ELSEVIER Geoderma 76 (1997) 263-283

GEODEI~MA

Properties, some criteria of classification and genesis of upland forest Podzols in Rwanda

E. Van Ranst a,*, G. Stoops b, A. Gallez c, R.E. Vandenberghe d J Laboratory of Soil Science. Unit'ersity of Ghent. Krijgslaan 281 ($8). B- 9000 Ghent, Belgium

h I-xzboratory nfMineralogy, Petrology and Micropedology. Uni~'ersity nfGhent. Krijgslaan 281 ($8), B-9000 Ghent. Belgium

Belgian Soil Map Project, B.P. 84. Kigali. Rwanda a Laboratory of Magnetism. Unirersity of Ghent, Proeftuinstraat 86. B-9000 Ghent, Belgium

Received 13 May 1996: accepted 22 January 1997

Abstract

In southwest Rwanda, upland forest soils developed on quartzites and micaceous sandstones along steep slopes have sometimes the macromorphological look of 'true" Podzols. An investiga- tion of the micromorphological, mineralogical and chemical properties, however, reveals only weak indications of illuviation of amorphous organic complexes. This process of cheluviation seems to be secondary relative to biological activity, Fe precipitation and weathering. The concept that cheluviation is the dominant process in the formation of spodic horizons and spodic materials would exclude these soils for the class of Podzols. Definitions of spodic horizon and spodic materials proposed in international soil classification systems, should give preference to macro- and micromorphological properties over chemical properties, because the former are directly related to different genetic processes (biological activity, oxido-reduction) which also could be responsible for their formation.

Keywords." Podzols; micromorphology: mineralogy: soil genesis: soil classification: Africa

I. Introduct ion

Podzols are mineral soils developed in coarse-textured and quartz-rich parent materi- als, and defined essentially as having a spodic B horizon (FAO-Unesco , 1990). They are found on all continents, but dominantly in the temperate and boreal regions of the Northern Hemisphere. A typical Podzol in these areas has a pale grey, strongly leached

• Corresponding author. Tel. 4-32 9264-4626: Fax: + 32 9264-4997: E-mail: eric.vanranst@rug.ac.be

0016-7061/97/$17.00 © 1997 Elsevier Science B.V. All rights re~rved. PII S0016-7061(97)00003-7

264 E. Van Ranst et al. / Geoderma 76 (1997) 263- 283

E horizon between a dark surface horizon with organic matter, and a brown to very dark brown spodic B horizon. Most Podzols have a surface litter layer of l to 5 cm thick, loose and spongy, and grading into an Ah horizon with partly humified organic matter. The Ah horizon consists of a dark grey mixture of organic matter and mineral material (mainly quartz) with many of the grains showing black isotropic coatings. The underly- ing bleached E horizon has a characteristic single grain structure. The structure of the brown to black Bhs horizon varies from loose through firm subangular blocky to very hard and massive (Driessen and Dudal, 1991).

Loose spodic B horizons have a predominance of pellets of polymorphic (discontinu- ous mass with variable colour and density) amorphous organic matter and aggregates, whereas coatings of monomorphic (continuous mass with uniform colour and density) amorphous organic matter prevail in the massive horizons (De Coninck et al., 1974). The loose horizons have many features suggesting major biological influences during their formation, viz.. high numbers of roots, thorough mixing of the organic units with elay and silt, and the presence of passage features. The massive horizons have features consistent with organo-metallic compounds immobilized in a gel-state, viz., the coatings are strongly cracked, indicating the transition of a gel into a solid: they contain large amounts of AI or AI plus Fe but very little or no Si. The two processes seem to operate simultaneously during the formation of spodic B horizons and their relative intensities determine the composition of each spodic B horizon at any moment in its evolution. As long as the biological activity predominates, the horizon remains loose. If the accumula- tion of mobile organo-metallic compounds starts to prevail, the horizon is gradually cemented and fossilized (De Coninck, 1980).

Podzols have limited occurrence in the tropics. They are rather uncommon in Africa. Tropical Podzols occur in two distinct ecological situations. The first one coincides with thick deposits of quartz sands in the lowlands; the second one is characterized by a cool isomesic temperature regime that prevails at high elevations in areas that have often been influenced by volcanic ash. The cool tropical Podzols are typical of mountain slopes that are hit by clouds producing orogenic rains (Barshad and Rojas-Cruz, 1950: Van Wambeke, 1991). Although most of the reports on Podzols in tropical areas refer to areas with udic or perudic soil moisture regimes (Klinge, 1965, 1969; Andriesse. 1968, 1969; Flexor et al., 1975), they are also known to occur in areas with ustic soil moisture regimes (Brammer, 1973: Schwartz, 1988). During the last decade, research into podzolic soils has been undertaken in the People's Republic of Congo (Schwartz et al., 1986: Schwartz. 1988), and Central Amazonia (Bravard and Righi, 1988. 1989; Righi et al., 1990). The soils studied in the Congo may well be Podzols, but some of those in Brazil are, most probably, albic Arenosols. although they contain a spodic horizon at great depth. It can be argued that many lowland tropical Podzols are in fact albic Arenosols in the FAO-Unesco Legend (FAO-Unesco, 1990), classified mainly on the basis of the E horizon, but the status of those in the mountains and on high plateaus is still problematic.

The soils discussed in this paper are part of a toposequence developed on quartzites and micaceous sandstones in the Nyungwe forest, in the southwest of Rwanda. All the pedons have a well developed bleached E horizon. Organic matter appears in various stages of decay: distributed throughout the surface horizons of all the soils, and as dark

E. Van Ranst et aL /Geoderma 76 (1997) 263-283 265

organic stains between and probably around sand grains in the Bhs horizon, especially of the soils of the lower slope. The upper part of this B horizon has the aspect of a spodic B horizon and shows clear evidence of Fe accumulation, even under the form of discontinuous Fe-pans.

The major objectives of this study are to investigate the different properties and to explain the genesis of these soils, more particularly of the horizon with a spodic field morphology.

2. Materials and methods

2.1. Encironmental conditions, .field description and sampling

The pedons chosen for this study are located in the Nyungwe forest (Fig. 1), situated in the southwest of Rwanda between latitudes 2°17 ' and 2°49 ' south and between longitudes 29005 ' and 29029 ' west. This forest occupies an area of 95,000 ha in a mountainous landscape with steep slopes, separating the Nile and the Zaire rivers. The vegetation between 1900 and 2250 m altitude is characterized by Entandrophragma excelsurn, Parinari excelsa, Prunus africana and Octotea usambarensis. The soils studied have developed on quartzite and micaceous sandstone. The annual rainfall in the

~ ~ ~ . . . ~ "I" ~?,.":~"-~ :i :..

t ~"'~"~~'

UGANDA T A N Z A N I A

° , ° , ° - 1 °30' R u h e n g e r i B y u m b a •

i ZAIRE Th~ma ,-,

KIGALI ®

K i b u n g o

Nyanza

Butare TANZANIA

29*00 ' I

o

BURUNDI 30°00 . 30=30 . j ,'

Fig. I. Location of the Nyungwe forest in Rwanda.

5 0 k m J

266 E. Van Ranst et al. / Geoderma 76 (1997) 26.7-283

UMWONGO 0 100m (2165m, 2%)

o ' ~ +d ~'

RWAMASAK A 0 (2140m, 4 2 ~ ~ 22

I / ~,m/&~l;~;~c'~A~O~ -11 + * -

sot. J / z -'- " I ~ - , s

/ -'--=--t-26 ??/++,+, / I " + + + + +

/ . I ++ ++-~o / Cu.AGA + +1

, l l (2065m. 60%) + I .= -9 ~ 65 /

P + ++ .I / ~ " ' ~ - ; 2 + + + + i / MANJE I + + +

12040m. 65°1.) 120cm

~ 70era ~ ~ 4 "'(//+'//I

::*/Uzy/~l L E G E N D HORIZONS ~ , 2 LT.~' +.+? : +'./+1 o + + BOUNDARY

• . 72 79 FLOCK

" ' : FRAGMENTS [ - ~ Bhs

110cm IRON PAN ~ B s

Fig. 2. Location of the soil pedons and idealized diagram showing the relationships among the morphology of pedons, altitude and geomorphology.

area ranges between 1800 and 2500 mm and the mean annual temperature is between 15 and 17°C.

Profile descriptions were made in detail from pits along a toposequence (Fig. 2) according to the FAO guidelines for soil profile description (FAO, 1977) and routine soil analyses of the different soil horizons were carried out in the laboratory of the +Belgian Soil Map Project" in Kigali, Rwanda. Some field characteristics, selected physico-cbemical properties and the classification of the soils along the toposequence are given in Table 1. In the Podzol soils (Curaga and Manje series), marked maxima of C, N, and cation exchange capacity occur in the B horizons relative to the other mineral horizons. Exchangeable basic cations are low and the pH-dependent exchange capacities of the B horizons are high, as indicated by the difference between CEC at pH 7 (values somewhat lower than those measured at pH 8.2 as specified for spodic horizon) and the sum of exchangeable basic cations plus AI. Organic matter in these B horizons has high C / N ratios (20 or more), indicative of low biological activity and a slow process of degradation of the organic materials.

E. Van Ranst et al. / Geoderma 76 (19971 263-283

Table 1 Partial description, some properties and classification of the soils along the toposequence "

267

Series/ Depth Colour Texture (%) Organic N C/N pH (1:2.5)

C Adsorption complex

horizon (cm) (moist) clay silt sand (g/kg) (g/kg) H,O KCI (cmol(+)/kg) basic x" AI CEC cations at

pH 7

Umwongo: (2°29'I"S. 29°05'35"E) Sam(v skeletal, siliceous, isothermic l~vpic Humitropept O II-0 - 490.5 20.90 24 3.3 2.1 5.30 3.6t) 116.7 Ah 0-22 7.5YR2/I 3 21 76 37.6 2.00 19 3.4 2.5 0.19 0.80 9.0 E 22-45 10YR5/2 1 29 70 2.7 0.14 19 3.9 3.2 0.01 0.28 0.7 Bs(h)/R 45-80+ 2.5YR 5/4 2 27 71 11.7 n.d. n.d. 4.4 4.0 0.04 1.63 8.0

Rwamasaka: ( 2°29'13"S, 29°05'38"E) Coarse loamy, siliceous, isothermic Oxic Dystropept O I I-0 - - 3 2 0 . 6 13.65 24 3.4 2.6 15.00 0.65 68.0 Ah 0-26 7.5YR2/I 2 19 79 19.6 (I.95 21 3.6 2.7 (I. 18 0.43 4.5 E 26-65 7.5YR5/1 4 20 76 2.7 0.32 8 4.0 3.3 0.01 0.05 1.0 C/R 65-12(1+ 7.5YR6/I 3 28 69 1.2 n.d. n.d. 4.4 4.1 0.01 0.04 0.4

Curaga: (2°29'16"S, 29°05'43"E) Coarse loamy over loamy-skeletal, siliceous, isothermic Typic Haplohumod O 9-0 - - 3(15.8 15.33 20 3.5 2.2 5.90 1.09 89.3 Ah 0-12 7.5YR 3/'1 3 19 78 18.0 0.95 19 3.5 2.6 0.13 0.46 4.3 E 12-23 7.5YR6/1 2 17 81 3.9 0.30 13 3.9 3.2 0.06 0.19 1.7 Bhs 23-32 7.5YR 2/2 11 22 67 68.2 2.80 24 3.8 3.1 0.22 5.17 38.0 Bs 32-70 7.5YR5/8 7 22 71 37.2 1.78 21 4.2 3.4 0.16 2.27 32.0

7~*pic Placarthod 6.90 39 4.2 3.1 18.30 0.02 60.9 2.00 13 3.9 3.1 0.18 1.19 9.2 1.16 15 3.9 3.1 0.11 0.91 3.7 0.18 13 4.2 3.3 0.02 0.06 1.1 1.41 20 4.0 3.1 0.20 1.29 21.0

Manje: t2°29'14"S, 29°05'46"E)Sam(v, siliceous, isothermic, O 8-0 - - 265.8 Ahl 0-14 7.5YR3/2 2 15 83 25.9 Ah2 14-42 7.5YR 2/I 4 19 77 17.6 E(g) 42-72 7.5YR 6/ I 2 14 84 2.3 Bhs 72-79 7.5YR 2/ I 4 33 63 28.4 (+Fe -pan) Bs 79-110+ 5YR4/8 3 35 62 24.2 1.19 20 4.1 3.4 0.16 1.72 17.9

Analyses carried out in the Laboratory for Soil Analysis of the Belgian project 'Soil Map of Rwanda" in Kigali, Rwanda. The soil classification names proposed by the project are based on Keys to Soil Taxonomy (Soil Survey Staff. 1990).

For the purpose o f this study, only the B and over ly ing hor izons of the Podzol soils

(Curaga and Manje series) were sampled for more specif ic mineralogical and chemical

analysis; undis turbed samples from these hor izons were taken with the aid of KubiEna

boxes for mic romorpho log ica l studies.

2.2. Micromorpholog ica l analyses

Oriented, undis turbed samples were air-dried and impregna ted with polys tyrene.

M e d i u m size thin sect ions (60 x 90 m m ) were made for direct study with the polar iz ing

microscope . Small (28 × 48 m m ) uncovered thin sect ions were used for select ive

268 E. Van Ranst et al. / Geoderma 76 ¢ 1997~ 263 - 283

extraction tests, applying the methods proposed by Arocena et al. (1989) for iron, and Babel (1964) for organic matter. Descriptions were made using the terminology pro- posed by Bullock et al. (1985).

2..¢. M i n e r a l o g i c a l ana lyses

X-ray diffraction (XRD) patterns of air-dried, parallel oriented silt and clay samples, separated by repeated dispersion using a Na_~CO 3 solution and sedimentation, and treated with dithionite-citrate-bicarbonate (DCB), were obtained with a Philips diffrac- tometer using Fe-filtered Co-K~ radiation.

Mi3ssbauer spectra of the fine earth were collected on a conventional constant-accel- eration spectrometer using a 57Co source in Rh matrix. Velocity calibration was obtained with a hematite absorber. All spectra were fitted taking into account distributions of hypeffine fields for the sextets and distributions of quadrupole splittings for the doublets (Vandenberghe et al.. 1994).

2.4. Chemica l ana lyses

Fluoride activity was determined after treating I g of soil in a plastic beaker with 10 ml 1 M NaF; the alkalinity developed by the reaction of F with the soil was titrated with HCI to pH 7 (Fieldes and Perrott, 1966: Hetier, 1969). The same treatment was used on samples from volcanic origin.

The total Fe content (Fe t) in the fine earth fraction was determined eolorimetrically with sulfosalicylic acid, after treating 100 mg of sample with HF + HNO 3 + HCIO~ until the solution was clear, and dissolving the evaporation residue in concentrated HCI (Ingamells, 1966; Omang, 1969).

Fe and AI were extracted from fine earth samples using ammonium oxalate-oxalic acid (OX), and dithionite-citrate-bicarbonate (DCB) as extractants. The OX treatment was carried out in the dark at 25°C for 4 h (Soil Conservation Service, 1984) with 0.2 M acid oxalate solution at pH 3.0, using a sample/solution ratio of 500 mg/100 ml. The DCB treatment (Mehra and Jackson, 1960) was carried out with solid Na-dithionite in a solution of citrate and NaHCO~, buffered at pH 7.3, using a sample/solution ratio of 500 mg/500 ml.

Fulvic (FA) and humic acids (HA) were extracted by repeated treatment with 0.1 M Na.~P_,O 7 and some Na~S0~ until the extract was completely colourless. The HA were separated from the FA by acidifying the extract with H_,SO 4 (pH 1.5) lbllowed by centrifugation. The precipitated HA werc redissolved in 0.1 M Na.~P20 7 (Righi, 1977). The sum of FA + HA is indicated as total extractable carbon (TEC). Humin is defined as the difference between total organic carbon and the sum of fulvic and humic acids (OC-TEC). The clear centrifugates were analyzed for Fe and AI by AAS.

3. Results and discussion

3.1. Micromorphoh>gica l p roper t i e s

The coarse material of the groundmass in both profiles, Curaga and Manje, consists of angular grains of quartz (dominantly medium sand size. but also coarser) and

E. Van Ranst et al. / Geoderma 76 (1997) 263-283 269

" 1

- .

.~. ,..,.. ,~, ~ , ~. 6 .

Fig. 3. Intergrain micro-aggregate microstructure in Ah horizon of the Curaga profile. Partially crossed polarizers.

fragments of quartzite and micaceous sandstone (up to 3 mm). In the upper horizons many very fine (< 30 txm) spherical phytolithes are noticed. In the Manje profile abundant mica flakes (50 rtm) occur throughout.

The Ah (Curaga) and the Ahl (Manje) horizon have a spongy to granular (peds of about 150 la, m in diameter) microstructure (Fig. 3). The micromass consists of a dark brown, dotted, mainly organic material with undifferentiated b-fabric. The c / f related distribution is single spaced enaulic (when the c / f limit is put at 50 I, zm). Several organic residues occur.

The underlying Ah2 horizon in the Manje profile has a similar microstructure, but the aggregates are less regular. Also the micromass is comparable, but somewhat darker. Many black-stained plant fragments (tissue and cell residues) occur. Selective extraction with Na-hypochlorite lightened the brown colours, whereas oxalate extraction had practically no effect, pointing to an organic nature of the colouring substance in this horizon. Porous rock fragments show clear intermineral impregnations with iron.

Fig. 4. Close porphyric c / f related distribution in E horizon (left) with l~x:al monic zones (right). pointing to a kxzalized leaching. Curaga profile. Polarized light.

270 E. Van Ranst et al. / Geoderma 76 (1997) 263-283

f

t

,],z: ~,'~,.: "~

Fig. 5. Rock fragment (below) with impregnatix, e capping (hypocoating) of isotropic organic matter. I- horilon of Curaga profile. Polarized light.

The E horizon of both profiles is quite different from the overlying horizons: the vughy microstructure with a few channels is much more compact, which is also expressed by a close porphyric c / f (limit at 30 ~Lm) related distribution. Only few spots

Fig. ¢~. Fe-pan in Manje profile. Polarized light.

E. Van Ranst et al. / Geoderma 76 (1997) 263-283 271

Fig. 7. Microstructure consisting of loose packing of angular micropeds in Bhs horizon. Polarized light.

of monic c / f related distribution point to a localized leaching (Fig. 4). The micromass is speckled and composed mainly of greyish brown fine sericite flakes, giving rise to a crystailitic b-fabric. Coarse rock fragments are capped by a thin (400 p,m) hypocoating of darker material (Fig. 5). In these hypocoatings small channels show crescent-like layered coatings of very dark, almost opaque, isotropic material. A few void coatings of coarse clay and fine sericite also point to a translocation of micromass. These coatings are quite often covered by a thin layer of opaque (humic) material. The boundary between the E and the Bhs horizon is sharp and undulating.

The Fe-pan in the Manje profile is a thin (2.5 mm) darker layer (Fig. 6) consisting of a diffuse clark brown impregnation of the groundmass (similar to that described above). Locally, its upper boundary can be followed through porous rock fragments present.

The Bhs horizon of both profiles consists of a packing of angular aggregates (Fig. 7) of about 250 p.m, composed of small spherical subaggregates, 50-70 Ixm in diameter. Their micromass is dark reddish brown with undifferentiated b-fabric. Bare quartz grains occur, giving rise to a single to double spaced enaulic c / f related distribution pattern, locally welded to a porphyric one. Some tissue residues are observed. In the same material, discontinuous subhorizontal bands of limpid, isotropic reddish material, 50 I~m thick, are observed, together with many roots and a few bleached zones showing mainly a monic c / f related distribution.

The Bs horizon of the Curaga profile has an intergrain microaggregate to granular microstructure with reddish brown limpid pellets, 50-200 lJ.m in diameter, with undifferentiated b-fabric, except for some sericite splinters. The pellets sometimes have a concentric fabric formed around a nucleus of darker material. These pellets are better expressed in the highest part of the horizon. After Na-hypochlorite treatment the fine material becomes lighter and a slightly reddish colour appears, which is not removed by the ammonium oxalate treatment. Rock fragments, frequently show yellowish red isotropic infillings of fractures (Fig. 8). After ammonium oxalate treatment (Arocena et al., 1989) these become lighter, but more reddish, indicating that they consist of weakly crystalline iron oxyhydrates and some hematite.

272 E. Van Ronst et M. / Geoderma 76 ¢1997~ 26.¢-28.?

; ; i { . 7 " : . ~ , . : ~ : 5oo ~.

Fig. 8. Quartzite fi'ugments with intermincral iufillings of iron oxyhydrates {left) in Bs horizon. Polarized light.

Although the Bs horizon of the Manje profile has a spongy to vughy microstructure with a double spaced, welded enaulic c / f related distribution, it also shows similarities with the same horizon in the Curaga profile, as the micromass is similar, and R)rms spherical subaggregates of 40 ~m in diameter. The brownish colour is bleached only partly, even after a prohmged Na-hypochlorite treatment, and even the DCB treatment does not remove it completely. Locally. spots of reddish limpid isotropic material occur as interstitial coatings and infillings. Rock fragments present in this layer do not show iron impregnations.

The most active process in the topsoil seems to be the formation, by biological activity, of pellets, partly composed of organic material, resulting in an enaulic c / f related distribution. The so-called E horizons are much denser, less porous and still contain a high amount of fine material (especially coarse clay and micaceous silt) as indicated by the porphyric c / f related distribution; only locally some evidences of leaching are observed (monic c / f related distribution) (Fig. 4). Hypocappings of dark isotropic (humic) material on top of gravels (Fig. 5) points to a vertical translocation of material. A limited translocation of coarse clay and silt has also taken place, as indicated by the presence of coatings and infillings. This process has been followed by a limited vertical translocation of black organic material. The Fe-pan (Fig. 6) is very thin (2.5 ram) and intersects porous rock fragments. In the Bs and Bhs horizons the accumulation of organic matter as pellets is still pronounced, indicating probably a high biological activity. Some illuviation of limpid reddish isotropic material (probably, a type of ferrihydrite) is noticed. No monomorphic organic material was observed, but the Bhs and Bs horizons bear characteristics of loose spodic B horizons.

3.2. Mineralogical properties

The qualitative mineralogical composition of the silt and clay fractions is rather similar in all the horizons studied. Both fractions contain mica. kaolinite, quartz and traces of feldspars. Traces of amphiboles are only detected in the silt fraction. In the clay fraction (Fig. 9), mica seems to be more abundant in the E and the Ah2 horizons, while

E. Van Ranst et al. / Geoderma 76 (1997) 263-283 273

CURAGA

Bhs

B s ~. ~

t

O

I I I I l 35 30 25 20 15

0 - 2pro M A N J E

L t I I

10 6 3 ° 2 O ( C o - K c 0

o

Ah2

Bhs ~

B s ~ ~ " ~ o

1 l I I I I [ I 30 25 20 15 10 6 3

Fig. 9. X-ray diffractograms of the clay fraction after DCB treatment

in the Bhs and Bs horizons the shoulder of the 1.0 nm peak towards the low angle side suggests a more advanced transformation of the mica layers. The E horizon of the Curaga profile contains also more kaolinite compared to the underlying B horizons.

The more advanced transformation of the mica layers in the clay fraction of the B horizons, compared to the overlying E (and Ah2) horizon, is in contradiction with what is normally found in true Podzols. In general, E horizons are more weathered than B horizons, and untransformed primary phyllosilicates usually are absent from the E horizon clay. The relatively rapid weathering in the E horizon of true Podzols is thought

Table 2 Alkalinity (cmol kg- ' soil) developed by treatment with I M NaF

Series/horizon Alkalinity (cmol~ kg- i)

Cltraga Bhs 38 Bs 49

Manic Bhs 13 Bs 27

Volcanic soil, Chili 1460 Voh'anic soiL Chili 1438 Volcanic soil. Chili 1043 Volcanic soil Chili 1301

274 E. Van Ranst et al. / Geoderma 76 (1997) 263-283

' I J

RT

80K CURAGA BS

RT

80 K MANJE Bhs 80 K MANJE Bs I

• 10 4 4 -4 -2 0 2 4 6 $ 10 -10 4 -6 .4 -2 0 2 4 6 s 10 -10 .e -6 -4 -2 0 2 4 6 e 10 v ( ram/s) v (mm/ I ) v ( t ony . )

Fig. 10. MOssbauer spectra of the untreated fine earlh of different B horizons.

to be the result of the continuing removal of weathering products from mineral surfaces by the complexing action of organic solutions. The common difference in the extent of clay mineral weathering in the E and B horizons of true Podzols is also thought to be due in part to the fact that mineral surfaces in the B horizon are presumably coated with sesquioxide-organic complexes and thus protected from weathering (McKeague et al.. 1983).

An admixture of volcanic ash in the B horizons of the soils studied seems unlikely, because compared to typical volcanic soil samples from Chili, the alkalinity (Table 2) developed by treating the B horizons with 1 M NaF can be neglected.

With respect to the Fe-bearing constituents, the M~ssbauer spectra (Fig. 10) and associated hyperfine parameters (Table 3) reveal mainly goethite in all B horizons, with only a small amount of hematite (5-8% of Fe in hematite) in the B horizon of the Manje profile. Some Fe(II) species are observed (around 5% of Fe). All the hematite spectra exhibit asymmetric lines at room temperature (300 K) and have lower hyperfine fields than expected (Hht = 516 kOe). Moreover, at lower temperature (80 K) all spectra show the features of a weakly ferromagnetic phase ( 2 ~ o - - - 0 . 2 ) . All this points to a relatively poor crystallinity of the Fe oxide minerals and an estimated particle size smaller than 20 nm (Vandenberghe et al., 1990). The goethite spectra at room tempera- ture exhibit similar features for all B horizons. They consist mainly of a large amount of superparamagnetic doublet and a partly relaxed and/or split spectrum inferring a relatively poor crystallinity. At 80 K, the goethite lines are still asymmetrically distributed and therefore it is expected that a large part of the remaining doublet at 80 K will arise from superparamagnetic goethite. However, the most probable value of A k,'op is about 0.6 mm/s at room temperature, pointing to superparamagnetic goethite, it is observed that the average quadrupole splitting (AEQ,,) is significantly higher. This might be an indication that the Fe(III) doublet contains a larger traction with higher quadrupole splitting. Although no spectra were recorded at temperatures lower than 80 K. this could be a very strong indication for ferrihydrite, because Fe-organic complexes

I;. I.%m Ranst et ul. . / Geoderma 76 ( I 9971 26.?-28.?

T~ble 3

Hyperf ine parameters of the Miissbauer spectra of the untreated fine earth of different B horizons

275

S e r i e s / T H.,, HI, 2,E~ 6 ~ k.'~.,, 3 k.~){, RA Ass ignment

horizon (K) (kOe) (kOe) ( r a m / s ) ( t u r n / s ) ( r a m / s ) ( r a m / s ) (q.¢)

('ttra.~,,a 300 - 0.36 0.72 0.62 I(X) D Goethite ( S P ) +

t:erri h.', drite (P)

B , ,~0 357 474 - 0.27 0.46 25 S (;oethite ( A F ) -

ferrihydrite (relax)

-. ().47 0.74 ().(',(} 75 D (;oethite ( S P ) +

fi.'rrih', drite (SP)

Ma;(ff" 300 478 407 0.2 0.39 - 8 S Hematite (WF)

Bh~. 236 298 .I).25 0.4 20 S Goethite (AF

relax)

- 11.37 0.74 0.61 57 D Goethite ( S P ) ,

Icrrih5 drite (P) - I .II 2.83 3.()e, 6 D Fc- '" doublet

80 531 529 - 0.2 0.46 - 5 S Henlatite (WF)

41 ~ 490 - 0.22 0.47 - 42 S ( ]oe th i te ( A F )

- ().4,~ 0.7g 0.61 47 I) Goethite ( S P ) +

ferrih?drite (SP) 1.21 2.94 3.27 5 D Fe z- doublet

,k/a.,Ue 300 494 496 0.2 0.38 - - 8 S Hernatite (WF)

B,. 245 337 0.28 0.37 - 37 S Goethite (AF 4-

re lax )

0.36 0.70 0.59 51 l) Goethi te ( S P ) +

fcrrihydrite (P)

- - 1.06 2.g9 3.13 4 D FeZ - doublet

81) 530 527 - O . 2 0 O.4g - - 0 S Hematite (WF)

421 4;49 - 0.22 0.46 - 51 S Goethite (Ak)

- - 11.48 0 7 8 0 7 2 39 D Goethite (SP) +

ferrihydrite (SP)

1.24 2.(.)7 3.30 5 D Fe z- doublet

H.,, = average hyperfme field: H e = most probable hyperfine field (peak value): 2 ~ = q u a d r u p o l e shift;

6 = i~,omer shift relative [o metallic Fe; ~/'.~),~, = a v e r a g e quadrupole '~plitting; A/z~) i ,=most probable

quadrupole splitting (peak value): RA = relative area: S = sextet. D = doublet: W F = weakly ferromagnet ic

pha~,e; A I r; = a n t i f e r r o m a g n d i c : P = r, a ramagnet ic : SP = superparamagnet ic .

generally have lower quadrupole splitting of the order of 0.5 m m / s (Schwertmann and Murad. 1988). However. the presence of minor amounts of Fe-organic complexes cannot be excluded. In the Bs horizon of the Manje profile, the average and most probable value is relatively high at lower temperature ( ~ 0.72-0.78 ram/s) inferring a strong presence of the ferrihydrite doublet. This is especially true for the Bs horizon of Curaga. where the spectrum consists mainly out of this ferrihydrite doublet. A ferrous doublet ('~ 5c/c Fe) in addition to the ferric one, apparently consists of a double bumped distribution, indicating the presence of Fe(ll) in two different positions in the lattice of silicate minerals.

276 E. Vim Ranst et al. / Geoderma 76 t 19971 263- 2,%'3

Table 4

Composi t ion ol the organic matter, and Fe and AI contents (in ~,; ) of the analyzed horizons

S e r i e s / 0(.7 TEC FA HA t tumin Fc, Fei Fe,. Fep Al<i AI,. AI r

horizon (c.;.) (</, OC) (<,~ OC) ('A OC) ('~,~ OC')

('ll?'tl~gl I- 0.39 18 Io x 82 I).22 o.07 li.{(2 II.02 (1.03 o.02 !).02

Bhs 6.82 58 41 17 4Z 4.1X 3.90 2.73 (I.85 0.20 0.16 0.0¢~

B~, 3.72 48 41 7 52 4.04 3.92 3.58 1.26 0.31 0.25 11.13

Ah2 2.59 21 4 17 74 0.48 0.42 0.1 I ().13 0.05 0.04 0.05

Bhs 2.:44 56 44 12 44 2.7(I 2.37 1.27 1.1)5 O.l() 0.08 0.09

B~, 2.42 56 45 I I 44 3.90 3.48 150 I.(M 0.14 1).0~.~ (I. lO

(.)(" = total tlrganic carbon: T E ( ' = organic e a r n s extractable in p.~ropho~,phale: FA = ful,.ic acid:,: H: \ =

humic acid~,: t = torah d = dithionile-citrate: o = a m m o n i u m oxalute: p = pyrophosphale .

The Mi~ssbauer spectra indicate a remarkable difference in the amount ot very poorly crystalline Fe oxides (goethite and especially ferrihydrite) between the studied B horizons. These constituents seem to be more abundant in the Bs horizon of the Curaga profile (75c~ Fe). compared to the B horizons of the Manic profile (47ek Fe in the Bhs and 37c~ • Fe in the Bs horizon). This difference, together with the occurrence of some hematite in the B horizons of Manje. points out a somewhat drier pedoclimate in the Manje subsoil, compared to the Curaga subsoil, and probably suggests a lateral downslope movement of Fe. The high goethite-to-hematite ratio in the B horizons of the Manje profile, however, still indicates a relatively important soil moisture activity (Schwertmann and Taylor. 1989).

3.3. Chemical properties

Table 4 gives some data on the organic matter, total Fc (Fct). and Fe and AI extractable in DCB (Fe d. Al<l), OX (Fe,,, AI,,). and Na-pyrophosphate (Fei,. Alp). Much (48 to 58%) of the OC of the Bhs and Bs horizons is alkali-extractable, and it consists predominantly of fuh'ic acids. These amounts are quite similar to the amounts found in weakly developed and loose spodic B horizons in temperate regions, having predomi- nantly pellets of polymorphic amorphous organic matter, but considerably lower than the amounts extracted lrom cemented spodic B horizons (De Coninck. 198(I). However. Higashi et al. (1981) found that high DCB-extractable Fe may reduce the extractability of organic carbon.

The absence of monomorphic amorphous organic matter in the studied B horizons may suggest in-situ formation ot + fulvic and humic acids by biochemical breakdown of root remains. As this process occurs not only at the soil surface+ but wherever roots are present, these organic compounds are formed not only in the litter layer, but also inside the mineral soil itself. In-situ transformation of root remains in the B horizons probably explains the high humin content or the complete fraction insoluble in Na-pyrophosphate. These organic acids, formed in the B horizons, are probably responsible for the more

l-. Van Ranst et al. / Gee>derma 76 ¢ 1997) 263-2,~¢3 277

advanced transformation of the mica layers in the clay fraction of these horizons, compared to the overlying horizons.

Marked maxima of extractable Fe and AI occur in the B horizons relative to the overlying mineral horizons. The different Fe and AI fractions seem to increase with soil depth, and no significant correlation exists between the Fe or AI amounts and total organic carbon or extracted organic carbon. In true spodic B horizons, the highest Fe and AI concentrations, especially of the mobile fractions (Na-pyrophosphate and OX). are expected in the Bhs horizon, relative to the other mineral horizons.

Pyrophosphate extracts have been evaluated by numerous workers (Jeanroy and Gt, illet. 1981; McBride et al.. 1983; Schuppli et al.. 1983: Valeriano Madeira and Jeanroy. 1984; Kassim et al.. 1984; Schwertmann and Murad. 1988). These studies mainly tound that, besides organic-Fe, pyrophosphate suspensions contain solid parti- cles. identified as ferrihydrite or even as goethite. Thus pyrophosphate not only extracts Fe-organic complexes but also peptizcs solid particles of Fe oxides, the relative proportions of which may vary. Pyrophosphate thus may not be selective for organic complexes of Fe (Farmer ct al.. 1983).

The OX and DCB methods are less problematic than the Na-pyrophoshate one. This is particularly true for DCB (Fe,~), which dissolves essentially all Fe bound in secondary oxides. Besides organically bound Fe. OX (Fe,,) extracts a certain fraction of the total Fe oxides which is reasonably well documented as being the group of ferrihydrite minerals (Schwertmann. 1959). The Fe,, amounts (Table 4) in the B horizons are considerably higher in the Curaga soil. compared to the Manje soil. which suggests a higher amount of ferrihydrite minerals in the Curaga soil. Fe,~ or Fe,,-Fe o. however, should not be equalled numerically with fcrrihydrite-Fe (Parfitt, 1983: McFadden and Hendricks. 1985: Shoji et al., 1988), because, on the one hand. some poorly crystalline goethite also may dissolve (particularly if 4 rather than 2 h of extraction are used), and on the other hand, pyrophosphate may extract part of the ferrihydrite. Although OX is also not fully specific, the method may be considered as suitable to extract the sum of organic-Fe and poorly crystalline Fe oxides. Because the goethites in the studied B horizons are poorly crystalline (Table 3). it is likely that they are associated with ferrihydrite. It has been stated that the occurrence of ferrihydrite in spodic B horizons is indicated by an often high ( > > 0.5) Fe,,/Fe d ratio (Blume and Schwertmann, 1969: Evans and Wilson. 1985). The relative higher Fc, amounts and Fe,,/Fe,j ratio (0.7 in the Bhs horizon and 0.9 in the Bs horizon) in the Curaga soil, compared to the Manje soil (ratio 0.5 in the Bhs and 0.4 in the Bs horizon), suggest that more poorly crystalline Fe oxides are present in the B horizons of the Curaga soil. which is confirmed by the Mi.issbauer spectra. This difference in relative amounts seems also to be indicated by the difference in F activity (Table 2). Poorly crystalline oxides will have a higher reactivity, and develop more alkalinity upon treatment with I M NaF.

3.4. Genesis processe.s in the toposequenc'e

The evolution in this toposequence is primarily based on the principle of reduction of free Fe(lll) compounds in the top soil. Soil fauna living in soils needs a source of energy for its development; it obtains this by oxidizing organic residues. The oxidans used for

278 E. Van Ranst et al. / Ge<>derma 76 < 1997~ 263-283

these biochemical reactions has to be present in the environment. If O~ is available in sufficient amounts, this strong oxidans is used preferentially. If. however. ()~ is not available, another oxidans has to be used: the high amounts of free Fe(llI) compounds present in many soils are read}, ti>r this reaction.

The absence of 0 . does not need the presence of some kind of watertable, but can be due to other [hctors: the presence of a top layer either preventing penetration of the air into the soil. or completely consuming the 0 2 present, e.g. by organic matter.

Field observations indeed show that in many places Fe compounds are removed out of the top soil without the presence of a watertable. This Fe is precipitated again deeper in the soil and can form an impervious horizon, on top of which a secondary perched watertable is formed.

Based on this principle, the genesis of the soils in the toposequence can be explained as follows:

(1) Micas and other silicates, especially Fc-Mg silicates are weathered with synthesis of kaolinite and precipitation of tree Fe(III) compounds. Comparable ratios of intensity of kaolinite and micas on XRD throughout the horizons studied suggest that clay migration previous to the weathering process is unlikely. The synthesis of kaolinite explains the low extractable amounts of AI. The fact that = 5c/, of the Fe compounds is in Fe(ll) form may be an indication that unweathered Mg-Fe(ll) silicates are still present.

(2) Due to the precipitation of free Fe(III) compounds a high porosity and stable structure develops, e.g. as expressed in an optimal form in Oxisols.

(3) Under the humid climate and the thick litter layer (up to I1 cm) a lack of O~ develops in the top of the mineral soil. This causes reduction of the insoluble Fe(Ill) into soluble ionic Fe(ll) compounds. These latter compounds migrate through the soil, both as a result of a concentration gradient and with moving soil water.

(4) When these soluble Fe(II) compounds arrive in an environment with higher redox potential, they are reoxidized and precipitate again as insoluble Fe(Ill) compounds. In the sequence typical forms of these reprecipitated compounds are the iron pan character- ized by a dense, opaque isotropic micromass, as well as the discontinuous subhorizontal bands of isotropic reddish material in the Bhs horizons. The high amounts of DCB-ex- tractable Fe in the Bhs and Bs horizons anyway suggest that some reduced Fe has migrated into these horizons, and is at least partially responsible for the micro-aggre- gated structure and the high porosity. Part of the reduced Fe may have migrated downslope on top of the soil.

(5) Where the free Fe compounds have been reduced and removed, a bleached or E horizon is formed. It acquires a much denser structure, typical for all E horizons, due to the absence of free Fe(Ill) compounds.

(6) The compaction of the E horizons favours reduction, as a result of the lowering porosity, so that the reduction of Fe gradually penetrates deeper into the soils and the E horizon thickens.

(7) The dense coarse textured E horizons retain only ~,mall amounts of water and fertilising substances causing roots to preferentially develop underneath in horizons w'ith high contents of free Fe(lII) compounds and better water retention possibilities. Break- down of these roots gives rise to high amounts of organic matter and a microstructure of

E. Van Ranst et al. / Geoderma 76 (1997) 263- 283 279

loose packing of angular micropeds. The presence of the organic substances with chelating properties can explain the more pronounced transformation of micas in these B horizons.

(8) The E horizons with low amounts of free Fe and AI compounds constitute environments in which soluble organic compounds move easily. But these soluble compounds may have been integrated into the micropeds in the Bhs horizons. Some may still be present in the E horizons as hypocoatings, although this is difficult to prove. The rather low content of extractable organic matter suggests that an important fraction of the organic matter is formed in situ.

3.5. Chemical criteria for spodic B horizon and spodic materials

Application of the chemical criteria, proposed in the recently revised international soil classification systems, for spodic B horizons (FAO-Unesco. 1990) and for spodic materials (Soil Survey Staff. 1992, 1994) leads to contradictory results (Table 5). None of the studied B horizons satisfies the chemical criteria of spodic B of the FAO system. and the Curaga and Manje soils cannot be classified as Podzols. On the other hand. the same B horizons meet the chemical criterion of spodic materials in the sixth edition of Soil Taxonomy (Soil Survey Staff, 1994), and the soils are Spodosols. As the World Reference Base for Soil Resources (WRB, 1994) adopted the chemical criterion of Soil Taxonomy, the studied B horizons will be spodic (Table 5).

The current chemical criteria for spodic B horizons in the FAO system are still based on the assumption that Na-pyrophosphate is a good extractant for organo-metallic compounds (Franzmeier et al.. 1965: McKeague, 1968). The ratio including clay percentage was intended to exclude Fe and AI extracted from the clay fraction by

Table 5

Some chernical criteria lor spodic B horizon (FAO-Unesco . 199(I), spodic materials (Soil Survey Staff. 1994).

and spodic horizon (WRB. 1994)

Series,/ FAO-Unesco(199(}) Soil Survey Staff(19941 WRB (1994)

horizon ( I ) (2) (3) (4) (5) 14) (6)

E - - 0.03 0.03 - Bhs 0.08 0.22 no 1.53 ves 1.53 yes

Bs 0.20 0.33 no 2.04 ves 2.04 yet

:]4~i11j~, Ah2 - O. IO (I. lO - Bhs (I.29 (I.46 no 0.72 yes 0.72 yes

B~, 0.38 0.31 no 0.84 yes 0.84 yes

(I): (Fel, + Alp J /c lay :> 0.2 it" Fel, >_ O. I.

(2): (Fet, + A l p ) / ( F % + A l l ) > _ 0.50. (3): Spodic B horizon.

(4): (AI,, +(].5 F-e,,) _> 0.50. and half that amount or less in an overlying umbric, ochric or albic horizon. (5): Spc, dic materials. (6): Spodic horizon.

2g0 E. Van Ranst et al. / Geodenml 76 ( 10071 2()3- 283

Na-pyrophosphate. The ratio including Fe,j and AI,~ was primarily proposed to distin- guish the spodic B from a cambic horizon developed in pyroclastic materials (Soil Survey Staff, 1975). However. it has been mentioned more than once that extraction of organo-metallic complexes with Na-pyrophosphate obviously give unreliable results.

The pyrophosphate-based criteria were abandoned in the latest revised versions ol Soil Taxonomy and in the World Reference Base for Soil Resources. and replaced by a chemical criterion based on the assumption that oxalate extracts the AI and Fe fractions most involved in podzolization. Spodic horizons and spodic materials are considered to contain active amorphous constituents, composed of organic matter and aluminum, with or without iron. However, this definition remains to() vague to be applied in practice, because several points are not evident: ( I ) How can the composition of these con- stituents be determined and how can they be distinguished from constituents accumu- lated under the influence of other processes, e.g. oxido-reduction? (2) How can translocated constituents be distinguished from constituents with similar composition but formed in situ? (3) If part of the constituents is illuviated and part formed in situ. what is given preference? (4) If the composition of the illuviated constituents is subsequently changed, e.g. by biological activity, are they still to be considered as spodic materials'?

None of the proposed and applied chemical extractions, including M8ssbauer studies, will give a definite answer to these questions. The only arguments that can be brought forward, against an intensive translocation of amorphous organic complexes in the soils studied, is the less advanced weathering stage of the clay fraction in the A and E horizons, and the absence of cracked monomorphic coatings in the B horizons. Only such coatings have properties which clearly indicate movement of soluble metal-humus complexes (chelates) out of the surface layer(s) to greater depth, and subsequent accumulation of A1- and Fe-chelates in a spodic horizon (De Coninck el, al., 1974: Van Ranst ct al.. 1980).

4. Conclusions

If we accept the concept that spodic horizons and spodic materials are predominanl,ly formed by movement of soluble metal-humus complexes out of the surface layer(s), and subsequent accumulation at greater depth, the soils studied will be excluded from being Podzols. In the soils studied indications of iJluviation of amorphous organic complexes are very weak and the process of cheluviation is only secondary relative to biological activity, Fe precipitation and weathering. The concept that cheluviation is the dominant process in Podzol formation should be forsaken definitely, because it excludes many soils which soil scientists want to be Podzols. It must be stressed that the presence of amorphous organo-A1 compounds in some soil horizon by itself does not prove cheluviation to be active. The chemical criteria actually proposed for spodic B horizons and for spodic materials do not always allow l,o distinguish loose spodic B horizons, present underneath a bleached E horizon, from cambic B horizons with friable or very friable consistence, a fJuf|" 5, or a crumb structure, high porosity and many regularly distributed roots. In the definitions of spodic horizon and spodic materials preference should be given to macro- and micromorphological properties over chemical properties.

1:'. Van Ranst et a l . / Geoderma 76 ~ 1997) 263-283 281

because the former are directly related to different genetic processes (biological activity. oxido-reduction) which also could be responsible for their formation. The importance of morphology over chemical spodic criteria was already emphasized by Shoji and Ito (1990) for tephra-derived Spodosols. More research is needed to formulate criteria for spodic B horizons and for spodic materials to include loose B horizons of soils with a podzolic field morphology, which soil scientists want to be Podzols.

Acknowledgements

We wish to thank F. De Coninck and D.P. Franzmeier for critical reading of the manuscript and for valuable comments. We are also grateful to the National Science Foundation (NFWO. Belgium) for the financial support of the research programme "Mineralogy and fertility of tropical soils" (No. 2.0017.93).

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