Clay Movement and Illuviation in Soils: Factors and Processes, Manuais, Projetos, Pesquisas de Agronomia

Baixe Clay Movement and Illuviation in Soils: Factors and Processes e outras Manuais, Projetos, Pesquisas em PDF para Agronomia, somente na Docsity! 205 CHAPTER 8 TEXTURAL DIFFERENTIATION 8.1. Introduction Textural differences between parent material, topsoil, and subsoil are common in soils of virtually all climates. Sometimes, such differences are inherited from the substrate (e.g., textural variations in sediments). Frequently, however, the differences are due to soil forming processes. Textural differentiation leads to topsoils and subsoils that are either finer, or coarser than the parent material. At least eight processes can result in textural differentiation: 1. Physical and chemical weathering of parent material, 2. Upward vertical transport of fine fractions by biological activity, 3. Downward transport of clay suspended in percolating soil water, 4. Superficial removal of clay by erosion without illuviation, 5. Superficial removal of clay due to tillage in wetland rice agriculture (‘puddling’), 6. Clay formation in the subsoil by precipitation from solution, 7. Weathering/dissolution of clay, 8. Vertical movement of soil (matrix) material. In soil classification systems, process (3) is emphasised, but other causes of texture differentiation may be more important in specific cases. The following discussion of the various processes leading to textural differentiation is adapted from Buurman (1990) and Soil Survey Staff (1975). 8.2. Processes of textural differentiation and their characteristics PHYSICAL AND CHEMICAL WEATHERING As discussed in Chapter 2, physical weathering caused by temperature differences, and ice formation reduces the size of rock fragments and mineral grains. Such physical weathering is strongest in the topsoil, where temperature differences are most pronounced. Physical weathering reduces the size of gravel, sand and silt fractions, but hardly produces any clay- 194 size material. Mild chemical weathering results in size reduction through, e.g., exfoliation of micas. Strong chemical weathering processes related to different soil forming processes are discussed elsewhere (Chapters 7, 10-13). BIOLOGICAL ACTIVITY Burrowing animals cannot directly move soil particles that much exceed their own size. By bringing fine material to the surface, however, they will indirectly cause a downward transport of coarser fragments. Slowly, the coarser fragments sink down to the lower boundary of animal activity. This is especially visible in soils with a conspicuous coarse fraction, such as gravel, in a fine matrix. Examples of this process are plentiful: - Ground squirrels in North America (pocket gophers) bury relatively large stones, while homogenising coarse-textured, gravelly soils. - Earthworms deposit part of their fine-textured ingested material as casts at the soil surface. Coarse fragments are not ingested and sink away. This is perfectly illustrated in the ruins of Roman villas in England. The wall foundations of such structures may still be in place, while the tile floors between the foundations are found 20-50 cm below the original surface, completely covered with a dark surface soil. After removing this soil, earthworms again deposited their excrements on top of the tiles (Darwin, 1881). - In tropical areas, termites build nests in the soil, or large mounds on top of the soil. Both nests and mounds are predominantly built of clay and silt. The nests decay again with time, causing finer textures in the topsoil. Coarse material is not brought up and may sink to a depth of more than 2 metres below the surface, forming distinct stonelines. In addition, mound-building termites carry fine material from the surroundings to the mound site, causing not only vertical differentiation, but also horizontal variability in fine material contents (Wielemaker, 1984). DOWNWARD TRANSPORT OF SUSPENDED CLAY (Eluviation and Illuviation) Eluviation (removal) and illuviation (addition) of clay in a soil depend on a number of conditions. In a dry soil, all clay is flocculated and bound in aggregates or on grain surfaces. Abrupt wetting of a dry soil may disrupt the aggregates and mobilise clay by air explosion. Air explosion is the sudden disintegration of a dry structural element when it is wetted: water enters the pores by capillary force and the entrapped air builds up sufficient pressure to blow the aggregate apart. For air explosion to be effective, the soil must be dry. This implies, that clay illuviation is favoured by climates with distinct dry periods (during which the soil dries out), followed by rainfall of high intensity. Air explosion occurs in all soils that are periodically dry, but it does not automatically bring clay particles into suspension. As outlined in Chapter 3, clay particles will disaggregate and remain in suspension when the electrical double layer expands. This expansion is favoured by 1) low electrolyte concentrations, and 2) a high concentration of monovalent ions (Na+). Expansion is hampered by di- and trivalent ions, such as Ca2+, 205 RECOGNITION OF CLAY ILLUVIATION In soils with clay eluviation and illuviation, the topsoil is depleted in fine clay. The subsoil with illuviated clay has the following properties: - It contains argillans along pores and on structural elements. In the field, argillans can only be identified with a hand lens in a soil that is not too wet. Argillans usually have surfaces that are smooth. They superficially resemble 'slickensides', shiny pressure surfaces that are formed by sliding structural elements in smectite clay soils that swell and shrink (see Chapter 10), but lack the striations (linear patterns) typical of slickensides. Many argillans are darker than the matrix and they do not contain coarse grains. Argillans are best-recognised micromorphologically, in thin section (Figure 8.2). Some argillans, however, have formed by chemical precipitation of clay minerals from solution (see ‘clay formation’, below), but these are usually not confined to linings along pores and peds. - It has higher clay content and higher fine clay to clay ratio than the overlying and underlying horizons. The fine clay fraction (<0.5 or <0.2 m) is transported preferentially, so that the illuvial horizon has a relatively high fine clay content. If the parent material is stratified, the clay content in the illuvial horizon is not necessarily higher than that of the horizon below it. - It has a higher CEC-clay than the overlying and underlying horizons. Because fine clay has a larger specific surface than coarse clay, it also has a higher CEC per unit mass of clay. This is reflected in a relatively high CEC-clay of the illuvial horizon in soils of mixed mineralogy. - Its clay fraction contains more smectite than that of the overlying and underlying horizons, because the finest clay fraction is usually dominated by smectite. Whereas illuviated clay is initially deposited as strongly parallel-oriented material in cutans, these argillans do not persist forever. Part of the cutans will pass through intestines of soil-ingesting fauna, and end up as small separate lumps of oriented clay, called ‘papules’. Given sufficient time, the orientation may disappear altogether. SUPERFICIAL REMOVAL OF CLAY BY EROSION: ELUTRIATION Overland flow of water is a common process in many landscapes, e.g., under tropical rainforest. Such overland flow may not result in features we commonly associate with erosion, such as gullies or sheet erosion, because it covers only short distances at a time, and the water is again taken up by the soil. However, in the long run, overland flow may cause significant removal of fine fraction from the topsoil; a process called elutriation (Eng.) or appauvrissement (Fr.). Canopy drip (concentration of rainwater by the leaf form or tree structure, resulting in localised drip of water instead of even distribution) usually results in increased dispersion of clay in the drip area, partly as a result of larger drops. During overland flow, the suspended clay moves with the runoff over the surface of the soil until the water infiltrates into the soil. Eventually, this process leads to removal of fine material from the surface horizon into the drainage system. Biological homogenisation spreads the effect of surface removal over the depth of the homogenised layer (usually the Ah horizon). The contrast in clay content between the Ah horizon and the underlying horizon may be quite sharp. 198 Soil fauna further stimulates the preferential removal of fine fractions from the surface soil because they bring relatively fine material to the surface (e.g. earthworm casts, termite hills). On old, stable land surfaces this may lead to a thick so-called faunal mantle (Johnson, 1990), consisting of a one to two meter thick medium-textured soil, deprived of both the very fine and very coarse soil particles. Within this faunal mantle, the Ah horizon may be markedly lighter-textured again. Question 8.3. a) Characteristics of surface removal of clay are different from those of clay illuviation. How can you recognise surface removal? (Consider argillans, fine/coarse clay ratios, clay and silt movement, clay balance, and CECclay). b) Why do 'faunal mantles' lack very coarse soil particles? TEXTURAL DIFFERENTIATION AS A RESULT OF PUDDLING Wet rice cultivation is widespread throughout the humid tropics. The practice calls for ploughing of waterlogged fields, to decrease permeability of the subsoil and to soften the soil before planting of the rice seedlings. This so-called puddling results in loss of clay from the surface soil if, as in many irrigation systems (e.g. in Indonesia, the Philippines), the water constantly flows downhill from one field to the next. The flowing water carries off all suspended matter, mainly clay. The long-term effect is a distinct removal of clay from the puddled layer. Question 8.4. How can you, by investigating the soil profile, distinguish such losses through puddling from losses through eluviation? CLAY FORMATION Clay formation accompanies weathering in practically all soils that have weatherable minerals. Although weathering is usually strongest at shallow depth, clay formation is usually stronger in B-horizons than in A-horizons. Reasons probably are that 1) clay formation in A horizons may be inhibited by organic matter (see Chapter 3.2), and 2) solutes are transported from the A to the B horizon, where their concentration may be increased by evaporation, causing stronger super-saturation with secondary (clay) minerals. This leads to B-horizons that have more clay than the overlying A-horizons. The amount of clay formed depends on the parent material and on the time and intensity of the weathering process. Strongly weathered soils, such as Ferralsols (Chapter 13) may have up to 90% clay, mainly due to new formation. Question 8.5. What is the effect of a) low contents of weatherable minerals in the parent material, and b) young soil age, on the amount of clay formed by weathering? Clay that is formed by crystallisation from the soil solution may be strongly oriented. In that case, it is difficult to distinguish from illuviation coatings. However, illuviation 205 coatings tend to have a zoned structure (Figure 8.2) with thin bands of slightly coarser particles, which is absent in clay precipitates (Figure 8.3). Recrystallisation of allophane and imogolite in Andisols may lead to clay coatings that are very similar to the (ferri)argillans that are due to translocation of clay. Because Andosols usually have a high Al-activity and because phyllosilicates are present in only minor amounts, 'argillans' observed in such soils should usually be ascribed to recrystallisation, or 'clay formation' (Buurman and Jongmans, 1987; Jongmans et al., 1994). See also Chapter 12. Differences in clay formation in stratified parent materials (unequal grain size, unequal amounts of weatherable minerals) are a common source of differences - but not of differentiation - in texture. Layers with higher contents of weatherable minerals will usually develop higher clay contents. This is a very common feature in layered volcanic deposits. Such stratification is also reflected in composition of the sand and silt fractions and is easy to spot when detailed grain-size analyses or sand mineralogy are available. If the coarser- textured layer is encountered in the topsoil, the difference with the subsoil is often (erroneously) attributed to clay movement. TEXTURAL DIFFERENTIATION THROUGH BREAKDOWN OF CLAY The weathering of clay that accompanies surface-water gleying in acid soils has been discussed in detail in Chapter 7. This breakdown usually enhances already existing differences in texture that were either formed by illuviation, or were due to stratification of the parent material. MOVEMENT OF MATRIX MATERIAL Downward movement of 'whole soil' material (usually fine silt, organic matter, and clay) is common in cultivated soils that have topsoils with low structure stability. In such soils, coatings of unsorted fine material can be found on the vertical walls of pores or, in sandy materials, as caps on the top of sand grains. Transport of matrix material occurs preferentially during heavy showers on freshly ploughed soil. This process is purely a suspension transport which ends when the water stops flowing in the larger pores. 8.3. Intensity and expression of textural differentiation by clay illuviation Clay movement is a purely mechanical process, which can be relatively rapid if the conditions are optimal. Clay illuviation has been demonstrated in deposits exposed by glaciers less than 200 years ago. In general, however, more than a thousand years will be necessary to develop a distinct illuvial horizon. 202 climates than in temperate climates, which may be due to both higher rainfall (deeper percolation) and usually longer time of soil formation in the tropics. In soils with swelling and shrinking clays, physical movements may be strong enough to obliterate effects of clay movement. Moreover, oriented clay is not visible in a matrix that is dominated by pressure structures. Many profiles with clay movement have younger overprints of other processes, such as podzolisation, surface water gley, or homogenisation. Figure 8.4. Stages of clay illuviation in river terraces. Left: Macley River; right: Gooroomon Ponds, Australia. K1 to K5 are successive stages of development. From Brewer, 1972. Reproduced with permission of the Geological Society of Australia. 205 8.4. Clay minerals in soils with clay eluviation and illuviation Fine-grained clay minerals, such as smectite, vermiculite, and illite, appear to be more mobile than kaolinite, which is relatively coarse-grained. This may lead to a mineralogical differentiation of eluvial and illuvial horizons. Weathering and clay mineral assemblage in soils with clay movement are variable. In temperate regions, clay mineral assemblages in the illuvial horizon usually resemble those of the parent material, but with higher contents of smectite minerals. In the tropics, profiles with clay movement can be strongly weathered and very similar to Oxisols. In these soils, kaolinite is the dominant - and sometimes the only - clay mineral. Many soils with a textural contrast in the tropics are acid, with high contents of exchangeable adsorbed aluminium. Few have been investigated micromorphologically, and it is not sure whether illuviation or elutriation has played a dominant role in causing vertical texture differentiation in such soils. Question 8.8. The presence of illuviated clay in soils with much exchangeable Al3+ suggests that clay movement is not an active process in these soils. Why? In temperate regions, weathering is much more restricted, e.g. vermiculite formation from illite in the eluvial horizon, and interlayering of illites to form soil chlorites in the illuvial horizons (see Chapter 3.2). 8.5. Texture-based diagnostic horizons in soil classification Changes in texture in a soil profile may dominate soil-forming processes and affect agronomically important soil properties, such as rootability, water availability, and water percolation. For these reasons, changes in texture are used in soil classification. Differences in texture due to movement and/or depletion of clay are quantified in the arg(ill)ic and natric (B) of USDA and FAO and in the kandic horizon of USDA (SSS, 1987 and later). The agric horizon of the USDA is due to transport of matrix material. Both the argillic and the natric horizon should have a certain amount of illuviated clay, either indicated by grain-size analysis, or recognised as oriented clay in thin section. The concept of these diagnostic horizons is based on clay eluviation and illuviation, but their definitions do not necessarily exclude textural differences that were formed in a different way. Many tropical soils have a relatively light-textured surface soil, but do not show clear clay illuviation cutans in the finer B-horizon. Such soils would not have an argillic horizon. The absence of cutans is generally explained by the theory that kaolinitic clays do not form birefringent cutans. To accommodate such soils, the kandic horizon was created (from kandites, a general term for 1:1 clays). The kandic horizon is defined by its upper boundary rather than by properties of the horizon itself: it should have a sharp increase in clay 204 content at its top, but need not have evidence of illuviated clay. Chemically and mineralogically, the kandic horizon is similar to the oxic horizon (see Chapter 13: Ferralitisation). Although there is only one argillic horizon for temperate and tropical climates, classification does take account of the weathering state (base saturation and CEC of the clay fraction) of the argillic horizon. In temperate regions, the illuvial horizon is usually mildly under-saturated and pH values are between 5 and 7, except if they have been affected by ferrolysis (see Chapter 7). Ferrolysed B horizons can be strongly depleted in exchangeable bases and strongly weathered, and have lower pH values. In tropical regions, argillic horizons tend to be low in exchangeable bases, and strongly weathered. The clay mineral assemblage is dominated by kaolinite, but some weatherable minerals may remain in the silt and sand fractions. Colours are usually redder than in temperate regions (see also Chapter 3). In the wet tropics, where the soil does not dry out seasonally, soils rarely show illuviated clay, and argillic horizons are scarce. If the textural contrast is sufficient, horizons are classified as kandic. The natric horizon is an argillic horizon high in exchangeable sodium. It is characterised by prisms showing typical rounded tops (see Chapter 9). These horizons probably form upon leaching of saline soils. When NaCl-rich soils are leached, dilution of the electrolyte results in peptisation of sodium clay. The dispersed clay is transported downward, where it may fill up the pore system and reduce permeability (see Chapter 7). Because adsorption of Na also disrupts the conformational structure of soil organic matter (see Chapter 4), clay illuviation is frequently accompanied by illuviation of organic matter. On stable land surfaces, soils with eluviation and illuviation of clay have an eluvial (E) and an illuvial (Bt; argillic) horizon. The E horizon is usually characterised by lighter texture and colour than the overlying and underlying horizons, but it is not white enough to qualify for an albic horizon. The albic horizon is clearly expressed in soils with clay movement that have also periodically stagnating water on the B-horizon, causing ferrolysis (see Chapter 7). In eroded soils, the E horizon has disappeared, and an Ah horizon may directly overlie the Bt. 205 Table 8C. Clay contents and oriented clay in the soil profile Heerlerheide (Typic Hapludalf, Orthic Luvisol) developed in loess. From Van Schuylenborgh et al., 1970. horizon depth cm Clay (mass%) oriented clay (volume %) total in situ ferri-arg papules Ah 0- 13 13.2 n.d. n.d. n.d. E 13- 21 13.6 0.3 0.0 0.3 Bt1 21- 33 15.5 2.2 1.1 1.1 Bt2 33- 92 21.5 4.1 2.3 1.8 Bt3 92-123 22.2 2.8 2.3 0.5 BC 123-307 19.2 1.2 1.0 0.2 C 307-327 15.0 0.3 0.3 0.0 Problem 8.5 Figures 8D and 8E (next page) give textural profiles and fine clay to clay ratios for a num- ber of soil profiles from West Kalimantan, Indonesia. Which processes could be respon- sible for the texture differentiation? Assume that the in each case the lower horizon repre- sents the C horizon. 8.7. Answers Question 8.1 a) The stable field below pH 5 is caused by Al3+ in solution; the stable field around pH 8 by the presence of calcium (and magnesium) carbonates, which cause both the presence of divalent cations in solution and a sufficiently high concentration of these cations to keep clays flocculated. b) The presence of gypsum (CaSO4.2H2O) has no influence on pH. It is more soluble than calcite and will therefore stabilise clays with exchangeable Ca2+ ions. 208 Figure 8D. Texture profile of profile Kalimantan 2. From Buurman and Subagjo, 1980 Figure 8D. Texture profile of profile Kalimantan 2. From Buurman and Subagjo, 1980. Figure 8E. Texture profile of profile Kalimantan 3. From Buurman and Subagjo, 1980. 205 Question 8.2 The main cause of major changes in soil forming processes is a change in climate. In the present case it should have been a change from a fairly moist to a drier climate. Question 8.3 a). Superficial removal of clay without illuviation can be spotted by the following features: i) Absence of argillans. Illuviated clay cutans are absent from the B-horizon or deeper in the profile. ii) Lower clay contents and fine clay contents in the topsoil without higher fine clay/clay ratios and fine clay contents in the subsoil. Fine clay to clay ratios should be more or less constant with depth. iii) Slight loss of fine silt from the topsoil. When detailed grain-size analyses are available, this effect can be measured. The patterns obtained for clay illuviation and clay removal are very different. iv) A negative clay balance: removal of clay from the surface soil exceeds the clay increase in the B-horizon. v) No increase in CECclay in the subsoil. If there is no accumulation of fine clay, the CEC of the clay fraction is not affected. b) Bringing only relatively fine soil material to the soil surface causes a downward movement of all coarser material. This tends to accumulate below the zone of faunal activity (stoneline). Question 8.4 The characteristics by which such a removal can be distinguished from clay loss and gain by illuviation are similar to those of elutriation. The process is essentially the same. Question 8.5 Low amounts of weatherable minerals result in a smaller potential to form clay. In young soils, clay formation has not reached its maximum development. Question 8.6 Process Topsoil Subsoil Remarks 1. Physical weathering finer unchanged little clay formation 2. Biological activity finer (coarser) stoneline formed 3. Clay illuviation coarser finer fine clay, CEC, coatings 4. Clay erosion coarser unchanged also silt loss 5. Puddling coarser unchanged also silt loss 6. Clay formation unchanged finer 7. Clay breakdown coarser unchanged ferrolysis 8. Matrix transport coarser finer cultivation Question 8.7 212 a) Profile 8D has a distinct increase in clay in horizons 3-5, which is offset mainly by changes in sand content. There is no loss of silt from the topsoil. Clay illuviation in these horizons should also have depressed silt contents. The fine clay/clay ratio increases with depth, but its maximum is below the clay maximum. The combination suggests a formation of clay from silt-size minerals, perhaps combined with some clay illuviation in the subsoil. There does not seem to be surface erosion. b) Profile 8E shows, towards the top, an increase in sand fraction and a loss in fine clay and silt fraction. Together these suggest a loss of fine fractions by superficial erosion. The fine clay/clay ratio seem to suggest an illuviation of clay in horizons 3-5, and it is possible that surface erosion and clay illuviation have acted together. 8.8. References Brewer, R., 1972. Use of macro- and micromorphological data in soil stratigraphy to elucidate surficial geology and soil genesis. Journal of the Geological Society of Australia, 19(3):331-344. Buurman, P., and A.G. Jongmans, 1987. Amorphous clay coatings in a lowland Oxisol and other andesitic soils of West Java, Indonesia. Pemberitaan Penelitian Tanah dan Pupuk, No. 7:31-40. Buurman, P., and Subagjo, 1980. Soil formation on granodiorites near Pontianak (West Kalimantan). In: P. Buurman (ed): Red soils in Indonesia, 106-118. Agricultural Research Reports 889, Pudoc, Wageningen. Buurman, P., 1990. Soil catenas of Sumatran landscapes. Soil Data Base Management Project, Miscellaneous Papers No. 13:97-109. Darwin, C.H., 1881. The formation of vegetable mould through the action of worms, with observations on their habits. John Murray, London, 298 pp. Johnson, D.L., 1990. Biomantle evolution and the redistribution of earth materials and artifacts. Soil Science, 149:84-102. Jongmans, A.G., F. Van Oort, P. Buurman, and A.M. Jaunet, 1994. Micromorphology and submicroscopy of isotropic and anisotropic Al/Si coatings in a Quaternary Allier terrace, France. In: A.J. Ringrose and G.S. Humphreys (eds.): Soil Micromorphology: studies in management and genetics, pp. 285-291. Developments in Soil Science 22, Elsevier, Amsterdam. Slager, S., and H.T.J. van de Wetering, 1977. Soil formation in archeological pits and adjacent loess soils in Southern Germany. Journal of Archeological Science 4:259-267. Soil Survey Staff, 1975. Soil Taxonomy. Agriculture Handbook No. 436. Soil Conservation Service, USDA, Washington Soil Survey Staff, 1990. Keys to Soil Taxonomy. Soil Management Support Services Technical Monograph 19. Blacksburg, Virginia. Van den Broek, T.M.W., 1989. Clay dispersion and pedogenesis of soils with an abrupt contrast in texture - a hydrochemical approach on subcatchment scale. PhD Thesis, University of Amsterdam, 1-109. 205 Van Reeuwijk, L.P., and J.M. de Villiers, 1985. The origin of textural lamellae in Quaternary coast sands of Natal. South African Journal of Plant and Soil, 2:38-44. Van Schuylenborgh, J., S. Slager and A.G. Jongmans, 1970. On soil genesis in temperate humid climate.VIII. The formation of a 'Udalfic' Eutrochrept. Netherlands Journal of Agricultural Science, 18:207-214. Wielemaker, W.G., 1984. Soil formation by termites - a study in the Kisii area, Kenya. PhD Thesis, University of Wageningen; 132 pp. 214 Plate Q. Massive calcite cement (arrow) in a river terrace, southern Spain. The weathered pebble (P) consist of serpentinite. Diameter of coin is 3 cm. Photograph P. Buurman P