Soil structure and classification

As a result of the various factors and processes outlined in Sections 4.6.1-4.6.5, over time soils develop stable and diagnostic features, many of which are recog nizable in the field. These features, particularly specific layers called 'soil horizons', are the basis for soil classification. An idealized soil profile, i.e. a vertical section, is shown in Fig. 4.21. Soil horizons are described using an internationally agreed system of abbreviations that are shown on Fig. 4.21 and used in the descriptions below.

The classification of soils is potentially complex. Two systems of soil classification are common, that of the US Department of Agriculture (USDA) and that of UNESCO's Food and Agriculture Organisation (FAO). In this book we mostly use the USDAs first tier of classification known as soil orders (Fig. 4.22), which can be related to factors such as degree of weathering, type of parent material and climate. Where possible the USDA name is followed by the equivalent FAO name in brackets. In some cases, for example vertisol, the name is the same in both systems. Most of the order names have the common ending -sol, from the Latin solum, meaning soil. Although soils are classified using stable features developed over time, the soils formed under dynamic conditions. It is variations in soil-forming processes that give rise to the vast number of different soil types. Three contrasting soils with distinctive diagnostic horizons are shown in Plate 4.2 (facing p. 138). Clearly a range of dynamic influences have controlled the development of each of these soils, and these are discussed separately below, emphasizing the role of soil chemistry.

4.9.1 Soils with argillic horizons

The term 'argillic' indicates that a soil has a clay-rich horizon (Bt in Fig. 4.23). The downward percolation of water through the soil controls most of the significant processes in the development of the clay-rich (argillic) horizon (Bt). The movement of water causes; (i) the leaching of calcium ions (Ca2+) from the A horizon; (ii) the washing of materials down profile (eluvation); and (iii) the deposition of these 'washed-down' materials at depth (illuvation) (Fig. 4.21). The leaching (decalcification) of Ca2+ that had bound together clay particles with excess negative charge (Fig. 4.23), causes destabilization of clay aggregates, allowing them to fall apart (deflocculate). The disaggregated clay particles are themselves then susceptible to translocation down profile (eluvation). The clay particles will re-flocculate lower in the profile, to form the argillic horizon, where sufficient divalent cations are present to re-bind them (Fig. 4.23). In the soil shown in Plate 4.2 the Ca2+ is supplied from weathering a CaCO3-rich 'C' horizon below the Bt horizon. Note that the chain of events leading to the development of the argillic horizon is analogous to those that translocate clays over larger scales in the formation of vertisols (Section 4.7). Argillic horizons can form in a number of soil types, for example ultisols (acrisols), mollisols (chernozems and kas-tanozems) and alfisols (luvisols).

4.9.2 Spodosols (podzols)

Spodosols (podzols) contain separate horizons from which material has been both removed and deposited (Fig. 4.24). However, spodosols form under very

Fresh

Litter

Partially decomposed litter

Fresh

Litter

Partially decomposed litter

O HORIZON SURFACE LITTER:

fallen leaves and organic debris

A HORIZON TOP SOIL:

organic matter (humus), living organisms, inorganic minerals

C HORIZON WEATHERED PARENT MATERIAL:

partially broken-down inorganic minerals

R HORIZON BEDROCK:

impenetrable layer

O HORIZON SURFACE LITTER:

fallen leaves and organic debris

A HORIZON TOP SOIL:

organic matter (humus), living organisms, inorganic minerals

E HORIZON 1 ZONE OF LEACHING ¿v, 1 (ELUVAL HORIZON):

[dissolved or suspended J materials move downward

B HORIZON SUBSOIL:

accumulation of iron, aluminium, humic "compounds and clay in illuval horizons leached down from the A and E horizons; more altered material than in C horizon

C HORIZON WEATHERED PARENT MATERIAL:

partially broken-down inorganic minerals

R HORIZON BEDROCK:

impenetrable layer

Fig. 4.21 Idealized soil profile showing master soil horizons and horizon abbreviations. The O, A, E and B master horizons can be further subdivided into subordinate horizons depending on composition (see Figs 4.23-4.25). Note that the O horizon is composed of fresh (L) and partially decomposed (F) organic litter. Soil profiles are typically 0.5-1.0m thick such that master horizons are typically centimetres to tens of centimetres thick. SOM, soil organic matter; CEC, cation exchange capacity.

Histisols (organic)

Deep accumulation of organic

Histisols (organic)

Deep accumulation of organic

Very cold permafrost

Desert shrubs, grasses, dry

Moist, mildly acidic fertile forests

Very cold permafrost

Desert shrubs, grasses, dry

Moist, mildly acidic fertile forests

Ultisols

(base-poor clays)

Wet tropical and subtropical forests, acid silicate and Fe, Al oxides

Entisols

(little or no profile development)

Mild weathering, various conditions Inceptisols (beginning of B horizon development)

Mild weathering on volcanic ejecta

Andisols (volcanic ash)

High base status, high CEC clays smectite, dry season

Semi-arid to moist grasslands, >0.6% SOM in thick upper horizon

High base status, high CEC clays smectite, dry season

Semi-arid to moist grasslands, >0.6% SOM in thick upper horizon

Wet, tropical forest, extreme weathering, low activity clays, Fe, Al oxides

Oxisols (oxides)

Cool, wet, sandy acid, coniferous forest

Spodosls (spodic horizon)

Degree of weathering and soil development

Slight

Intermediate

Strong

Fig. 4.22 Diagram relating the 12 USDA soil orders to degree of weathering, degree of soil development, climate and vegetation conditions. SOM, soil organic matter. Approximate FAO names where different are: entisol (arenosol, fluvisol, regosol); inceptisol (cambisol); andisol (andosol); ardisol (xerosols); alfisol (luvisols); ultisol (acrisol); spodosol (podzol); oxisol (ferralsol). Gelisol and mollisol have no simple FAO equivalent. Modified from Brady and Weil (2002), reprinted by permission of Pearson Education Inc., Upper Saddle River, NJ.

different conditions to those described for argillic horizons, the materials being removed and deposited are generally metal ions and/or organic matter rather than clay particles. Spodosols are sandy forest soils, forming beneath a horizon of tree litter, (horizon 'L' in Fig. 4.24). In pine forests, for example, large annual inputs of dead pine needles cause an accumulation of organic matter in both the litter horizon 'L' and the organic-rich 'Ah' horizon. The low pH in these horizons, resulting from the release of organic acids, often limits organic matter processing by soil organisms allowing thick accumulations to form.

Spodosols are typified by a prominent eluvated horizon (E) that has lost many of its metal ions, particularly iron, manganese and aluminium. This horizon shows clearly in Plate 4.2 as the grey Ea horizon. Rather insoluble metal ions like iron and aluminium are leached because their solubility has been enhanced by a 'com-plexing' or chelating agent. In spodosols the 'free' metal ions are chelated (see Box

Water

Water

Divalent cations, e.g. Ca2+, bind negatively charged clay particles together, facilitating clay particle aggregation

When divalent cations are leached from upper soil horizons they no longer bind clay particles. Consequently, the negatively charged clay particles repel each other

Cation loss causes larger clay aggregates to fragment (deflocculate). Resultant smaller clay particles are more readily washed down profile (Eluvation)

Deflocculated clay particles may re-flocculate where sufficient divalent cations are present (Illuvation)

Repulsion

Repulsion

CaCO3 rich from limestone bedrock

Fig. 4.23 Schematic diagram explaining the formation of argillic horizons in soils. The resulting horizons are comparable to those in the alfisol (luvisol) shown in Plate 4.2(a). The master horizons are typically centimetres to tens of centimetres thick.

6.4) by soluble organic compounds such as fulvic acids, derived from the breakdown of organic matter in the 'L' and 'Ah' horizons (Fig. 4.24). Dissociated functional groups for example -COO- and -O- in fulvic acids, along with lone pairs of electrons (see Box 6.4), for example on the N atom of NH2, coordinate with metal ions, thereby increasing their solubility. These soluble complexed ions are then translocated downward with percolating water. At depth however, the metal ions reprecipitate to form an illuvial 'Bs' spodic horizon. This metal-rich horizon can also be organic-matter-rich, in which case the horizon is denoted 'Bsh'. Repre-cipitation of the metal ions may occur because the metal chelates become unstable due to lowered pH as they move down the soil profile. It is also possible that the organic chelates are degraded by microorganisms as they move down the profile. Clearly if the organic chelates are removed the metal ions will precipitate.

L Acidic leaf litter

_Ah Organic layer formed by soil macro- and micro-flora processing litter

|—Bs Spodic horizon: metal ions reprecipitate (illuvation) due to oversaturation or degradation of fulvic acids

L Acidic leaf litter

_Ah Organic layer formed by soil macro- and micro-flora processing litter

Formation of water-soluble fulvic acids

Fulvic acids chelate metal ions

(e.g. Fe and Mn) enhancing their solubility

Downward percolating water transports metal complexes (eluvation)

|—Bs Spodic horizon: metal ions reprecipitate (illuvation) due to oversaturation or degradation of fulvic acids

Fig. 4.24 Schematic diagram of a spodosol (podzol) with an explanation of horizon formation. The resulting horizons are comparable to those in Plate 4.2(b). The master horizons are typically centimetres to tens of centimetres thick.

4.9.3 Soils with gley horizons

Gley horizons form when the water table is present within the soil profile. Soils with gley horizons (denoted 'g') are typically aquepts (gleysols), a suborder of the inceptisols (Fig. 4.22). Gley horizons may be either Bg and/or Ag horizons, depending on the height of the water table influence in the soil profile (Fig. 4.25).

Ah horizon in Fig. 4.24 not diagnostic of gleysols

Mottled red/brown and grey horizon diagnostic of gleysols

rn CO2

Fe3+

Fe3+

Fe3+

Fe3+

Fe3+

Fe3+

Fe3+

Fe3+

Fe3+

_Water_ table A

Fe (HCO3)2(aq)

O

X

C

table B

FeCO

Fig. 4.25 Schematic diagram of a soil with a gley horizon based on the mollisol (mollic gleysol) in Plate 4.2(c). The upper Ah horizon contains oxygen and carbon dioxide in the soil atmosphere, while the lower Bg horizon is very low in oxygen. Under conditions of high water table (A) and low oxygen, Fe2+ species are stable and soil colours are grey. A fall in water table (B) allows further ingress of oxygen into the Bg horizon causing oxidation of Fe2+ species and precipitation of iron oxides as red/brown patches (mottles). The master horizons are typically centimetres to tens of centimetres thick.

The mollisol (mollic gleysol) in Plate 4.2, for example, has a Bg but no Ag horizon. Gley horizons have a mottled red-brown/grey appearance on account of both reduced (Fe2+, grey) and oxidized (Fe3+, red/brown) iron species being present together. Both iron species are present because of the continuous change in redox conditions (Box 4.3) within the soil profile as the water table rises and falls.

The geochemical cycling of iron species in gley horizons (Fig. 4.25) is mediated strongly by microbiological reactions (Section 4.6.5). Most permanent groundwater has a high Ca2+ concentration, near neutral pH and very low dissolved oxygen (Eh near zero). Under these conditions iron is stable in the reduced mineral FeCO3 called siderite (see Box 5.4). However, due to the ingress of atmospheric CO2 into the soil (or CO2 that has been generated as the result of biological activity) the rather insoluble FeCO3 can be converted to a reduced aqueous species Fe(HCO3)2 and mobilized:

As water moves through the soil, either by fluctuating groundwater levels or capillary action, this mobile aqueous iron species is dispersed. If the water table drops, however, the reduced Fe(HCO3)2 is exposed to more oxygenated condition, resulting in the formation of oxidized (Fe3+) minerals such as Fe(OH)3 (Fig. 4.25).

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