Journal ofsoil Science, 1984,35,447-452

Proto-imogolite and the process of podzol formation: a critical note

P. BUURMAN & L. P. VAN REEUWIJK* Department of Soil Science and Geology, Agricultural University, Wageningen, and

*International Soil Rejerence and Information Centre, 'f' Wageningen, The Netherlands

SUMMARY

The presence of imogolite or other poorly crystalline aluminosilicates in the lower B horizons of podzols can be explained by neoformation from silica in the percolating soil solution and aluminium species liberated from their organic bond by activity of micro-organisms. There are physicochemical arguments refuting the alternative concept that iron and aluminium are transported down the profile as short range order aluminosilicates such as proto-imogolite which would thus play an important role in podzol formation. There is ample evidence to attribute the mobilization, translocation and precipitation of sesquioxides to complexing organic compounds.

INTRODUCTION

The occurrence of allophane in podzol B horizons has been known since Franzmeier et al. (1963) reported its presence in Michigan podzols, but its role in the podzolization process has only recently received sustained attention. Traditionally considered a by-product of the transport of material in podzols, allophane is now attributed a crucial function in the podzol- ization process itself (Farmer, 198 I ; Farmer et al., 1980; Anderson et al., 1982). These authors abolish the theory of transport of sesquioxides by complexing organic compounds. Instead, they argue that sesquioxides are transported as short range order silicates, thus forming an eluvial and illuvial (Bs) horizon, and that organic matter is subsequently precipitated on these silicates.

Stability of organic complexes One of the assumptions on which the alternative theory is based is that not sufficient iron and aluminium could be set free by microbial decomposition of organo-metallic complexes to account for the accumulation of these elements in the B horizon: because breakdown of these complexes would be too slow. Arguments for this theorem are taken from the great ages of sesquioxide fulvates in fossil (buried) volcanic ash soils (Farmer et al., 1980). The basis of these arguments, however, is incorrect since in fossil soils the breakdown of organic matter and the incorporation of fresh I4C are impeded or stopped altogether. In fact, this is one of the characteristics that allows determination of the approximate age of such soils with I4C.

The age of fossil humus in buried soils has nothing to do with the mean residence time (MRT) of such humus in active soils. Scharpenseel et al. ( I 968) list MRTs of podzol Bh humus varying between 930 and 2570 years. De Coninck (1980) reports similar values (between 1500 and 4000 years), and remarks that the faster turnover occurs in better drained soils. Much lower values were reported by Guillet (1972) and Guillet & Robin (1 972) from forest podzols in the Fontainebleau Forest (Paris Basin): MRTs vary between 180 and 6 10 years in the upper Bh horizon. Tamm & Holmen (1967) mention mean residence times between 330 and 465 years for the Bh horizons of podzols from Middle Sweden. These figures are evidence disprov- ing the value of 10,000 years adopted by Farmer et al. (1980) and, therefore, the assumption

t Formerly International Soil Museum

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448 P. Buurman & L. P. Van Reeuwijk

that metal fulvates would be too stable to account for the free sesquioxides in the B horizons of podzols cannot be valid. Nonetheless, even if sesquioxides were not set free-or only slowly-from fulvate complexes, they are also transported as complexes of relatively short- lived organic acids of lower molecular weight. Bruckert (1970a, b), reporting on the dynamics of podzols and brown earths, demonstrated that in podzols there is a year-round supply of simple organic acids such as citric, lactic, oxalic, malic, succinic, glucuronic, vanillic, p-coumaric and p-hydroxybenzoic acid. It appears that these acids can transport Fe and A1 from the podzol A into the Bh and Bs horizons. Traces of such acids are frequently found in podzol-B horizons. Anderson et al. (1 977) found p-hydroxybenzoic acid, 3,4-dihydroxy- benzoic acid, vanillic and syringic acid down to the A22 horizon of a Scottish podzol.

The continuous production of such complexing acids, albeit in small amounts, combined with their rapid turnover, makes them potentially important transporting agents. It should be stressed that an apparent MRT ofpodzol humus, or even ofa humus fraction, would hardly reflect the effect of such compounds.

Although organic matter may precipitate on sesquioxide surfaces, this fact is not in contra- diction with the concept that metal fulvates remain soluble when undersaturated with metals and become insoluble when the metal/fulvate ratio rises to saturation. Nor does it disprove the fact that fulvic acids are able to dissolve and transport aluminium and iron (for references see review by Schnitzer & Kodama, 1977).

Stages in podzo1,formation and transport ofproto-imogolite Anderson et al. (1982) propose three successive stages in podzol formation, viz: stage A: the formation of a B2 (Bs) horizon with imogolite-type materials and iron oxides (A1 and Fe illuviated as silicate complex without organic matter); stage B: migration of organic matter and its precipitation on these oxides; and stage C: leaching of iron in ferrous form from the Bh horizon and its precipitation as a thin iron pan.

We agree with these authors that transport of a hydroxy-aluminium silicate complex in their stage B is not feasible. This would require a certain stability of allophane in the eluvial horizon. Since the dissolving effect of organic acids on crystalline clays, leading to the removal of aluminium from the octahedral sheets and the formation of swelling clay minerals in eluvial horizons, is well documented, it is inconceivable that aluminium in allophane structures would be exempt from the complexing action of organic acids. Therefore one should conclude that allophane cannot exist in podzol eluvial horizons, and, to the authors’ knowledge, it has never been found in such horizons. Farmer (1 98 1) and Farmer et al. ( I 980) report that fulvic acid forms stronger complexes with Al than silicic acid does, and can even dissolve proto- imogolite allophane. Synthesis of proto-imogolite was achieved only in the absence of com- plexing organic acids (Farmer et al. (1977)). It has not been identified in podzol leachates thus far, its supposed presence being based on in vitro synthesis only. Very recent experiments con- firmed that formation of allophane and imogolite is indeed effectively counteracted by organic ligands (Inoue & Huang, 1983). Therefore, Anderson et al. (1982) had to postulate that during stage A Al and Fe are brought into solution by non-complexing organic and inorganic acids or by readily biodegradable small complexing organic acids. The significance of the former is likely to be relatively small; the sufficient length of life of the latter for transport to the B horizon was discussed above. Yet it seems unrealistic to assume that at some stage of podzol formation decomposition of (fresh) organic matter will only produce small organic acids and no polymeric organic acids. Scheffer et al. (1 983) report that the organic matter of an 0 and A horizon of a heath podzol in Lower Saxony contains roughly 20% fulvic acids and 20% humic acids.

Although it has been amply verified that in most cases the organic matter ofthe Bh horizon has a shorter mean residence time than that of the Bs horizon, this does not mean that the Bh itself is necessarily younger. This implies that stage A does not necessarily precede stage B, but that the two ‘stages’ could well operate simultaneously, rendering their distinction impossible and meaningless.

Formation stage A without consecutive stages B and C would lead to soil profiles with

Process of podzol formation 449

an E horizon depleted in aluminium, iron and silica and a B horizon with imogolite-type material and accumulation of iron compounds, but without illuvial organic matter. Such intermediate profiles have not been encountered. Profiles with a well-developed E horizon and appreciable accumulation of sesquioxides in B horizons with low organic matter content are better explained by the rapid turnover of organic matter in the B horizons of such soils.

Also, the prominent zonal occurrence of podzols in colder climates can be explained very plausibly with the organic complex concept. Bruckert (1970b) found that at 0°C where bio- degradation is very weak, transport of complexes of small organic acids in podzols was much more effective than at 20°C. This inverse temperature effect is difficult to explain with the proto-imogolite concept.

Presence of imogolite Farmer et al. (1980) and Anderson et al. (1982) consider the presence of imogolite-type material in the clay fractions of B2 horizons of podzols proof for the theory that aluminium is transported as an aluminium orthosilicate complex. Obviously the mere presence of a com- pound cannot be proof that this compound was illuviated as such. Anderson et al. (1982) apply this logic to the presence of organic complexes in the B horizon, but not to the case of imogolite. Although imogolite was found in several podzols and brown soils on eutrophic parent materials in Scotland (Tait et al., 1978), it could not be traced in Bs horizons of Dutch and Belgian podzols on siliceous parent materials (Mokma & Buurman, 1982). In these soils, oxalate-extractable aluminium virtually equals pyrophosphate-extractable aluminium indi- cating that all aluminium is organically bound (McKeague et af., 197 1; Parfitt & Henmi, 1982; Parfitt et al., 1983). In fact, of some 30 non-gleyic podzolic soils for which both Al, and Alp data of Bs horizons are given by Mokma & Buurman only half have any appreciable A1 attributable to imogolite or allophane (Al, - Alp > 0. I O i o ) . Thus, podzolization is not necessarily linked with the presence of imogolite.

Transport of iron Contrary to that of aluminium, the movement of iron in well-aerated soils in the absence of organic matter is too slow to form any recognizable iron-containing B horizon within some hundred thousand years (Van Schuylenborgh & Bruggenwert, 1965), so it is generally accepted that iron movement takes place in the form of organic complexes. Schnitzer & Hansen (1970) found that Fe forms even more stable complexes with fulvic acid than A1 does; Kerndorff & Schnitzer (1 980) reported a pH-dependence of the complexation by humic acid, Fe being preferred below pH 4.8.

The first model for podzol formation with proto-imogolite makes no provision for iron movement since proto-imogolite is an aluminosilicate (Farmer et al., 1980). T o provide for this imperfection Farmer & Fraser (1982) succeeded in synthesizing stable A1203- Fe203-SiOz-H20 sols with Fe : A1 molar ratios up to 1.5. However, the formation of these sols in A horizons of podzolic soils can be effectively hindered by complexing organic com- pounds in the same way as was discussed for proto-imogolite. These sols can probably not be designated proto-imogolite. It is unlikely that the imogolite structure allows appreciable substitution, if any, of Fe or Al and in view of the separation of Fe and Al in B horizons there is no obvious genetic link between soluble Al-FeSi units and imogolite. Precipitation of A1 and Fe-saturated organic complexes and subsequent biodegradation seems more plausible than precipitation from stable sols.

Farmer & Fraser (1982) take the constancy of the A1 : Fe ratio (1.5-2.0 wt/wt) in oxalate extracts of Bs horizons of three Scottish podzols as evidence for transport of Al-Fe-Si sols since alternative mechanisms such as transport of fulvate complexes would not account for such constancy. However, as discussed earlier, oxalate extracts of soils (not treated with peroxide) include organically bound as well as amorphous inorganic Fe and Al and the sug- gested reason for such constancy can therefore not be valid. Indeed, execution of a statistical F-test on the oxalate extract data of podzolic soils given by Mokma & Buurman (1982) indi- cates that the AI,iFe, ratios between 1.5 and 2.0 have no statistically significant importance

450 P. Buurman & L. P. Van Reeuwijk

with respect to the whole collection of data (ranging from 0.1 to 12). Considering the rather wide range of compositional stability of the Al-Fe-Si-sols it is not clear why a supposed con- stancy should carry more weight than perhaps reflecting a similarity in soil forming factors. Such conditions might easily be found within a limited area and then transport of Fe and Al as organic complexes (which will be of similar nature and present in similar amounts) could just as well lead to a certain constancy in the ratio of deposited Fe and Al.

Transport of silica in podzols Little is known about the transport of silica in podzols. One of the few investigations into this matter, by Schnitzer & Desjardins (1969), showed that after separation of humus fractions, silica was encountered in a fulvic acid fraction that was virtually free of aluminium. The untreated leachate from a podzol Ae horizon studied by these authors contained, among other, 15% SiO2, 7% A1203 and 22% CaO in the ash fraction (2 1.5% of the dry matter). After passage over a H-resin and centrifugation, the leachate was split up into a soluble fraction with a low ash content (1.2% ash) and a precipitate that contained 47.6% organic matter and 52.4% ash. This ash consisted for 98% of SiO2, and contained only 1.7% A1203. Although these results do not prove that silica was transported as an organic matter complex, no more do they suggest that the silica was bound to aluminium.

Fieldes ( 1 955) suggested that complexation of alumina by organic matter prevented forma- tion of allophane and thus could cause silica to remain discrete. Support for this was found by Shoji & Masui (197 1) and Tokashiki & Wada (1975) in A horizons ofandosols which show many genetic similarities with podzols. Flach et al. ( 1 980) report a microprobe analysis of ‘monomorphic’ coatings in an iron-poor spodic horizon revealing the association of organic matter and aluminium whereas silicon appears to be present in sand and silt grains only. These authors state that ‘because the morphology of the coatings suggest a direct process of leaching and deposition, their elemental composition should reflect the elemental composition at the time of deposition’. This implies that silica moves separately from A1 and organic matter and that, if it is precipitated, this occurs with a different mechanism.

Precipitation of amorphous organic matter on allophane The precipitation of amorphous organic matter on allophane has been used to explain the occurrence of amorphous organic matter in the Bh horizon of Humus Iron Podzols. Anderson et al. (1982) quote De Coninck (1980) to substantiate this theory. It turns out, however, that while Farmer et al. (1980) and Anderson et al. (1982) suggest the presence of amorphous organic material (monomorphic coatings?) in the upper Bh of a Humus Iron Podzol, De Coninck (1980) does not describe such coatings from the upper Bh horizon (where he finds ‘polymorphic’ organic matter) but from the lower Bh horizon, and from B horizons of hydro- morphic podzols. As a rule, the upper B horizon in well-drained profiles has polymorphic organic matter, whereas the lower B (Bs) has monomorphic coatings (Altemiiller, 1962; Eswaran et al., 1969) although the reverse has also been encountered (Robin & De Coninck, 1978). Thus, the form of organic matter accumulation in the upper B horizons of podzols is not conclusive evidence of the chemical precipitation’ of humus colloids on allophane.

Compound coatings of organic matter precipitated on allophane/imogolite coatings which would lend support to the stage A and stage B theory of Anderson et al. (1982) have not been reported so far.

Alternative model of podzol development Finally, an alternative model for the development of podzol morphology and the formation of allophane in the illuvial horizon could be as follows: In the initial stage of podzolization, sufficient amounts of sesquioxides, mainly of iron, are available in the profile to saturate organic complexes at short distance from the soil surface. Weathering mainly affects iron bear- ing minerals. If a Brown Forest Soil stage (Sol brun acide) has preceded podzol development, ‘free’ alumina may have a maximum lower in the profile.

Upon continued production of organic acids, the uppermost organo-metallic precipitates

Process of podzol.formation 45 1

will redissolve because the fresh acids will remove part of the sesquioxides, and reprecipitate after renewed uptake of sesquioxides at slightly greater depth.

As soon as the bottom of the eluvial horizon arrives below the zone of intensive mixing of surface humus, the eluvial horizon becomes visible. The intensity of weathering gradually increases, affecting feldspars and clay minerals and setting free larger amounts of silica, while the aluminium and iron are transported downward in the form of organic complexes. Although microbial breakdown of organic matter is active in all of the B horizon, it will not result in deposition of hydrous alumina and iron oxides in the upper B because redissolution by fresh acids will immediately remove such freed sesquioxides. Lower down in the B, where redissolution is not yet active, sesquioxides are set free by microbial breakdown of organic complexes and alumina can combine with silica from the soil solution to form allophanic material and imogolite. Iron will form ferric oxihydroxides.

Because the mean residence time of organic matter in well-drained forest podzbls is in the order of magnitude of 300-1000 years, appreciable amounts of sesquioxides may accumu- late while organic matter contents remain low. Due to the generally stronger organic complex- ation of Fe with respect to Al, the former is preferentially removed from the upper B horizon upon dissolution of the B and its downward movement. This results in an aluminium maxi- mum in the upper B and iron maximum in the lower B. If the aluminium maximum is found in the lower B, this could either be an inheritance from the Brown Forest Soil stage, or be ascribed to pH dependence of preferential complexation of A1 and Fe.

Allophane will not be found in the eluvial horizon because Al in organic complexes is more stable than in orthosilicate. Nor will allophane be found in the upper B (Bh) horizon as this is the most active, dissolving, part of the B horizon. Only in podzols without a Bh horizon allophane may be found throughout the B.

Placic podzois, finally, are not a normal end member in podzol formation. Although a thin iron pan may form upon reduction of iron and its removal from the upper B in Humods (Righi et al., 1982), the main agent in the formation of the placic horizon is surface gleying. The ‘Bh’ horizon above the placic horizon should be compared with illuvial humus under peat, and not with the Bh horizon of podzols.

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(Received I1 January 1983)