Aust. J. Soil Res., 1979, 17, 371-81
The Development of a Red-Brown Earth. I
A Reinterpretation of Published Data
D. J. ChittlebovoughAand J. M. OadesB
A
Department of Agriculture and Fisheries, Box 1671, G.P.O. Adelaide, S.A. 5001;
present address: Division of Soils, CSIRO, Private Bag No. 2, Glen Osmond, S.A. 5064.
Department of Soil Science, Waite Agricultural Research Institute,
The University of Adelaide, Glen Osmond, S.A. 5064.
Abstvact
Previously published chemical and physical data for the Urrbrae loam, and the conclusions drawn
from these data concerning profile development, have been appraised. Translocation of clay seems
likely to have played a significant role in the development of the Urrbrat: loam, a red-brown earth
(calcic rhodoxeralf). Both A and B horizons apparently did not originate as distinct sedimentary
deposits nor was much additional clay formed in position. Further experimental evidence is required
to substantiate the clay illuviation hypothesis.
Introduction
Whereas in early pedogenic studies (Stephens 1953), the high clay content of
the B horizons of red-brown earths was ascribed to the movement of clay from the
surface and its deposition lower in the profile, recent studies in Australia have
concluded that clay translocation played only a minor role in the development of
red-brown earths. Instead it has been concluded that the horizons of these soils
have originated as distinct sedimentary layers and by the formation of clay in position
(e.g. Green 1966; Oertel and Giles 1967; Brewer 1968; Oertel 1974).
The critique presented here is part of a study to investigate the processes responsible
for the development of texture contrast soils and to develop criteria to distinguish
between these processes. The first phase of this investigation involves the reappraisal
of existing data. In this regard, the Urrbrae loam is a good starting point because
of the extensive chemical and physical data available. Some of these data have been
published by Brewer (1968) and Oertel(1974); the remainder have been derived from
Technical Memoranda of the Division of Soils, CSIRO (Nos. 12, 13 and 39/68,
and 1, 2, 24 and 25/69)."
Data from two contiguous profiles, A1003 and A1004, were studied. Except where
otherwise stated, all analyses discussed in this paper are for profile A1004 and are
contained in the Technical Memoranda listed above.
The Urrbrae Loam Profile
The soil studied is a red-brown earth (Stephens 1953; Stace et al. 1968) formed on
alluvium of the southern Adelaide Plains. Mean annual rainfall is 635 mm, most of
which falls in the months of May to September. The site is 110 m above sea level,
and there is an even 5 % slope to the west.
* Available on request from the Chief, Division of Soils, CSIRO, Adelaide.
D. J. Chittleborough and J. M. Oades
372
The profile is described and illustrated in Table 1 and Fig. 1 respectively. A plot
of the clay distribution is given in Fig. 2.
Table 1. Brief description of Urrbrae loam profile
Depth
(cm)
Brief description
A1
0-15
A2
15-26
B1
26-35
B2
35-80
B2ca
80-110
Dark reddish brown (5YR3/3 moist), fine sandy loam; moderate subangular
blocky; very friable; small amounts of gravel; clear to
Red (2.5YR5/8 moist) fine sandy loam to fine sandy clay loam with some
dark reddish brown (5YR3/4 moist) patches; moderate subangular blocky;
very friable; moderate amounts of gravel and stone; sharp to
Dark red (2.5YR3/6) moist light medium clay; moderate subangular
blocky; friable; small amounts of gravel and stone; clear to
Dark red (2.5YR316 moist) heavy clay; strong angular blocky with
smooth ped faces; very firm; slight amounts of gravel and stone; clear to
Yellowish red (5YR4/6 moist) medium clay; calcareous; discrete particles
of carbonate; subangular blocky; firm to very firm; diffuse to
Yellowish brown (10YR5/6 moist) medium clay; calcareous; occasional
concretions of carbonate; weakly subangular blocky; firm to very firm
Horizon
-
C
-
110-
--
Fig. 1. The Urrbrae loam profile.
Discussion
Three processes, alone or in combination, have been proposed to explain the
particle size distribution of the Urrbrae loam,
Development of a Red-brown Earth. I
373
1. Sedimentary layering. The differences in particle size distribution are the
result of an initially non-uniform parent material.
2. Formation of clay in position by differential weathering. Under this mechanism,
there has been no clay movement.
3. Downward translocation of clay within an initially homogeneous material.
The significance of each of these processes is discussed below.
Uniformity of the Parent Sediment
In considering the development of a duplex soil such as a red-brown earth, the
first point to decide is whether or not the soil has formed from a uniform parent
material. If so, variations in the profile can be ascribed to soil-forming processes.
The usual method of assessing uniformity is to measure the concentrations of minerals
Clay (%)
Fig. 2. Clay distribution of the
Urrbrae loam.
known to be resistant to weathering, e.g. zircon, rutile, anatase, and to calculate
ratios between the concentrations of the mineral in different size fractions (e.g.
~ i r c ~ n , , , ~ / ~ i r cand/or
~ n , ~ ~ratios
~ ) of concentrations of different minerals in the
same fraction (e.g, zircons,nd/rutile,and). In either case, a constant ratio down the
profile indicates uniformity. A change in ratio would indicate a different resistant
mineral assemblage and hence a different parent sediment. Measurements of elements
known to occur only in one mineral, e.g. zirconium in zircon, or a particular suite
of resistant minerals, will give a similar result and are generally preferred, since
they can be readily measured by X-ray fluorescent spectrometry, obviating the
necessity for tedious mineral grain counts.
Consideration of the ratios TiSand/Tisilt,
Zrsand/Tisand,
Tisand/Alsand
and Tisiit/Alsilt,
leads to the conclusion that the parent material was originally uniform throughout
(Table 2, Fig. 3). The ratios are essentially constant throughout, although there are
some fluctuations in the ratios involving zirconium. Because of the relatively small
D. J. Chittleborough and J. M. Oades
amount of zirconium in the profile, these fluctuations can be attributed to sampling
and analytical error. Significantly, the ratios are constant across the boundary
between A and B horizons.
Table 2. Elemental ratios for the Urrbrae loam
Profile A1004
Horizon
Depth
(cm)
Tisand/Ti,ilt
I
a
n
Profile A1003
T i d A 1 1 ~ Zr~d/Tisand ZrsandITisand
It is concluded that A and B horizons did not originate as distant sedimentary
deposits.
Fig. 3. Ratio of titanium concentrations
in sand and silt fractions of the Urrbrae loam.
Formation of Clay in Position
A subsidiary explanation put forward by Oertel for the high clay content in the B
horizon of the Urrbrae loam is that clay was formed in position, i.e. minerals in the
Development of a Red-brown Earth. I
375
B horizon weathered more rapidly than in the A. This has been the suggested origin
of the reddish clayey B horizons of Desert soils (Nikiforoff and Drosdoff 1943) and
those of Reddish Brown and Chestnut soils (Byers et al. 1938).
Beavers et al. (1963) suggested a method of testing this hypothesis based on the
observation that each mineral species has a different stability or resistance to weathering. Calcic plagioclase minerals are considered relatively susceptible to weathering,
whereas zircon and rutile are highly resistant (Jackson and Sherman 1953). Several
workers have calculated the calcium/zirconium molecular ratios in the silt fraction
and concluded that these were a sensitive measure of weathering (Beavers et al. 1963;
Jones and Beavers 1966; Smith and Buol 1968).
Table 3. CaO/Zr02 and CaO/TiO, ratios for the Urrbrae loam
Horizon
Depth
(cm)
CaO/Zr02
sand
CaO/TiO,
sand
CaO/Ti02
silt
The calcium oxide/zirconium oxide ratio in the sand fraction and the calcium
oxideltitanium oxide ratio in the sand and silt fractions, were calculated for the
Urrbrae loam (Table 3). There is no significant change in the ratios throughout the
profile, and therefore no evidence of a greater intensity of weathering in the B horizon.
If a substantial portion of the clay in the B horizon had arisen through more intense
weathering of the minerals in that horizon, the calcium oxide/zirconium oxide ratio
should show a marked decrease in the region 30-60 cm. The increase in the ratios
below 80 cm is due to the presence of calcium carbonate.
There is no evidence that the B horizon developed to any significant extent by
the formation of clay in position.
Clay Translocation
Several criteria are commonly applied to test whether clay illuviation has occurred.
Two will be considered in this discussion, viz. the presence of void cutans and the
chemistry of the clay fraction.
Void cutans. Void cutans (or more specifically, void argillans) have been taken as
incontravertible evidence that clay illuviation has taken place. Brewer (1968) studied
the Urrbrae loam and was unable to detect any void cutans. He concluded that clay
movement was not responsible for the formation of the B horizon.
D. J. Chittleborough and J. M. Oades
While the presence of cutans is clear evidence of movement, their absence does not
necessarily mean that there has been no movement. Nettleton et al. (1969) have
shown that, in soils with a high shrink-swell potential, cutans are not observed-at
least when the coefficient of linear extensibility is >4%. The B and C horizons of
the Urrbrae series have a very high shrink-swell potential (Aitchison et al. 1954).
Thus, one would not expect to observe cutans. Either they did not form or were
rapidly destroyed by pedoturbation.
Potassium (%)
19
2.1
2.3
25
2.7
2.9
Fig. 4. Concentrations of potassium
in the soil (*), sand ( 0 ) and
clay ( 0 ) of the Urrbrae loam.
Chemistry of theJine and coarse clay. From the data in Table 3, it was concluded
that the formation of clay in the B horizon, or in fact any part of the profile, has not
been a significant factor in the development of the Urrbrae loam. Further evidence
to support this conclusion can be obtained by reconsidering the distribution of the
elements potassium, aluminium and titanium as reported by Oertel(1974). From these
distributions it can be shown that (a) only a small part of the clay in the B horizon
can be accounted for by clay formation in place, and (b) these data are compatible
with clay illuviation by making two assumptions.
The first assumption is that the clay fraction is not chemically or mineralogically
homogeneous and should be considered as being composed of a coarse (2-0.2 pm)
and fine ( < Q e 2ym) fraction.
Development of a Red-brown Earth. I
377
Some of the best work showing that the clay fraction cannot be considered as
homogeneous is that of Jackson et al. (1948), who proposed a weathering sequence
of minerals of thirteen stages, from gypsum (stage 1, least resistant) to anatase,
ilmenite, corundum (stage 13, most resistant). They showed that the chemical composition of the soil colloids varied according to particle size: the coarse clay invariably
contained minerals of earlier stages of weathering than fine clay. It was commonly
observed that, in the same < 2 p m fraction, feldspars such as albite (stage 5,
silicon : aluminium = 3 : 1) were more abundant in the coarse clay, while smectites
(stage 9, silicon : aluminium = 1 - 5 : 1) were more common in the fine clay.
The second assumption is that fine clay is more mobile than coarse clay. Dixit
(1978) has shown that the smaller the particle diameter the greater the relative mobility,
and furthermore, that this effect is more pronounced for kaolinitic than for illitic
colloids.
If clay formation in situ has been limited and clay illuviation has been the dominant
process, then the chemistry of the fine clay should be similar throughout the profile
and likewise, the coarse clay. Any chemical and mineralogical differences between
the two fractions should be consistent throughout the profile.
The considerations above are presented separately for the elements potassium,
aluminium and titanium.
Potassium. The data for the Urrbrae loam show that: (1) the amount of potassium
in the whole soil is practically constant throughout the profile; (2) the concentration
in the sand and silt reveals a slight overall tendency to increase with depth; (3) the
content in the clay is constant in the A horizon but sharply decreases to a constant
value in the B (Fig. 4).
If the clay in the B horizon was formed in position by breakdown of potassium
minerals, e.g. the potassium feldspars, in the sand and silt fraction, then the potassium
in the sand would increase. As a result of weathering, the sand fraction would show
an enrichment of quartz, zircon and other resistant minerals. However, the potassium
content of the sand remains constant and even shows a slight increase in the B horizon.
Thus, Oertel's conclusion that the clay in the B horizon was formed in positisn seems
unlikely.
The most satisfactory explanation is that there has been clay illuviation. If fine
clay, relatively mobile and lower in potassium than coarse clay, has moved into the
B horizon from the A, the potassium content of the clay in the A horizon would show
an increase and, in the B, there would be a decrease in potassium content of the clay.
The potassium content of the whole soil should not change significantly, despite the
movement, and there would be no change in the potassium concentration of the sand
throughout the profile since this does not move or weather.
Aluminium. The amount of aluminium (percentage of aluminium by weight) in the
soil, sand and silt remains practically constant throughout the profile, but shows a sharp
increase in the B horizon for both soil and clay (Fig. 5). For reasons similar to those
given above for potassium, it does not seem likely that the clay of the B horizon was
formed in position. If this were so, the clay could only have come from the breakdown
of aluminous-rich minerals in the sand and silt fractions, leaving behind a sand+ silt
fraction lower in aluminium and higher in resistant minerals such as quartz and
zircon than in the A horizon. However, the aluminium in sand silt remains constant
throughout the profile, despite an absolute 8 % increase in the aluminium content
+
D. J. Chittleborough and J. M. Oades
of the B horizon of the soil compared to the A. Mean aluminium in sand is 3 . 2 %
(s = 0.2) and in silt is 4 . 5 (s = 1.0).
The aluminium distribution may be the result of illuviation of fine clay, high in
aluminium leaving a coarser clay, lower in aluminium, but no data are available to
substantiate this.
Titanium. The graphs of titanium distribution for the Urrbrae loam are similar to
those for potassium, i.e. titanium is practically constant with depth for sand, silt
and soil, whereas the clay of the A horizon has a much higher concentration than
in the B (Fig. 6).
Aluminium (%)
Fig. 5. Concentrations of
aluminium in the soil (*),
sand (o), silt ( 0 ) and
clay ( * ) of
the Urrbrae loam.
Oertel proposed that the distribution is an inherited feature of an initially nonuniform parent material and has remained unchanged during the soil-forming process.
This explanation seems most unlikely. On the basis of elemental ratios discussed
earlier in this paper, there is strong evidence that A and B horizons did not originate
as distinct sedimentary deposits.
The titanium distribution may have arisen by the translocation of fine clay, lower
in titanium and higher in aluminium than coarse clay, from the surface to form the
B horizon. This would also explain the negative correlations between aluminium in
soil and titanium in soil (r = -0.98) and aluminium in clay and titanium in clay
(r = -0.98).
Development of a Red-brown Earth. I
379
Another reason for these observations may be that aluminium and titanium are
present in different minerals. There is some evidence to suggest that not all the titanium
in the clay fraction is present as a substitute for silicon in the aluminosilicates. In
a titanium-rich clay from Scotland, Bain (1976) found that the titanium is present
almost entirely in the form of cryptocrystalline anatase. If only part of the titanium
is associated with aluminium and the remainder is present as a separate mineral (e.g.
cryptocrystalline anatase), it is probable that the mo5ility of the aluminosilicate clay
is different from that of the titanium mineral. Given that the aluminous minerals are
more mobile, titanium will show a relative enrichment in the A horizon and a relative
decrease in the B, in both the whole soil and the clay.
Titanium (76)
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Fig. 6. Concentrations of titanium
in the soil, sand and clay
of the Urrbrae loam.
General remarks. For clay illuviation to be a tenable hypothesis, the fine clay must
be lower in potassium and titanium and higher in aluminium than the coarse clay.
There is good reason to expect this. Turchenek (l975), studying organo-mineral
associations in the A1 horizon of the Urrbrae loam, found that the medium and fine
clay fractions were significantly lower in titanium and much higher in aluminium than
coarse clay. The data for potassium were equivocal. In the aluminium horizons of
pasture plots, potassium was higher in the fine clay, but in wheat-fallow plots, potassium was lower.
D. J. Chittleborough and J. M. Oades
Other workers (e.g. Joffe and Kumin 1942; Lichmanova 1962; Kuz'min 1968;
Genrich 1972) have reported much higher concentration of aluminium in the fine
clay than coarse clay or silt fractions. Genrich (1972) reported that the potassium
content of the clay fraction decreased in the order, coarse clay > medium clay > fine
clay, whereas in the 10 profiles studied by Maclean and Brydon (1963) the potassium
contents of the coarse and fine clays were similar, six having contents somewhat
greater in the coarse clay than fine clay, four having contents somewhat less. Few
studies have been made on titanium distribution within fine fractions. The few that
have (e.g. Joffe and Kumin 1942; Genrich 1972) reported much higher concentrations
of the element in the coarse clay than fine clay.
Conclusion
From a reconsideration of the chemical and physical data available for the
Urrbrae loam, ('a typical red-brown earth'), it is concluded that the A and B horizons
did not originate as separate sedimentary layers, nor that there was a significant
amount of clay formed in position. Clay illuviation or clay decomposition cannot
be discounted as likely mechanisms, but further work is necessary to test these
hypotheses.
Acknowledgments
Particular thanks are due to Mr J. T. Hutton for his long-standing interest and
helpful comments, and for criticising numerous drafts of this paper.
Mr K. H. Northcote also made valuable suggestions for the improvement of the
manuscript.
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Manuscript received 14 June 1979