Grouping soils by their morphological characteristics

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

Table 4-Average coefficient of variation of selected soil properties by • AVERAGES FOR SAMPLES W I T H I N INDICATEO SOIL MAPPING UNITS

mapping units and geomorphic surface. G „,

____0-20 cm depth________60-80 cm depth____ Q '-° " «Soil mapping Geomorphic Soil mapping Geomorphic z " D g [~~|

Soil property unit surface unit surface § 60 • * o • z 0.8 •———————————————————————————————————————————————— 5 « B o . ,_, U A

———————————————— % ————————————————— a ' ~1 n* J E B flpH 4.7 4.8 6.9 8.2 < 40 ' S M S 0'6 ' F g, [W AApH 31.4 28.9 38.6 40.0 1C £ " •Organic carbon, % 32.6 40.0 - - m 20 • * 171 ^ 0.4 • fExchangeable Cat 73.3 79.6 70.9 87.2 S • E _•_Base saturation, % 23.6 51.2 56.6 70.7 v? r*i M ICEC/lOOgcIayf - - 29.6 28.6 ' V 'J V V ' | ' ' V ' |V' II II IClay,% 32.4 33.9 31.0 22.4

fmeq/ lOOg. lo £ g 60r- J°* > °

yielded populations more homogeneous than grouping soils ° os . G E ji A 40 - «according to geomorphic surfaces. 2 F * * o ' • fl

Generally, soil characteristics more related to soil fertility ^ o.e • F • U so • D [Jwere the ones having more variability when the two sample = * 3 " F *groupings of Table 4 are sompared. Base saturation, for in- 2 °'4" f5! ° Z° * [*| H B ,*,stance, was a property that snowed one of the highest dif- H o * , ' H Mferences in the average coefficient of variation. °'2 — - — — jjj —^— 7 - I— - —y—LJ—U—LL

Figure 5 shows relationships between age of surface andsoil properties. A similar method was used by Daniels et GEOMORPHIC SURFACE - INCREASING TIME ——al., (1970) to illustrate relationships among coastal plain Ffe- 5-Relationship between geomorphic surface and (a) average

., , , . ,- • ILT i ,-. i- TU- base saturation in the 60-80 cm depth; (b) organic carbon in the 0-20soils and geomorphic surfaces in North Carolina. In this cm depth. (c) textural ratio (% clay 0-20 cm/% clay 60-80 cm), andpaper, where only relative ages between surfaces are (d) apparent clay CEC for 60-80 cm depth.known, bar graphs are preferred to line graphs since the lat-ter implies a knowledge of time interval and a continuous CEC values. This is because surface V cuts both highlychange between the soils of the geomorphic surfaces. weathered sediments (sandy loam surficial deposits) and

Base saturation varies with surface age (Figure 5) but fresh Subarkosic Bauru sandstone. Younger surfaces, espe-there is no regular increase or decrease of this property with cially erosional surfaces that expose a variety of material,time. The younger surfaces (III, IV, and V) show a ten- are expected to have greater variation in soil properties thandency for decreased base saturation but an older site, like old depositional surfaces. Advanced weathering tends tosurface II, has base saturation values as high as the average overcome some of the differences in original materialsfor the youngest site. These data suggest that under the If we assume an approximately equal time interval be-prevailing climatic conditions, there is a tendency for bases tween the surfaces, the graph of apparent clay CEC valuesto be leached from the soil profile but there are some posi- indicates that the change of this property in time is curvihn-tions on the landscape where the combination of stra- ear- This Pr°Perty reaches a somewhat constant value ontigraphy and geomorphology combine to reverse the trend. surfaces III to I pointing out that after a certain time theThe exception to the rule of increased leaching with time weathering of soil minerals reaches a stage where sub-may be either the influence of lateral moving water enriched sequent changes proceed at a much slower rate than duringwith bases or to a difference in parent material since a single the first stages of soil development,geomorphic surface may cross more than one kind of sedi-ment.

The textural ratio, the 0-20 cm clay content divided bythe 60-80 cm clay content, has no regular increase or de-crease with surface age (Fig. 5). The very young soils ofsurface V have, on the average, identical textural ratios tothe soils from the oldest sites. In this area, the degree ofclay translocation cannot be used as an indication of soil ageas it has been sometimes considered elsewhere.

Organic carbon contents in the A horizons are not relatedto surface age (Fig. 5b) although there is a trend toward anincrease in carbon from the youngest to the oldest surface.Probably carbon contents are more related to environmentalconditions such as drainage, vegetation, and soil texturethat are unrelated to soil age..

Figure 5d illustrates how apparent clay CEC decreases assoil age increases. The graph points out that more soil varia-tion is expected on younger than on older surfaces. SurfaceV, the youngest surface, contains three different mappingunits which show a considerable range in apparent clay

LEPSCH ET AL.: SOIL-LANDSCAPE^N THE OCCIDENTAL PLATEAU OF SAO PAULO STATE, BRAZIL: II. 109