Baixe Livro - Soil Formation e outras Manuais, Projetos, Pesquisas em PDF para Agronomia, somente na Docsity! 257 CHAPTER 11 PODZOLISATION 11.1. Introduction The striking and dramatic soil profiles of podzols have intrigued generations of soil scientists. This is reflected by a voluminous literature on the subject. From top to bottom, a podzol profile consists of up to five major horizons, two of which may be absent (in italics): - a litter layer (O); - a humose mineral topsoil (Ah), caused by biological mixing; - a bleached, eluvial layer (E); - an accumulation layer of organic matter in combination with iron and aluminium (Bh, Bhs, Bs); - thin bands with organic matter accumulation in the subsoil. In thin section, podzol O- and A-horizons show the normal transformation of plant litter into humic substances (see Chapter 4). In addition, organic remnants develop diffuse boundaries upon decomposition, which indicates the transformation of solid to soluble organic substances. E horizons usually have thin and scattered remnants of coating-like material. Organic matter in B-horizons is present either in polymorphic or in monomorphic form. Polymorphic organic matter (Figures 6.2; 11.8) consists of mesofauna excrements in various sizes, and is frequently linked to decomposing root remnants. Monomorphic organic matter (Figure 11.7) consists of amorphous coatings around sand grains and in pores. Polymorphic organic matter may grade into monomorphic, but not vice-versa. Organic matter in the thin bands below the B-horizon usually consists of monomorphic organic matter. Polymorphic organic matter in B-horizons is associated with de decomposition of root material. Podzols occur as zonal soils associated with a boreal climate, mainly on glacial, mixed parent materials, including those with relatively high amounts of weatherable minerals. Outside the boreal zone, podzols occur on poor, siliceous parent materials, both in temperate zones (e.g. on periglacial sand deposits) and in the tropics (e.g. on uplifted coastal sands). THREE THEORIES Podzols form through transport in solution, from the surface to deeper horizons, of organic 258 matter, iron and aluminium. The process consists of a phase of mobilisation and one of immobilisation of these compounds. There are various theories concerning the mechanism of podzolisation. The main three are: a) The fulvate theory b) The allophane theory c) The low molecular weight acids theory The three theories ignore the influence of root-derived organic matter accumulation in the B-horizon. In the following, we will integrate this aspect and show that it is dominant in some podzols. The fulvate theory (e.g., Petersen, 1976) postulates that unsaturated fulvic acids in the topsoil dissolve Fe and Al from primary and secondary minerals, and that the dissolved metal-organic matter complexes precipitate upon saturation (charge compensation) of the organic ligand. The precipitate has a specific C/metal ratio (Mokma and Buurman, 1982; Buurman, 1985,1987). The allophane theory (Anderson et al., 1982; Farmer et al., 1980) postulates that Fe and Al are transported to the B horizon as positively charged silicate sols, where they precipitate as amorphous allophane and imogolite (see Chapter 12) through an increase in pH. After this, organic matter would precipitate on the allophane and thus cause a (secondary) enrichment in the B-horizon. The low molecular weight (LMW) organic acid theory postulates that LMW acids are responsible for the transport of iron and aluminium to the subsoil, and that precipitation of Fe and Al is caused by microbial breakdown of the carrier (Lundström et al., 1995). Buurman and Van Reeuwijk (1984) have made clear that amorphous Si-Fe-Al sols cannot be stable in an environment with complexing organic acids. Nowadays, podzolisation is mostly regarded as the result of a combination of process a) and c), whereas the formation of allophane in process b) may precede or accompany podzolisation but is not essential to it. This view will be detailed in the following. 11.2. Conditions favouring podzolisation Conditional to the formation of podzols is the production of soluble organic acids, in the absence of sufficient neutralising divalent cations, such as Ca2+ and Mg2+. Conditions in which production of soluble organic acids is high are usually related to impeded decomposition of plant litter and increased exudation of organic acids by plants and fungi, resulting from one or more of the following factors: - Low nutrient status of the soil and unpalatable litter. Sandy parent materials with low amounts of weatherable minerals have insufficient buffer capacity to neutralise organic acids that are produced during litter decomposition. In addition, low availability of nutrients such as N and P (1) strongly 261 Figure 11.1. Seasonal variation in amount of three low-molecular-weight organic acids in litter of a Podzol (closed symbols) and a Dystric Cambisol (open symbols). Oxa = oxalic acid; gluc = glucuronic acid; van = vanillic acid. Data from Bruckert, 1970. 262 on evapo-transpiration. A precipitation event on relatively dry soil has a certain penetration depth, beyond which dissolved and suspended material cannot be transported. If the soil dries out again from above, dissolved and suspended material accumulate at the moisture front. Especially non-complexing substances, substances that are not precipitated by complexed metals and suspended organics are immobilised this way. The suspended organics are not easily remobilised because they are predominantly hydrophobic and will be difficult to rewet once precipitated on a grain surface or on a pore wall. Question 11.3. Show, from the stoichiometry of the reaction, how consumption of Al- oxalate by aerobic microbes can lead to precipitation of Al(OH)3. Use the 2:1 complex Al(C2O4)2-. INFLUENCE OF pH Complexation of Fe and Al is pH-dependent for two reasons: 1) these metals form various complexes with OH-, depending on pH (e.g., dominant aluminium species shift from Al3+ to Al(OH)2+, Al(OH)2+, Al(OH)3o, and finally Al(OH)4-, as pH increases from <3 to >8, and 2) soil organic matter has acidic groups with a range of pK values so that the number of dissociated groups increases with increasing pH. Therefore, complexation of metals by, e.g., fulvic acids, is dependent on pH and preferential binding may change with pH (Schnitzer & Skinner, 1964-1967). At pH 4.7: Fe3+>Al3+>Cu2+>Ni2+>Co2+>Pb2+=Ca2+>Mn2+>Mg2+ At pH 3.0: Cu2+>Al3+>Fe3+ The two sequences indicate that at pH 3, aluminium is more strongly bound than iron, while at pH 4.7 the reverse is true. Elements other than Al and Fe play a negligible role in complexation in podzols. Question 11.4. If organic matter precipitates upon saturation with metal ions, this means that at a fixed pH, the precipitate will have one typical C/metal ratio (e.g. 10, as stated above). What is the effect of a decrease in pH on the C/metal ratio at which the complex precipitates? Consider the effect of pH on dissociation of organic acids and on hydroxylation of metal ions. Immobilisation of soluble organic compounds in a B-horizon leads to monomorphic organic matter. Actually, polymorphic organic matter dominates the B-horizons of most zonal podzols and of many well-drained intrazonal ones. This suggests that decay of roots is important in the accumulation of organic matter in many podzol-B horizons. The following paragraphs describes this effect as part of the podzolisation process, following Buurman and Jongmans (2002). 263 11.4. The podzolisation process in stages Podzolisation involves simultaneous production, transport, precipitation, redissolution, and breakdown of substances in different parts of the soil profile. The process itself changes with time, because pH of the soil tends to decrease with continuing depletion of weatherable minerals (acid neutralising capacity, see Chapter 7). This causes a change in microflora and microbial activity, and in the dissociation of organic acids. The changes with time and the resulting profile morphology are best understood when we describe the podzolisation process in various stages. It should be clear, however, that processes acting in early stages may continue throughout the development. a. Decomposition of plant litter and roots, and exudation by roots and fungi, results in humic substances and LMW organic acids. Both are effective in weathering of primary minerals, and percolating water transports part of the organic substances. Weathering results in liberation of metal ions and silica. Most of the dissolved silica (H4SiO4) is removed from the profile by percolating water, together with monovalent and divalent cations. b. During downward transport, the LMW acids are largely decomposed. Trivalent metal ions bound by these acids can be precipitated as hydroxides or silicates, or transferred to organic larger molecules. This is probably the source of allophane or imogolite in the B-horizon of podzols on rich parent materials in northern Scandinavia (Gustafsson et al., 1995). Allophane, however, is only found in podzols formed in relatively rich parent materials. The metal - HMW organic matter complexes migrate downward in the profile, taking up more metals on the way. The complexes precipitate upon 'saturation', forming an illuvial horizon that is enriched in organic matter and metals. Organic matter precipitates as monomorphic coatings. When topsoils still contain mobilisable Fe and Al to neutralise and precipitate the organic matter, migration of HMW acids is absent or shallow. Biological mixing also counteracts downward migration of other dissolved and suspended material. As a result, the initial surface horizon is enriched in organic matter plus associated Fe and Al (Ah-horizon). As the process continues, however, larger amounts of organic matter are produced and transported, the soil acidifies and biological homogenisation decreases both in intensity and in depth. Ah horizon formation is decreased while transport of soluble material reaches deeper into the soil. This result in the formation of an eluvial (E) horizon, just below a progressively thinner zone of biological homogenisation, and of an illuvial layer immediately below the E-horizon (see also Figure 11.2). Formation of an E-horizon at the site of a former B-horizon implies that the organic matter of the B-horizon must disappear (see c). Fe(III)-oxides usually coat the dominant soil minerals (quartz, feldspars), and cause most well-aerated soils to have yellowish-brown colours. The removal of these oxide coatings by complexing organic acids is responsible for the ash grey colour of the eluvial horizon of podzols. c. As the production of transportable organic matter in the litter layer continues, the progressively weathered topsoil further loses its capacity to provide metal ions to 266 Question 11.6. Figure 11.2 depicts the horizon development with time of a podzol in a porous medium with good drainage. Give horizon designations (consult Appendix 1) for the horizons 1-6. Question 11.7. How can podzolisation occur even when the A- and E-horizons still contain appreciable amounts of weatherable minerals? Consider the kinetic aspects of different processes involved in podzolisation. 11.5. Influence of parent material, nutrients, and hydrology on the podzol profile. Figure 11.2 suggests that the gradual deepening of the profile with time stops after a certain period, after which the profile morphology does not change further. The final thickness of the E horizon and the expression of the B horizon depend on many factors, such as composition of the parent material, nutrient status of the subsoil (decomposition of organic matter) and composition of the organic fraction itself. It is possible that the maximum depth at which dissolved organic matter is decomposed would determine profile equilibrium with respect to depth of the E horizon. The maximum content of organic matter is determined by its Mean Residence Time (see Chapter 6). Question 11.8. Explain why equilibrium may be reached under the circumstances outlined above. Parent material influences podzolisation through its content of weatherable minerals, Fe and Al oxide contents, and nutrient content. Content and kind of weatherable minerals Figure 11.2. Changes with time in the thickness (depth) of six different horizons (labelled 1-6) in a podzol. Arbitrary scales. 267 determine the neutralising capacity of the soil for organic acids, and thereby its buffer capacity against podzolisation. Question 11.9. How would the development of a podzol profile be influenced by a) large amounts of easily weatherable Al-silicate minerals; b) large amounts of slowly weatherable Al-silicate minerals; and c) low amounts of weatherable minerals. As discussed above, availability of nutrients strongly influences organic matter dynamics. Higher levels of nutrients cause faster decay. Faster decay leads to a lower Mean Residence Time of the organic fraction. A lower MRT implies less accumulation of organic matter. Mean residence times of organic matter in B-horizons of zonal podzols tend to be much lower than that in intrazonal podzols. Mean residence times of B-horizon humus vary from a minimum of 400-500 years in some Scandinavian (zonal) podzols through 2000-2800 years in cemented B-horizons of (intrazonal) podzols in SW France, up to 40.000 years for some tropical podzols (Schwartz, 1988). In the Netherlands, MRT's measured so far range between 2000 and 3000 years. Question 11.10. Parent materials of zonal podzols are often relatively rich in weatherable minerals and lithogenic nutrients. Give the (geological) reason for this phenomenon. Question 11.11. Two podzols have equal net production of organic acids in the O/Ah horizon, but organic matter in soil (1) has a lower MRT than that of soil (2). a) What do you expect about the relative accumulation of organic matter and sesquioxides in the B- horizons of profiles 1 and 2? b) Does the MRT influence the total amount of sesquioxides that is accumulated in the B-horizon? In a given climate, the speed of podzol formation strongly depends on parent material. On poor parent materials it is a fairly rapid process. In Dutch inland sand dunes, which do not contain calcium carbonate and are very low in weatherable silicates, clear podzol morphology is already present after a hundred years of vegetation development. Periodic high groundwater in porous soils favours podzolisation through three main factors: inhibited decomposition, inhibited mixing, and reduction/removal of iron. Extreme wetness may fully inhibit podzolisation if downward water movement is absent. In coarse-textured podzols, hydromorphism (reduction of iron) combined with lateral groundwater flow usually leads to complete depletion of iron. Dissolved Fe2+ is hardly bound by organic matter and is easily removed by flowing groundwater. In such podzols, aluminium is the dominant complexed cation. A hydrosequence of sandy podzols is shown in Figure 11.3. 268 At the left-hand side, a well-drained podzol exhibits Ah, E, Bhs and Bs horizons. Where the highest groundwater level touches the Bs-horizon, a concretionary Bs(g) horizon is formed. Further downward, the Ah horizon becomes peatier and the Bh merges with the Ah, while the E-horizon disappears. Iron and iron-bound phosphate, that are solubilised by reduction, are leached from the soil and transported to depressions where they may accumulate depending on aeration (iron hydroxide at anoxic/oxic interfaces, blue vivianite, Fe3(PO4)2, or siderite, FeCO3, in anoxic zones). In poorly drained podzols, lateral transport of organic matter may cause accumulation of dense humus pans. If this organic matter reaches open water, it causes 'black water rivers'. Part of the laterally transported organic matter accumulates as bands in the subsoil, where it accentuates lithological discontinuities (Figure 11.4). In fine-textured parent materials, such as glacial tills, stagnant groundwater causes reduction and redistribution of iron, but seldom leads to iron depletion. In loamy soils, there is a whole range of transitions between podzols and gley soils in which iron concretions can be found either in the topsoil, the albic horizon, the spodic horizon, or below the spodic horizon, depending on groundwater fluctuations and parent material characteristics. Figure 11.3. A podzol hydrosequence in cover sand of the Netherlands. Explanation see text. From Buurman, 1984. 271 Question 11.13. Which processes cause the transformation from mica to beidellite (see Chapter 3.2)? The fraction of expanding 2:1 minerals in the A- and E-horizons tends to increase with time, temperature, drainage, and acidity. In very strongly weathered profiles, however, smectite may be removed completely by weathering. Clay weathering is stimulated by low concentrations of free, uncomplexed Al3+. In the eluvial horizon, complexation and removal by leaching depress Al3+ concentrations, and weathering rates are high. In the B-horizon, most of the aluminium occurs in the form of organic complexes, but microbial degradation increases the concentration of free Al3+. If the pH in B-horizons is sufficiently high, aluminium may polymerise and form Al-hydroxy interlayers in vermiculites and smectites (formation of 'soil chlorites'). In tropical and subtropical podzols, where weathering is very intense, kaolinite may dominate the illuvial horizon. It is not clear whether this kaolinite was formed during podzolisation. 11.7. Recognising podzolisation DIAGNOSTIC HORIZONS Because of the intense colour contrast between Ah, E, and B-horizons, podzols are easily recognised in the field. If an eluvial horizon is absent, as in incipient podzols and in Figure 11.6. Dissolution features in the top of a podzol from the coastal lowlands of eastern Malaysia. The E-horizon is about 70 cm thick. Photo P. Buurman. 272 transitions to Andisols and some Cambisols, micromorphologal and chemical criteria are used to identify podzolisation. The spodic and albic horizons are used to classify Podzols (Spodosols). The presence of a spodic horizon reflects the dominance of the podzolisation process over other soil forming processes, such as weathering, ferralitization, and clay illuviation. The spodic horizon is a B-horizon that shows a certain accumulation of humus, iron and aluminium (sesquioxides). Al and Fe arrive in podzol B-horizons as organic complexes. Upon decay of the organic carrier, the metal is set free and forms an amorphous component, which may crystallise with time. Therefore, we can expect organically bound, and amorphous and crystalline sesquioxides of Fe and Al as well as amorphous Al-silicates in podzol B-horizons. Chemical criteria (see below) for this diagnostic horizon are minimal because of the difficulty of defining appropriate boundaries between Andisols and Spodosols. The present definition (Soil Survey Staff, 1994; Deckers et al., 1998) assures that almost any recognisable podzol B-horizon is a spodic horizon. The World Reference Base and SSS have the following criteria in common: Colour: B-horizons can be either black or reddish brown Organic matter: more than 0.6% C accumulation pH-water:  5.9 Sesquioxides: Alox + 1/2Feox  0.5 Optical density oxalate extract:  0.25 Thickness:  2.5 cm In addition, SSS has the following criteria: Cementation: A horizon cemented by organic matter and Al, without Fe. Coatings: 10% or more cracked coatings on sand grains. Question 11.14. What aspect of the podzolisation process is reflected by each of these criteria? The albic horizon in podzols, which is defined as a bleached horizon with a colour determined by uncoated mineral grains (without iron coatings), is virtually equivalent to the E-horizon. Question 11.15. E horizons often contain small remnants of organic matter resembling amorphous coatings typical of B-horizons. Why? MICROMORPHOLOGY Weatherable minerals in E-horizons of podzols under coniferous forest are criss-crossed by tunnels with a diameter of about 5 m, formed by (presumably ectomycorrhyzal) fungi. Tunnels are rare or absent in young podzols and tunnel frequency regularly increased with time over 8000 years of soil formation (Hoffland et al., 2002). Tunnels appear to be rare or absent in related soils (e.g. Dystric Cambisols) that lack and E-horizon. 273 Figure 11.8. Polymorphic organic matter from a Humod on a river dune, Blitterswijk, the Netherlands. Height of picture 1 mm. Photo A.G. Jongmans. 276 Assume that ‘fulvic acids’ and ‘humic acids’ represent LMW and HMW components, respectively, and that ‘humin’ in these profiles is mostly undecomposed plant litter. Ignore the ‘acetyl bromide fraction’. a. Explain the lateral changes in profile development. Consider vertical and lateral water movement. b. What form of organic matter do you expect in the various B-horizons? c. Study the trends in humus fractions (for the meaning of the fractions, see Chapter 4.5). Explain differences between A and B-horizons. A ground water sample from below the B22h horizon was analysed for organic matter, aluminium and iron. The find out whether the metals were transported as organic matter complexes, the water was filtered over two different gels (Figure 11D). Small molecules move faster through the gel than large molecules, and therefore, the first eluent (away from the Y-axis) carries predominantly small molecules, while the larger molecules follow later (peaks close to the Y-axis). Each type of gel has a so-called 'cut-off value', which indicates which size of molecules can pass. Molecules larger than the cut-off value are retained in the gel. For gel G10, the cut-off value is 700 Daltons; molecules of weights between 100 and 5000 D pass through G25. The vertical axis in Figure 11D gives the amount of a component in the eluent; the horizontal axis is the eluted volume. d. Discuss the composition of dissolved organic matter (DOC, DOM). Explain the different behaviour of Fe and Al on one hand and Ca on the other hand. Figure 11B. Organic matter fractions in a podzol sequence of Les Landes (France). From Righi, 1977. 277 Figure 11C. 14C ages of organic matter in the sequence of Figure 11B. Numbers indicate years BP. From DeConinck, 1980b. 278 Figure 11D. Elution curves of groundwater components through gels Sephadex G10 and G25. From Righi, 1976. 11.9. Answers Question 11.1 Because hampered decay of litter is one of the prerequisites of podzolisation, we should look for ecosystems with a litter layer (brushwood tundra, spruce forest, oak forest). For better prediction, data on percolation and parent material should be available. Question 11.2 Because of a permanent litter layer, production in the podzol may continue throughout the year. In addition, decay is probably faster in the brown soil because pH and nutrient content are probably higher. Question 11.3 Al(C2O4)2- (aq) + O2 + 2 H2O  Al(OH)3 + HCO3- + 3 CO2 Question 11.4 'Upon saturation' means that all negative charge is compensated by metal ions. Because dissociation decreases at lower pH, lower amounts of metal can be bound. This causes an increasing C/metal ratio of the precipitate upon decreasing pH. On the other hand, hydroxylation of metal ions decreases with decreasing pH, so that fewer metal ions are necessary to compensate the negative charge of organic matter. Also this increases the C/metal ratio of the precipitating organic matter. 281 These are relatively small fractions, especially for Fe. This suggests that the organic carrier is broken down rapidly, so the MRT is probably low. Relatively small amounts of Al and especially Fe are (still) bound to organic matter. b) The different theories predict different fates for dissolved substances and allophane formation: i) The fulvate theory predicts that 1) HMW-Al and -DOC are produced below the O horizon, and 2) are transported downwards to be precipitated in the B horizon ii) The allophane theory predicts that 1) HMW-Si is produced below the O horizon, and 2) are transported downwards to be precipitated in the B horizon as allophane or imogolite iii) The LMW acids theory predicts that 1) LMW-Al and -DOC are produced below the O horizon, and 2) are transported downwards to be decomposed in the B horizon where Al precipitates, as amorphous product, e.g. allophane or imogolite iv) The LMW acid/roots theory predicts that sesquioxides are transported by organic carriers and precipitate when the carriers are broken down. The hypotheses can be tested by determining the fluxes of these different solutes as a function of depth. If soil solution concentrations would represent annual flux-weighted means, calculating them with the percolation rates gives solute fluxes in mmol/m2.yr: (mmol/l) * mm/yr = mmol/m2.yr. The resulting solute fluxes at the bottom of a number of horizons are given below. The difference between total and HMW-DOC, -Si and -Al would represent LMW species: horizon water flux mm/y r fluxes, mmol/m2.yr DOC tot. DOC HMW Si tot Si HMW Al tot Al HMW Fe tot O2 200 6400 3840 23 2.3 9.6 5.3 1.8 E2 150 600 330 49 3.9 10.5 6.8 2.0 B1 120 360 126 32 1.6 3.8 2.3 1.3 B2 100 100 30 22 0.44 0.1 0.001 0.1 B3 90 45 16 15 0.15 0.009 0.001 0.001 The data are consistent with all three mechanisms: both HMW and LMW species disappear from solution as it moves from the E into the B-horizon. However, the contribution of HMW-Si (which may be inorganic colloidal Si) appears to be very small (3.9-0.15 = 3.75 mmol/m2.yr of HMW Si are removed against 34 - 3.75 = 30.25 mmol/m2.yr of LMW-Si). Therefore, ii) would be relatively unimportant if it exists at all. Possibly, HMW-Si is partly organically bound, so further work would be needed to test this. LMW and HMW forms of dissolved organic matter and of Al are about equally important, indicating that both i) and iii) apply. Note first, that the drop in DOC concentration is much larger between the O and E1, than between the E2 and B1 horizons, although accumulation of solid C is larger in the B1 than in the E1 horizon (see Table 11A 282 C %). This indicates that most of the decrease in DOC with depth must be attributed to decomposition to CO2, not to precipitation as solid organic material! This is also compatible with the LWM/root model. Note, furthermore, that most of the soluble Al and Fe and about half of the soluble Si that percolate from the E into the B horizons are produced already in the organic O horizon, not in the E horizon as postulated by these two theories. A likely explanation is that mycorrhizal fungi bring Fe, Al and Si, mobilised by them in the E horizon (see Figure 4.12), towards mycorrhizal roots in the O horizon (where most of them reside). These substances (unwanted by plants, contrary to P, Ca, Mg and K that are taken up by these fungi too) are exuded or released, and next percolate back into the E horizon. Alternatively, Si may be produced by weathering of mineral components mixed with the litter in the O- horizon. Theories ii) and iii) predict the formation of allophane or other forms of amorphous Al. Allophane contents are calculated using a Si mass fraction of 14%. Therefore, Sio concentrations have to be converted to weight percentages. The Al that can be attributed to allophane is Alo-Alp. Because both Al and Si are given in mmol.kg-1, the molar ratio can be calculated directly: (Alo-Alp)/Sio. Thus, allophane contents and Al/Si ratios in allophane are: B1 B2 B3 B4 B5 C14 Allophane % 0 2.9 3.3 2.6 1.8 0.3 Al/Si ratio 4.5 2.7 2.6 2.3 2.3 2.6 Because of low contents, molar ratios for B1 are not reliable. Allophane may be formed by recombination of aluminium that is set free by decay of organic complexes with silica in solution. Note that the distribution of allophane with depth does not agree with that expected from the changes with depth of the Al and Si fluxes. These fluxes would result in higher allophane contents in the B1, and much lower contents in the B3 horizon. Probably, the fluxes based on solution concentrations in June 1996 are not representative for centuries of annual fluxes. Problem 11.2 a) In the left part of the drawing, the podzol has an E horizon, which disappears towards the right. In addition, the B-horizon is higher in the profile at the right hand side. This suggests that there is more vertical percolation at the left side, and that restricted vertical water movement and increased influence of ground water fluctuations dominate at the right hand side. The B21h and B2h-horizons are loose, which means that they are aerated part of the year. However, it is illogical that the loose B2h disappears under the cemented B22h. This may be an error in the drawing. b) The friable parts will have dominant polymorphic (excrement) organic matter, while the cemented parts have monomorphic (cutan) organic matter. c) There is a strong shift in composition from the A to the B horizons. The most obvious differences are: - Higher humin contents in the A; - Much higher fulvic acid contents in the B; - A smaller acetyl-bromide soluble fraction in the B. 283 Humin consists of strongly mineral-bound and of incompletely decomposed organic matter. In sandy soils, the latter is low. The difference in humin is therefore mainly due to incompletely decomposed plant material. The fraction of undecomposed plant material is, of course, mainly restricted to the topsoil and to the friable B-horizons, where roots decompose. Roots are probably much scarcer in the cemented horizons. Fulvic acids are water soluble, and are therefore easily transported to the B horizon, where they precipitate with metal ions (in this case Al, because it is a hydromorphic sequence). d) Both elution curves indicate a dominance of high-MW organic molecules. In the G10 curve, all Fe and Al appears to be bound to the largest fraction, while in G25 it is bound both to the smallest and larger fractions (the smallest fractions on G25 are the largest on G10). The movement of Ca in G10 seems to be related to the low-MW organic fraction (adsorption), but it may also be independent of organic matter. The G25 curve, however, suggests that Ca is adsorbed to the low-MW organic fraction. 11.10. References Alban, D.H., and E.C. Berry, 1994.Effects of earthworm invasions on morphology, carbon and nitrogen of a forest soil. Applied Soil Ecology, 1:243-249. Anderson, H.A., M.L. Berrow, V.C. Farmer, A. Hepburn, J.D. Russell, and A.D. Walker, 1982. A reassessment of podzol formation processes. Journal of Soil Science, 33:125- 136. Bal, L., 1973. Micromorphological analysis of soils. PhD Thesis, University of Utrecht, 174 pp. Barrett, L.R., and R.J. Schaetzl, 1998. Regressive pedogenesis following a century of deforestation: evidence for depodzolization. Soil Science, 163:482-497. Bruckert, S., 1970. Influence des composés organiques solubles sur la pédogénèse en milieu acide. I. Etudes de terrain. Annales Agronomiques, 21:421-452. Buurman, P. (ed), 1984. Podzols. Van Nostrand Reinhold Soil Science Series. 450pp. New York. Buurman, P., 1985. Carbon/sesquioxide ratios in organic complexes and the transition albic-spodic horizon. Journal of Soil Science, 36:255-260. Buurman, P., 1987. pH-dependent character of complexation in podzols. In: D. Righi & A. Chauvel: Podzols et podzolisation. Comptes Rendus de la Table Ronde Internationale: 181-186. Institut National de la Recherche Agronomique. Buurman, P., and A.G. Jongmans (2002). Podzolisation - an additional paradigm. Proceedings 17th International Congress of Soil Science, Bangkok. In press. Buurman, P., and L.P. van Reeuwijk, 1984. Allophane and the process of podzol formation - a critical note. Journal of Soil Science, 35:447-452. De Coninck, F., 1980a. The physical properties of spodosols. In: B.K.G. Theng (ed): Soils with variable charge. 325-349. New Zealand Society of Soil Science, Lower Hutt. De Coninck, F., 1980b. Major mechanisms in formation of spodic horizons. Geoderma, 24:101-128. Deckers, J.A., F.O. Nachtergaele, and O. Spaargaren, 1998. World Reference Base for Soil Resources. Introduction. ISSS/ISRIC/FAO. Acco, Leuven/Amersfoort, 165 pp. Farmer, V.C., J.D. Russell and M.L. Berrow, 1980. Imogolite and proto-imogolite allophane in spodic horizons: evidfence for a mobile aluminium silicate complex in