Soil profiles and properties

Soil profiles of the kind shown in Fig. 2.2 occur in many of the world's temperate zones (see also Box 2.1). Soil reaction or pH is an important property influencing the availability of important mineral elements in different

Soil Profile Rendzina

Figure 2.2 Profiles of common British soil types. The organic O horizon can be divided into three distinct horizons as shown depending upon the degree of decomposition of the litter and its integration into the soil below. The A and Ai horizons shown in Fig. 2.1 are here further subdivided into those without eluviation (A and Ap - the latter not shown in the figure) and with eluviation (Ea and Eb). Similarly, the B horizon is differentiated into four discrete horizon types. The three humus types forming the O/A horizons (mull, moder and mor) are discussed in this section. Rankers, like rendzinas, are lithomorphic soils (that is, having a shallow AC profile) but in this case the soils are non-calcareous and over hard rock. (Redrawn from Burnham and Mackney, 1964. Modified from Packham and Harding, 1982. Ecology of Woodland Processes. Edward Arnold.)

Figure 2.2 Profiles of common British soil types. The organic O horizon can be divided into three distinct horizons as shown depending upon the degree of decomposition of the litter and its integration into the soil below. The A and Ai horizons shown in Fig. 2.1 are here further subdivided into those without eluviation (A and Ap - the latter not shown in the figure) and with eluviation (Ea and Eb). Similarly, the B horizon is differentiated into four discrete horizon types. The three humus types forming the O/A horizons (mull, moder and mor) are discussed in this section. Rankers, like rendzinas, are lithomorphic soils (that is, having a shallow AC profile) but in this case the soils are non-calcareous and over hard rock. (Redrawn from Burnham and Mackney, 1964. Modified from Packham and Harding, 1982. Ecology of Woodland Processes. Edward Arnold.)

Ea Bleached or pale horizon which has lost iron and/or aluminium

Eb Relatively pale brown friable horizon which has lost some clay

Illuvial horizons enriched in clay or humus or iron and aluminium

Bs Orange or red-brown horizon, enriched in iron and/or aluminium

Bw Weathered subsoil material, not appreciably enriched in clay, humus or iron, distinguished from overlying and underlying horizons by colour or g The addition of 'g' denotes mottling or greying thought to be caused by

Mull is a characteristic A horizon, which may be covered by a thin L horizon,

Moder characteristically has an H horizon thicker than the L and F combined

soils. It is measured on the pH scale, pH 7 being chemically neutral. Since soils in humid zones are inherently acidic (due to carbon dioxide dissolving to form carbonic acid, organic acids produced by roots, fungi and decomposition, and the ready uptake by plants of nutrients, most of which are positive ions), an ecologically neutral soil has been regarded by various authors as having an average pH of between 5 and 6.5. Soil pH values normally vary between 4 and 10 with most forest plants growing best between values of 5.5 and 8.5. Strongly acidic soils below pH 4 often have toxic levels of soluble aluminium, iron and manganese and fewer nutrients since these are very soluble and readily leached. In strongly alkaline soils with a high pH, many of the essential nutrients are

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British soil class

FAO/ISRIC World Reference Base for Soil Resources

1st level (2nd level where necessary)

USDA Soil Taxonomy

Order

Connotation of order name

Suborder & Great Group

Connotations of lower level names

Notes

Humo-ferric Podzol podzol

Ground- Gleysol water gley

Argillic brown earth

Rendzina

Luvisol

Acrisol

Brown earth Cambisol

Leptosol (Rendzic)

Spodosol PODzol Orthod

Inceptisol INCEPTion stage Endoaquept of soil development Entisol RecENTly deposited Endoaquept parent material Alfisol Has some free Udalf

AL and Fe

Ultisol ULTimate stages Udult of leaching

Inceptisol INCEPTion stage Eutrudept or of soil development Dystrudept

Mollisol

Soft, easily worked ('MOLLify')

Rendoll

Orthodox podzol

Tod' comes from Russian for ash, because of grey horizon USDA recognizes wetness only at 2nd level

Especially calcareous brown earths For more leached and acid soils Most British brown earths are in less leached 'Eutr-' groups

'Rendzina' from Serbo-Croat

Brown Earth Soil Profile

Figure 2.3 Influence of soil pH on soil formation, mobilization and availability of mineral nutrients, and the conditions for life in the soil. The width of the bands indicates the intensity of the process or the availability of the nutrients. Plant needs are divided into macronutrients and micronutrients, a somewhat arbitrary division since all are important but macronutrients are needed in larger amounts. Macronutrients: Nitrogen (N), Potassium (K), Calcium (Ca), Magnesium (Mg), Phosphorus (P) and Sulphur (S). Micronutrients: Chlorine (Cl), Iron (Fe), Boron (B), Manganese (Mn), Zinc (Zn), Copper (Cu), Molybdenum (Mo) and Nickel (Ni). (After Schroeder 1969. Redrawn from Larcher, 1975. Physiological Plant Ecology. Springer.)

Figure 2.3 Influence of soil pH on soil formation, mobilization and availability of mineral nutrients, and the conditions for life in the soil. The width of the bands indicates the intensity of the process or the availability of the nutrients. Plant needs are divided into macronutrients and micronutrients, a somewhat arbitrary division since all are important but macronutrients are needed in larger amounts. Macronutrients: Nitrogen (N), Potassium (K), Calcium (Ca), Magnesium (Mg), Phosphorus (P) and Sulphur (S). Micronutrients: Chlorine (Cl), Iron (Fe), Boron (B), Manganese (Mn), Zinc (Zn), Copper (Cu), Molybdenum (Mo) and Nickel (Ni). (After Schroeder 1969. Redrawn from Larcher, 1975. Physiological Plant Ecology. Springer.)

insoluble and so unavailable to plants (Fig. 2.3). Neutral to acidic soils, such as the brown earths, often possess mull humus (where the humus is mixed with the mineral soil, primarily by soil animals - see Section 2.2.2) and may be quite rich in calcium ions adsorbed by colloids of the clay-humus complex. Calcium carbonate is especially abundant in rendzinas, alkaline or nearly neutral soils whose organic A horizon rests directly on calcareous parent rock.

Soil texture is also important in differentiating how different soils behave. The three important size ranges of particles are clay (< 0.002mm), silt (0.002-0.02mm) and sand (0.02-2mm). Different combinations of these three soil components and their interaction with organic material important in their aggregation create the range of soil textures from fine clays to coarse sands. Sand is important for drainage but in terms of effect on plant nutrition, the clay component is the most crucial. Clays are normally 'secondary minerals': that is, formed from minerals dissolved in the soil water. Along with some of the humus, the clays form colloids, suspensions of particles too small to settle out. Both these inorganic clay colloids and the organic humic colloids have a very large surface area and also have a net negative charge on their surface. The negative charge means that cations (positive ions that include many of the essential plant nutrients such as calcium, magnesium and potassium) are absorbed from solution and, as conditions change, can be exchanged for others. The total amount of exchangeable cations that a soil can hold is measured as cation exchange capacity (CEC) or base exchange capacity. Broadleaved trees normally require soils richer in cations than do conifers. Base saturation is a measure of the extent to which soil exchange sites are filled by cations or bases rather than aluminium or hydrogen ions. Basic soils are rich in alkaline minerals, especially the cation calcium, and are typically rich in other nutrients as well.

The dominant exchangeable cation in temperate soils is calcium, which causes clay particles to aggregate into stable lumps: Ca clays are flocculated. In contrast sodium (Na) clays, which form when the sea penetrates coastal barriers and in depressions in arid zones, are highly dispersed into very small particles and percolate down the profile in suspension (illuviation). Such a 'clay shift' can also occur under other conditions, especially in neutral or slightly acid soils. However, the presence of free bicarbonate ions (HCO3) inhibits such movement in more alkaline soils, while aluminium and dissociating ferric hydroxide (Fe(OH)3) do so in acidic soils.

Soil colour is greatly affected by the presence of sesquioxides, the free oxides and hydroxides of iron, aluminium and manganese, which tend to coat sand particles. Partial or occasional waterlogging can dissolve these oxides, however, leading to an iron pan formation lower in the soil profile (see below and Fig. 2.1). Soils of the humid tropics are generally red or yellow because of the oxidation of iron or aluminium, while the combination of organic matter and iron oxides gives the brown colour typical of many temperate soils.

The amount of water available also makes a big impact on forest soils, especially when they are acidic. Podzols (meaning ash-like) are base-deficient, strongly acid and often support conifers. They develop relatively rapidly on acidic sandstones, gravels and other permeable parent materials when substantial amounts of water are available for leaching. Soils developing over acidic rocks have few earthworms (mostly absent in temperate soils below pH 5.0) and so develop a thick layer of relatively undecomposed mor humus on top of the soil (see Section 2.2.2). This is reinforced by the nutrient-poor, decomposition-resistant litter of the northern conifers and other evergreens typically associated with this type of soil in a circumpolar belt. Note though that if climatic conditions are wet enough with sufficient downward leaching, a peaty layer of mor humus will even form on boulder clay and other substrata. Water passing through the surface humus becomes so acidic with dissolved humic and carbonic acids that it can strip the iron and aluminium oxides from the underlying mineral particles, leaving a layer of quartz sand grains bleached to an ashy grey colour (an eluvial Ea layer). Quartz grains remain, often accompanied by some secondary silica. As the influence of mor humus increases and leaching becomes stronger the Si/(Al + Fe) ratio rises. As conditions change further down in the soil, the mobilized oxides and illuviated humus are deposited to create an orange or red-brown tiger-stripe (Bs) horizon or even a hard humus or iron pan (Bh) typical of the humus-iron podzols ((a) in Fig. 2.2). These pans can be impermeable enough to hold a perched water table, leading to waterlogging of the surface.

Long periods of waterlogging cause soils to become anaerobic and lose the bright red and ochre colours caused by the presence of ferric iron. Iron and manganese are reduced to ferrous and manganous forms, which are soluble and more mobile. If such gleyed soils, which are very common in low-lying areas and valleys, undergo seasonal drying, re-oxidation causes black manganese concretions and the yellow and rusty spots associated with ferric iron. Gley (g) horizons are often neutral or mildly acid, besides being normally deficient in phosphate. Stagnopodzols (peaty-gleyed podzols, (e) in Fig. 2.2), which possess a thin dense iron pan and a layer of peat, are very common in upland Britain and similar wet environments.

In addition to water, climate as a whole makes a big difference to soils. The British Isles exemplify this nicely. There is a strong correlation between the soil regions of Britain and the five climatic regimes defined by Burnham (1970) on the basis of mean annual precipitation (over or under 1000 mm), and mean annual temperature. Very cold was taken as under 4 °C, cold as 4.0-8.3 °C, and warm as above 8.3 °C. Clay eluviation under the warm, dry regime results in argillic (leached) brown earths ((c) in Fig. 2.2)thathavepreviouslybeenusedfor agriculture rather than forestry. Attitudes are changing and we may well in future establish plantations on the better soils. This eluviation process is initiated by the summer dryness so common in south and east England. Early winter rains bring clay particles near the surface of the soil profile into suspension. These are washed into the subsoil, in a mechanical eluviation process called argilluviation (arg = clay, luviation = washing), and the water is drawn into the dry lumps of soil (peds) leaving clay films on their external surfaces in a clay-enriched horizon.

Under a cold, dry regime (found, for example, in parts of north and east Scotland, the North York Moors, and areas on the eastern slopes of the Pennines) rainfall and potential evaporation are both low so what water is available tends to leach down through the soil. Parent materials in these areas are of low base content, so most soils are acidic, have mor humus and podzolization is widespread. On lower ground where the organic matter is more decomposed and integrated into the A horizon (mull humus or a less integrated transitional moder) soils tend to be less acidic and podzolization is not as strong. These sites, which originally would have supported oakwood, show intergrades (semi-podzols = brown podzolic soils) between brown earths and podzols. Heathlands now occupy most of the podzols on higher ground but, before the combined influence of climatic change and human interference, pinewoods were frequent here.

Under the warm, wet regime of south-west England, most of Wales and coastal areas in north-west England and south-west Scotland, there is a high degree of leaching but the warmth leads to more biological activity. Over acidic rocks weak podzolization can occur in places. Mostly, however, the litter dropped by plants is nutrient rich and easily decomposed, and the active soil fauna, especially earthworms, readily mix the humus into the mineral soil to create a neutral to moderately acidic (pH 4.5-6.5) brown earth ((d) in Fig. 2.2). Until cleared for cultivation almost all the area under this climatic regime was covered by deciduous woodland. Trees, especially of Sitka spruce Picea sitchensis, are planted quite extensively on some of the better acid brown earths and forest yields are high, although some of the more podzolized areas often need deep drainage ditches. The acidification of brown earths is helped by leaching by rain leading to a lower pH and calcium content at the surface than further down the profile. This surface acidification is particularly obvious where parent materials are uniform as in Hayley Wood, Cambridgeshire, whose soils are derived from chalky boulder chalk with a small admixture with loess (Rackham, 1975).

Areas with a cold, wet regime are covered by stagnopodzols. Here a deep layer of undecomposed litter builds up leading to intense podzolization and often waterlogging. The humus layer, aided by wet conditions, may become thick enough to be called peat. Such soils coated with peat on all but the steep slopes are classified as blanket peats characteristic of a cold, wet regime. Most of these areas are too exposed for tree growth and forestry. More favourably sheltered areas often require ploughing to mix in the surface peat layers and improve drainage before trees can be planted. Leaching is strong and weathering weak; and so the recycling of nutrients is slow. Fertilizer additions, especially of phosphates, considerably improve tree growth. Sitka spruce, sometimes planted with lodgepole pine Pinus contorta, grows well under these damp conditions, and is very successful in less exposed parts of this region. In contrast, the alpine humus soils developed under very cold, wet conditions are near or above the natural treeline in Britain and have never been forested.

The British Isles are relatively small and so do not show the even greater diversity of climate associated with such areas as North America or Eurasia. Here variation in rainfall, potential evaporation, irradiation, temperature and length of growing season lead to major differences in the soil types and vegetation present. The most striking of these different climates are those of the wet tropics.

Around one-third of tropical rain forests are in the seasonal moist or humid tropics and have some period when soil processes and plant growth are affected by moisture shortage (see Section 1.6.1). By contrast, soil processes in the aseasonal or perhumid tropics, which are characterized by high even temperatures and abundant rainfall year round, proceed rapidly and without prolonged and regular interruptions. These processes include the leaching of solutes, the weathering of primary rock minerals, the production and weathering of secondary clay minerals, and many rapid biological processes such as the comminution (breaking up) and chemical decomposition of organic matter. They produce what are commonly called oxisols. In the last few decades the sheer heterogeneity of these soils has gradually been appreciated. Unfortunately a generally agreed scheme of classification has not been produced but an idea of their diversity is given in Box 2.2; Baillie (1996) should be consulted for more detail on this complex and developing subject. Large areas of the tropics have geologically and tectonically stable land surfaces bearing old and intensively weathered soils. Typically these ancient soils are visually striking: deep and rather uniform and with a topsoil that has little organic darkening, but a bright reddish or yellowish subsoil. Few original minerals remain near the surface, while the well-weathered clay minerals include substantial amounts of sesquioxides, giving the soils their characteristic bright colours. The main group of clay minerals are the kaolinites. These are chemically and physically less active than other clay minerals, having low electrical charges on their surfaces and so a low cation exchange capacity. This is why these soils are generally very nutrient poor and have been referred to as 'wet or green deserts' or even a 'counterfeit paradise', a phrase coined by

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  • Sara Barese
    What is the Soil Profile for tropical rainforest?
    6 years ago
  • lukas
    What is soil profile and properties?
    8 months ago

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