LAND USE, LAND COVER AND SOIL SCIENCES Vol. VI Soils and Soil Sciences - Willy Verheye

Transcription

1 SOILS AND SOIL SCIENCES Willy Verheye National Science Foundation, Flanders/Belgium, and Geography Department, University of Gent, Belgium Keywords: Bulk density, cation exchange capacity, color, mapping unit, organic material, permeability, ph, regolith, soil profile, soil survey, structure, texture, water retention, weathering Contents 1. Introduction 2. Soils and Soil Science 3. Soil Formation and Soil Forming Processes 4. The Soil Profile 5. Soil Composition and Soil Properties 6. Soil Survey and Classification Glossary Bibliography Biographical Sketch Summary Soil science is a relatively new discipline which has mainly developed since the 1880s. It uses terms, methods and processes borrowed from other basic disciplines like climatology, geology, chemistry, physics and biology, but with a direct application to soils. At present, it is difficult to speak about one single science but as soil sciences, as they cover several fields including: pedology (or pedogenesis), soil survey (or mapping), classification and applied soil sciences like soil fertility, soil conservation, land evaluation or soil and land management. In this chapter an overview is given of the concepts of modern soil sciences. The process of weathering and gradual evolution of a regolith towards a mature soil profile is described. Soil composition and main soil properties, as well as their inter-relations with other characteristics and impact on land use are discussed. Finally, a summary is given of the basic principles of soil survey and soil classification. As an overhead chapter on the topic of soil sciences in this Encyclopedia, this article provides a synthetic overview, with direct references to the more detailed information included in the 17 chapters which make up this topic. 1. Introduction The term soil, derived from L. solum, has many definitions. Geologists and road engineers consider the soil primarily as an inert unconsolidated weathering product of the underlying rock, a nuisance that must be quarried and removed before reaching the material of their interest, i.e. the basement for construction. Alternatively, soil (or dirt) can also be used for filling excavations or providing foundations. For many other users,

2 however, including farmers and earth science specialists, the soil is primarily a medium for plant growth or crop production and water storage, and a major source of living. Hence, these users and farmers in particular, pay automatically more attention to the inherent characteristics of soils and their management, because to them soil is more than useful, it is indispensable. Throughout history farmers have learned, through trial and error, to observe differences in soils and to improve, wherever possible, their properties. Long before our era the Greeks were aware of the beneficial effects of applying manure or using ash or sulfur as soil amendments. Since the Roman Empire traditional land use practices have been passed from generation to generation, but always in a conception that soil is a rather inert material to which fertilizers and water has to be added for producing crops. This concept dominated people s minds until as recent as the late nineteenth and early twentieth centuries, and it is only since von Liebig in 1840 discovered the role of nutrients in crop production and Dokouchaiev in 1880 made the link between soil properties and bioclimatic zones that the soil is considered a dynamic body with variable properties and potential depending on variations in climate, vegetation and parent material. In the same line Jenny in 1941 defined the 5 soil forming factors which, still today, guide pedogenetic thinking and research (see: A Brief History of Soil Science). In the present-day concept the soil is considered a product of evolution and changes over time, with an own morphology and properties. The morphology of each soil, as expressed by a vertical section of different layers or horizons, is a direct reflection of the effects of the five genetic factors responsible for its development. This dynamic and evolutionary nature is embodied in the universal definition of soil as: a natural body, located at the interface between the atmosphere, lithosphere and biosphere, consisting of layers of unconsolidated mineral and/or organic constituents of variable thickness which have been subjected to and influenced by genetic and environmental factors of: parent material, climate (including moisture and temperature effects), macro- and microorganisms, and topography, all acting over a period of time and producing a product-soil that differs from the material from which it is derived in many physical, chemical, and biological properties and characteristics. The upper limit of the soil is air or shallow water. Its lower limit coincides with the lower limit of biologic activity, as reflected by the rooting depth of native perennial plants. This active soil section in between corresponds with what is commonly defined as the solum. As soils differ in their properties both in the vertical and horizontal sense, their study and characterization should involve both the vertical succession of overlying horizons and spatial variations. The first aspect requires the observation and study of a soil profile pit, approximately 1m x 1m x 1m in size and being considered representative for the soil around. This small basic entity, from which one can observe variations in properties and extract samples for analytical investigations, is called a pedon. It is the smallest volume that can be called a soil, but large enough to exhibit a full set of horizons.

3 The combination of various pedons with minor differences within a larger landform is called a polypedon. Such minor differences may relate to the nature and arrangement of horizons, or to the degree of expression of one or more horizons below the depth of normal plowing. In fact, a polypedon corresponds usually with what is often described as a soil series. Soils of the same series have a similar horizon sequence and nearly identical properties of the horizons. 2. Soils and Soil Science Soil science is the study concerned with observing and describing, collecting, establishing and systematizing facts, principles and methods in order to acquire an indepth knowledge of the soils, their properties and potential for production and conservation. Soil science uses an integrated multidisciplinary approach in the sense that it borrows concepts, techniques and processes from other sciences, but with a focus on soils. Soil science relies on 7 major supporting sciences or sub-disciplines (Figure 1): Climatology which affects the pedo-climate in terms of moisture and temperature conditions in the solum, and thus influences physical and chemical soil processes and plant and animal life; Geology which determines the nature and constitution (mineralogy) of the parent material from which the soil profile develops; Geomorphology (landform evolution) and hydrology which have a major impact on runoff, erosion and sedimentation processes, and differential warming up of soils; Physics, the basic laws of which determine the nature, intensity and interrelationships between the solid, liquid and gaseous soil components; Chemistry, concerned with the chemical constitution, chemical properties and chemical reactions in the soil, and their direct effect on soil fertility and nutrient supply to plants; and Soil (micro) biology dealing with the soil fauna, the vegetation above and below the soil surface, as well as the microscopic soil population, and their role in various transformations and the liberation of nutrients. Soil science as a discipline is relatively young compared to other sciences like mathematics or astronomy, the origin of which dates back for more than years. The first who started to systematically study soils was the Russian geographer Dokouchaiev in the late 19 th century, but his work became only known at the international floor after it had been published in German and disseminated by Marbut and Jenny in the USA (see: A Brief History of Soil Science). The disastrous Dust Bowl in the Mid West in the 1930s gave an additional impetus to the study of soils and was the start of the Soil Conservation Service in the US. Elsewhere in the world the growing interest in soils work received a major push after World War II when in a number of European countries a national soil survey institute was established (in 1947 in Belgium, 1952 in France, 1966 in The Netherlands, etc.) while in the tropics the rapid development and increased demand of plantation crops asked for an better knowledge and understanding of soil-plant relations in these areas.

4 Figure 1: Supporting basic sciences and current sub-disciplines of soil science At present, there are four major disciplines within soil science focusing on different applications and users (Figure 1): Pedology, focusing on the formation (pedogenesis) and development of soils as recognized in the characteristics of the soil profile. It deals with fundamental and academic research aspects of soils, and includes the description and analytical characterization of the soil; Soil survey which describes the soil properties (making use of field and laboratory observations as referred to above) and delineates the geographical distribution of the different soils; Soil classification which organizes the soils and their particular properties on the basis of a hierarchical system of pre-defined criteria and classes. Though these criteria may vary as a function of the objectives, most international classification systems have a pedogenetic background; and Applied soil sciences which interpret the soil properties in function of their ranking and potential for a specific objective. This includes applications (1) for housing and construction, focusing mainly on physical and much less on chemical properties; (2) for agronomic and crop production objectives, including soil fertility, suitability for drainage and irrigation, etc.; (3) for soil conservation and the protection of soils against physical loss by erosion or by chemical deterioration; and (4) land evaluation which assesses the production and use potential of the soil, including the development and monitoring of land use practices. The latter domain has since the 1980s gradually become a key issue

5 for land resource management and land use planning. The various aspects of soil sciences described above are discussed at large in different separate chapters in this section: Soil Physics, Soil Chemistry and Soil Fertility, Soil Biology and Microbiology, Soil Biochemistry, Soil Mineralogy. 3. Soil Formation and Soil Forming Processes Soil formation takes place in two consecutives stages, starting with a simple weathering (disintegration and decomposition) of rocks and minerals giving rise to an unconsolidated regolith (from Gr. rhegos, covering, and lithos, stone), and followed by a soil profile development, whereby the regolith material is gradually modified and a horizon sequence develops under the combined action of climate, vegetation, topography and time Weathering and Regolith Formation Weathering is basically a combination of destruction and synthesis. It breaks up rocks, modifies or destroys their physical and chemical characteristics, and carries away the soluble products and some of the solids. These changes are accompanied by a continuous decrease in particle size and by the release of soluble constituents, which are subject to loss in drainage waters or recombination into new (secondary) minerals. There are three major forms of weathering: physical, chemical and biological weathering. Physical or mechanical weathering takes places under conditions where water is no active agent to enhance chemical reactions. It is particularly active in deserts or in polar areas where temperature changes create internal pressures in the rock and produce cracks (see: Dry Lands and Desertification, and Soils of Arid and Semi-Arid Areas). Chemical weathering is mainly related to the concerted action of water, oxygen and organic chemicals released by higher plants and microorganisms. While physical weathering results generally in a broad breakdown of soil and rock components, chemical weathering affects much more intensively the composition of soil material. The three major weathering processes related to water are hydrolysis (the dissociation + from H and OH ions from H2O ), hydration (addition of a water molecule to the mineral) and dissolution (the solubility of a compound and its elimination from the environment). Biological weathering processes are activated by living agents (animals, higher plants, microorganisms) and are mainly responsible for both the decomposition and disintegration of rocks and minerals. The processes related to these weathering forms have been discussed at large in: Dry Lands and Desertification Soil Profile Formation and Horizon Development Soil profile development is basically a re-arrangement of soil particles into soil horizons, each of them with specific properties. Soil formation can proceed rather fast in aggressive humid tropical climates, but is much slower in cold or dry climates; when the surface layers are eroded, the (active) root zone comes nearer to the regolith and soils

6 are rejuvenated. Conditions that hasten the rate of soil development are: (1) a warm, humid climate, (2) forest vegetation, (3) permeable unconsolidated material low in lime content, and (4) flat or lowland topography with good drainage. Factors that tend to retard development are: (a) a cold or dry climate, (b) grass vegetation, (c) the presence of impermeable consolidated material high in lime, and (d) a steeply sloping topography. Weathering and soil formation can be studied by changes of color, structure and texture in the field, by laboratory analyses and by microscopic observations and techniques (see: Soil Mineralogy, and Soil Microscopy and Micromorphology). The processes involved in soil profile formation and horizon development are: (a) gains or additions of water, organic and mineral material, (b) losses of such material from the soil, (c) transformation of mineral substances within the soil, and (d) translocation or movement of soil material from one point to another, involving movement in solution (leaching) or in suspension (eluviation) of clay, organic matter or hydrous oxides. Conditions that retard or offset horizon differentiation are due to: (a) mixing of material by burrowing animals, (b) removal of surface soil by water or wind, (c) creep, and (d) accretion of sediments in floodplains. There are 9 fundamental processes that affect profile differentiation: Humification - The process of transformation, i.e. decomposition of raw organic material into humus under the influence of soil microorganisms. During this process, the soluble organic substances regroup themselves into large molecules and become poorly soluble. In the strict sense, the term focuses in particular on the phase which follows the decomposition of the organic debris and which consists mainly of processes of synthesis and building up of new molecules through microbial or physicochemical pathways. Eluviation and illuviation - The process of removal of soil constituents in suspension or in solution by percolating water from the upper to the lower layers. It encompasses mobilization and translocation of mobile constituents (mainly clay) resulting in a textural differentiation, or leaching of soluble elements like salts. Calcification and decalcification- The movement of soluble calcium carbonate in the soil, involving their leaching, movement, precipitation and accumulation in various soil layers. The general reaction which controls the movement of carbonate is: CaCO + H O + CO Ca(HCO ) If CO2 and H2O are present, i.e. under an active biological activity, the reaction proceeds in the right direction, with the formation of soluble bicarbonate. When CO 2 and water are not active, i.e. in the dry season when the biological activity is largely reduced, the reaction proceeds to the left and insoluble calcium carbonate precipitates. This is, for example, what happens in Mediterranean soils which develop under an alternate humid winter and dry summer period (see: Mediterranean Soils).

7 Podzolization - The process of extreme leaching of bases in an acid environment, relatively poor in weatherable minerals, characteristic of regions with a (very) humid boreal or tropical climate. It involves the eluviation of acid and complex-forming humus that becomes mobile and gets leached from the upper part of the profile, and their subsequent deposition in the lower horizons. The process is the most active under pine tree forests (see: Forest, Range and Wildland Soils). Lateritization (currently replaced by the connotations allitization and ferrallitization) - The process that removes silica and soluble bases from the upper layers of the soil, creating a relative accumulation and concentration of sesquioxides (Fe and Al-oxides) in the soil. As the alkaline bases are removed from the seat of their formation, the residual soil is acidic in reaction. Though considerable eluviation takes place, there is no marked horizonation as the eluviated materials are not re-deposited in the lower layers. Depending on the intensity of the weathering process the residual soils are dominated by Fe and Al compounds (ferrallitization) or by Al-(hydr)oxides only (allitization). These processes act most intensively in warm and humid tropical climates with mm annual rainfall and high temperatures (>22 C) throughout the year (see: Soils of the Humid and Sub-Humid Tropics). Gleization - A process of soil formation under an anaerobic environment and leading to the development of a gley horizon with green-blue colors, related to the reduction of soluble ferrous iron under water-logged conditions. Where the groundwater fluctuates considerably with the season, the gley shows distinct mottling of yellow and rusty brown colors caused by alternate oxidation and reduction phenomena. Salinization - The process of accumulation of salts, such as sulfates or chlorides in the form of a salty horizon. It is active under conditions of highly saline or brackish groundwater, and evaporation being higher than precipitation, so that salts move up by capillary action from the groundwater. Desalinization is the removal, by leaching of excess soluble salts from horizons that contain enough soluble salts to impair plant growth (see: Salinity and Alkalinity Status of Arid and Semi-arid Lands). Alkalinization - A process involving the accumulation of sodium ions on the exchange complex of clays, resulting in the formation of a sodic soil (Solonetz). At this moment a high soil ph (>8.5) develops, soil colloids are dispersed and a very poor soil structure develops. The organic matter dissolved under alkaline conditions forms black organoclay coatings on the ped surfaces giving the soil a dark-colored appearance (Black Alkali Soils). + Solodization or de-alkalinization - The removal of Na from the exchange sites and the 2+ dispersion of clay, promoted by the addition of Ca to the formerly alkaline soil, often under the form of easily soluble gypsum (see: Dry Lands and Desertification). Pedoturbation - The process of mixing the soil due to faunal activity (ants, earthworms, moles, termites, etc.), plant roots, natural swell-shrink processes or by man-made land management practices. It is very active in boreal areas covered by long-time and usually undisturbed forest vegetation (see: Forest, Range and Wildland Soils).

8 The rate of soil development varies with the intensity of the processes involved, and with the age of the soil. In dry climates there is almost no water for either chemical processes or organic material production. Hence, soil formation is mainly limited to an incomplete physical breakdown of soil components. In a humid, warm climate hydrolysis, dissolution and leaching are much more intensive, and soil properties rapidly change into a material that is composed of stable mineral components. Individual processes vary also in intensity over time. Under ideal conditions a recognizable soil profile may develop within a couple of centuries. But under less favorable environments, as is the case in deserts, the time taken for soil development may extend over several thousand years. 4. The Soil Profile An examination of a vertical section of a soil in the field reveals the presence of more or less distinct horizontal layers. Such a section is called a profile, and the individual layers are regarded as horizons. The horizons above the parent material are collectively referred to as the solum (from Gr. Solum, soil, land or a parcel of land). The topsoil corresponds with the upper part of the solum which is enriched in humus and is the major zone of root development. When the soil is cultivated this topsoil may correspond with the plow layer. The underlying layers between the topsoil and the regolith or parent material are referred to as the subsoil. Figure 2 is an example of a deep profile developed over aeolian loam. The slightly darker topsoil extends over the upper cm. The subsoil includes a B-horizon with a well-developed structure (25-60 cm) overlying a less well-structured and rather compact C horizon (60-110cm+) Profile descriptions in the field make a distinction between master horizons named with capital letters like O, A, E, B, C, R (Table 1). Subordinate distinctions within these master horizons are indicated by lower case letters used as suffixes to designate specific kinds of characteristics (Table 2). Surface or near-surface horizons relatively high in organic matter (OM) are designated O or A horizons, the difference between the two being determined by the amount of organic matter present. The surface horizon is considered organic if its OM content is >30% and the mineral fraction has >50% clay, or if the OM content is >20% and the mineral fraction has no clay. Wetness generally favors the increasing thickness of O horizons and may qualify as peat when it is > 40cm deep and its OM is >50%. The mineral horizons on the other hand, are low in OM and may overlie either a B or C horizon, or even directly the rock mass. Beneath the O or A horizon, in some environments, there is a light-colored horizon relatively leached of iron compounds called the E horizon; this is identical to the A2 horizon of some (older) classifications. The B horizon commonly is beneath the surface horizon(s), unless the latter has been eroded and the B horizon appears on top of the new soil. It encompasses a multitude of soil characteristics relatively to those of the assumed parent material. Among the B horizon characteristics are: clay accumulation, a (yellowish) red color, the accumulation of iron compounds with or without organic matter, and the residual concentration of resistant materials following the removal of

9 more soluble constituents under conditions of intensive weathering and leaching. Figure 2: A typical soil profile developed over aeolian loam, Belgium The slightly weathered C horizon is beneath the B horizon, and forms the transition with the partly weathered saprolite (see: Soils of the Humid and Sub-Humid Tropics) or unaltered bedrock, the R horizon. In desert environments, carbonate buildup plays an important role in soil morphology and genesis, and horizons high in carbonate are designated K (see: Soils of Arid and Semi-arid Areas). Soil horizons can be further subdivided by adding a number to the master horizon designation. A3 and B1 are used for horizons transitional from the A to the B, with A3 having properties closer to A, and B1 having properties closer to B. Transitional

10 horizons can also be designated AB or AC if a more detailed subdivision is not possible nor warranted. B2 is used for that part of the B horizon that displays the maximum expression of the properties upon which the B horizon is defined (e.g. B2t for the maximum expression of an argillic B horizon). A B3 horizon still retains many properties of the B2 horizon but is transitional to the underlying horizon. A second number can be added for even further subdivision (B21t, B22t and B31), based on subtle changes in such properties as color or texture. Within the C horizon, numbers are used to denote a vertical sequence of layers (C1, C2, etc.). Since 1960 the Soil Conservation Service of the USDA has introduced a new set of names for soil horizons because the more traditional terms, such as an A or B horizon, were neither precisely defined nor used in the same sense by all workers. This new terminology has now largely been taken over by other worldwide classification systems, the World Reference Base for Soil Resources for example. The new terms, such as a mollic or epipedon or an argillic subhorizon, are precisely defined, so much that at times recognition of such horizons may require laboratory analysis. The boundaries between these units and those of the A, B and C horizons in a profile do not necessarily coincide. At present, both systems are in use: the traditional A, B, C nomenclature is mainly used for profile descriptions and field-related investigations, whilst the modern terminology is mainly applied for soil classification purposes. Table 1 summarizes the major properties of the different master horizons. As the correspondence with the Soil Taxonomy nomenclature is not always evident (because criteria partly overlap), the correlations are only approximate. Table 2 defines the subordinate distinctions. Symbol H O A Layer/horizon description and corresponding horizons in Soil Taxonomy (*) Layer dominated by organic material, formed from the accumulation of undecomposed or partially decomposed organic material at the soil surface. Saturated with water for prolonged periods or once saturated, but now artificially drained. *Histic epipedon: Surface horizon saturated for 30 or more consecutive days in some time of most years, with 75% or more sphagnum, and low bulk density. Surface accumulation of organic material, consisting of un-decomposed or partially decomposed litter ( leaves, needles, twigs, moss, lichens, etc.) overlying generally a mineral soil. Not saturated with water for a prolonged period. The mineral fraction is only a small percentage (generally < 50%) of the volume of the material. *See mollic or umbric horizons below. Mineral surface horizon in which all or much of the original rock structure has been obliterated, having one of the following: (a) organic matter accumulation intimately mixed with mineral soil, (b) properties resulting from cultivation or similar human disturbance, (c) a morphology different from underlying B or C horizons. *Mollic epipedon: Dark colored (chroma < 4.0, value < 3.5 moist), with > 1% organic material, base saturation >50%. Often grass vegetation. *Umbric epipedon: Similar to mollic epipedon except that base saturation is < 50%. Often associated with (tropical) forest vegetation.

11 *Ochric epipedon: Too light in color and too low in organic matter than mollic or umbric epipedons. Often associated with young soils and/or semiarid vegetation. E Or A2 horizon. Mineral horizon below O or A and above B horizon, characterized by loss of clay, Fe, Al or a combination of these, leaving a concentration of sand and silt particles. Light colors are mainly due to the color of primary mineral grains. *Albic horizon: Surface or lower horizon with light color determined by the color of the sand or silt particles: value, moist, is 4 or more, or value, dry, is 5 or more. If value, dry, is 7 or more, or value, moist, is 6 or more, chroma is 3 or less. If value, dry, is 5 or 6, or value, moist is 4 or 5, chroma is close to 2. B Subsurface horizon under O, A or E horizons, showing little or no evidence of original rock structure and having one or a combination of the following: evidence of illuviation, removal of carbonates, residual concentration of sesquioxides, coatings, well developed structure, brittleness. *Argillic B horizon: Subsurface horizon with more silicate clay than in the A or E: 3% or more if overlying horizon has <15% clay; 1.2 times the amount of clay if overlying horizon has 15-40% clay; 8% or more if overlying horizon has >40% clay. Clay in B horizon is translocated (illuviation) from overlying horizons or has formed in place, or both. Clay translocations are recognized in the field by oriented clay films on mineral grains, small channels or ped surfaces *Natric B horizon: Like argillic horizon, but with columnar or prismatic structure and ESP > 15 in some sub-horizon. *Spodic B horizon: Occurs generally below an E horizon, holds a concentration of organic matter and/ or sesquioxides translocated from E horizon. *Oxic B horizon: Highly weathered subsurface horizon with low CE and dominated by hydrated oxides of iron and aluminum and 1:1 clays. *Cambic B horizon: Subsurface horizon enough altered to eradicate most rock structure, form some stable soil structure and remove or redistribute primary carbonate. Has higher chroma or redder hue than underlying horizons. C Subsurface horizon, excluding bedrock, holding material from which the soil has developed. Lacks properties of A and B horizons, but includes weathering as shown by mineral oxidation, accumulation of silica, carbonates, or more soluble salts, and gleying. R Consolidated bedrock underlying soil. Sufficiently coherent when moist to make hand digging with a spade impractical. * More complete and updated definitions can be found in USDA Soil Taxonomy (1975) and related website. Table 1: Soil horizon nomenclature and brief description of master horizons (adapted from FAO Guidelines for Profile Description, 1990 and Soil Taxonomy, 1975) Symbol b c ca Description Buried soil horizon. Concretions or nodules that have accumulated in a significant amount. Accumulation of calcium carbonate in amount greater than the parent material is presumed to have had; can occur in A, B, C and R horizons.

12 cs f g h ir j k m n o p q r s si t v w x y z Accumulation of gypsum in amount greater than the parent material is presumed to have had; can occur in A, B, C or R horizons. Frozen soil, with layers that contain permanent ice or are perennially colder than 0 C Strong gleying or reduction of iron, so that colors approach neutral, with or without mottles; can occur in A, B and C horizons. Accumulation of organic material (humus), appearing as coatings on grains or as silt-size pellets; common in podzols. Illuvial concentration of iron, appearing as coatings on grains or as siltsize pellets; common in podzols. Jarosite mottles. Accumulation of carbonates, commonly calcium carbonate. Strong irreversible cementation (>90% cemented) or induration, for example by accumulation of iron, calcium carbonate or silica. Accumulation of exchangeable sodium. Residual accumulation of sesquioxides, different from s (below) which indicates illuvial accumulation of organic matter + sesquioxides. Plowing or other tillage disturbance by Man. Accumulation of secondary silica (quartz). Strong reduction. Illuvial accumulation of sesquioxides in combination with amorphous dispersible organic matter. Cementation by silica, as nodules or as a continuous medium; if cementation is continuous the horizon is called a duripan or silcrete. Accumulation of translocated clay, like in argillic B horizon. Occurrence of plinthite, iron-rich and humus-poor material, firm when moist, and hardening irreversibly when exposed to air. Development of color or structure, in particular in B horizons. Characteristics of a fragipan, related to firmness, brittleness or high bulk density. Accumulation of gypsum. Accumulation of salts more soluble than gypsum. Table 2: Subordinate soil properties (adapted from FAO Guidelines for Profile Description, 1990; Soil Taxonomy, 1975, and recent updates) 5. Soil Composition and Soil Properties 5.1. Soil Composition An average soil is composed of unconsolidated mineral (inorganic) compounds and decayed plant (organic) material. Depending on their organic matter content soils can therefore be differentiated into two major types. Soils with less than 20% organic matter (or 30% if the clay content is high) are called mineral soils; those that have more organic material are termed organic soils. The latter are by no means as extensive as are mineral soils, yet their total area is more than 300 million ha, worldwide.

13 Organic Soils Organic soils occur when high amounts of organic matter accumulate in poorly drained areas. After plant decay water replaces air in pores and voids, thus preventing rapid oxidation. Peat deposits are mainly found in cold climates (Russia, Canada, Finland) though they can also be extensive in the tropics (12 million ha in Indonesia and Malaysia, 1.2 million ha in Louisiana, US). The composition of the peat varies depending on its origin and content: sedimentary peat, fibrous peat, woody peat. Peat is often used, either mixed with mineral soil for potted plants and for home flower and vegetable gardens, or as a substratum for gardens and lawns. Its use as domestic fuel is declining, though it is still prevalent in remote areas of the Soviet Republic, Ireland, Finland and Scotland (where it is an important component for giving a specific flavor in the production of whiskey). Finally, it provides also an excellent substratum for field vegetable production. The term muck is used to describe peat that is markedly decomposed whereby the original plant parts can (almost) no more be identified. In Soil Taxonomy (USDA, 1975) peat soils are called Histosols, and they include at suborder level Fibrists (where the fibrous organic material can still easily be recognized), Saprists (where the original plant fibers have mostly disappeared) and two intermediate types: Hemists and Folists. The most important characteristics of organic soils are: a dark brown to intense black color; a high content of at least partly un-decomposed organic matter; a low bulk density, of the order of g/cm 3 ; a high water holding capacity on a weight basis, i.e. 2 to 4 times their dry weight (though part of this water might not be available to plants); a loose structure and physical condition in general; most peat soils are porous, and easy to cultivate, which makes them very desirable for vegetable production; a high surface area and corresponding high CE, 2 to 10 times higher than mineral soils, on a weight basis; this advantage disappears however on a volume basis; and a relatively low ph Mineral Soils All other soils are called mineral soils. These are composed of a solid phase and of pores which might be filled either by water or by air. The solid phase is made up of a dominant mineral fraction and a less important organic fraction. A good or high quality soil holds 48% of mineral fraction, 2% organic fraction, 25% water and 25% air. The relative proportion of pores filled with water or air varies. The mineral fraction contains a more or less inert part of coarse elements, sand and loam, which provides a foothold for roots and forms the soil skeleton. Besides, it holds an active fraction (clay and fine silt) with specific surface properties; the physicochemical properties of these are at the basis of electrical charges and ion

14 absorption, and the formation of structural elements which influence the pore volume and the air and water conditions of the profile. The organic fraction is mainly concentrated in the surface layers and originates from the decay of organic products of fauna and flora. It affects the sorption capacity of the topsoil and the soil structure of the profile as a whole, and thus also the moisture status of the root zone. The relative importance of the water and gas phases in the soil is determined by soil porosity. It varies between 25 and 50% and is influenced by soil texture (particle size), soil structure and soil compaction. These properties are discussed in more detail in the following sections Soil Texture Soil texture refers to the relative proportions of various size particles in a given soil. Particle size analysis enables to fix adequately the percentages of the various constituents of the soil. This soil characteristic has an important impact on the soil moisture status and aeration, as well as on other qualities like workability, root penetration and anchorage, cation retention, etc. Sandy soils are considered as light, clayey soils as heavy since they are either easy or more difficult in tilling and cultivation. In soil sciences, the soil texture is quantified in terms of particle size composition of the fine earth fraction, in particular in their relative proportion of mineral particles of less than 2 mm in size. Particles with a diameter > 2mm are considered as belonging to the so-called coarse fraction and are not part of the fine soil fraction. Coarse fragments are categorized as gravel (particle size diameter 2mm- 7.5cm), stones (diameter cm) and cobbles (diameter >25cm). For engineering and construction applications, equal attention is given to both the fine and the coarse fractions, and a somewhat different classification of texture is taken into consideration (see: Soil Physics). The fine soil fraction is classified, after destroying the aggregates, according to size categories corresponding to an international scale: clay : < 2 μm; fine silt : 2-20 μm; coarse silt (sometimes also called very fine sand): μm; fine sand: μm; medium sand: 200 μm - 1 mm; and coarse sand: 1-2 mm. It is recalled that some national classification systems (Germany, Russia, many Anglo-Saxon countries) still use slightly different size limits, but there is a general tendency to rely to one uniform system as described above. Measurement - Soil texture is primarily estimated through a (rapid and cheap) field test followed by a more detailed (more time-consuming and more costly) laboratory analysis of a selected number of samples. The field test is conducted through a finger test, i.e. by rubbing a moist or moistened sample between thumb and fingers. Experienced field soil scientists are able to estimate the relative proportions of sand, silt and clay at a <5% error. Clay feels as a smeary paste, silt is smooth and soft like talc powder, and sand feels abrasive with individual

15 grains being observed with a 10-power hand-lens. Laboratory tests do generally confirm well these field estimations, though there are situations where this is not so. A good example of this is the highly weathered tropical soils (Oxisols) where individual clay particles are cemented by iron to form pseudo-silt and pseudo-sands. The field texture of these soils is therefore rather coarse and the soil profile behaves as such in terms of moisture retention and water permeability characteristics. The laboratory tests which apply a preliminary destruction of the iron bounds as a standard procedure, give generally a much finer texture. Both estimations have their value as the field texture is well correlated with the soil hydraulic properties (water retention, permeability) while the laboratory data are more in line with the physicochemical characteristics of the soil. The commonly used laboratory methods for particle size analysis are the ones designed by Bouyoucos and Robinson; for engineering purposes the focus is more on sedimentation methods (see: Soil Physics). The Bouyoucos hydrometer method is relatively accurate (though somewhat underestimating the clay content) and fast. It determines the 3 textural fractions clay, silt and sand without separating them. The sample is first dispersed with a sodium pyrophosphate solution, treated with a highspeed soil mixer, and then poured in a cylinder to which distilled water is added to bring the contents up to volume. With the help of a stirrer the soil suspension is thoroughly mixed and the time noted. The rate of fall of suspended particles is related to size: sand settling faster than silt and silt settling faster than clay, based on Stoke s law: V = gr d d η 2 2 ( 1-2)/9 where: V = velocity of fall (cm/sec); g = acceleration of gravity (cm/sec 2, say 981); r = equivalent radius of particle (cm); d = density of particle (gram/cm 3 1, say 2.65 for soil); d = density of medium (gram/cm 3 2, say 1 for water) and η = viscosity of medium (dyne sec/cm 2 ). Two hydrometer readings are taken of the soil suspension using a special soil hydrometer. A reading taken after 40 seconds determines the weight of silt and clay remaining in suspension, since the sand has settled to the bottom. Subtraction of the 40 seconds reading from the sample weight gives the grams of sand. Another reading after 2 hours gives the weight of clay in the sample. The silt is calculated by difference: add the percentage of sand and the percent of clay and subtract from 100. The Robinson method is based on the same principles. A given weight of oven-dry soil is sieved through a series of different sand-size sieves, from where the sand fractions are determined, and the supernatant liquid is put into suspension. An aliquot of the suspended material is taken at different times and at corresponding depths in the suspension, based on the Stoke s equation. The air-dried fractions are expressed on a percentage basis. Textural diagram - The relative importance of the different particle size fractions, generally regrouped into 3 main classes: clay (0-2 μm), silt (2-50 μm) and sand (50 μm-

16 2 mm) is represented on a triangular diagram, whereby the proportion of each of the 3 fractions is marked and the characteristic point of a soil or a given horizon is the intersection of 3 lines parallel to the sides obtained by plotting on each side the values in percentage of clay, silt and sand. There exist a number of different textural diagrams but the most currently and nowadays almost exclusively used diagram is the so-called USDA or FAO triangle composed of 12 classes (Figure 3). For small-scale identification and mapping at national or continental scales these classes may be regrouped. In the FAO-UNESCO Soil Map of the World or the WRB Soil Classification only 3 major classes are withheld: coarse texture : containing < 15% clay and > 70% sand; medium texture: containing <35% clay and < 70% sand; and fine texture: containing > 35% clay Figure 3: Soil textural triangle, composed of 12 classes Impact on classification and land use Clay translocation through the profile is an expression of soil profile evolution. The presence of an argillic horizon in the profile

17 and of clay coatings on ped surfaces are important diagnostic criteria for the classification of soils. Texture has also a major impact on the hydraulic properties of soils, both in terms of water infiltration (permeability) and water retention capacity, as well as on consistence and tillage properties, porosity and aeration, etc. It determines also management practices, as for example the type of irrigation to be applied (flood irrigation is not economical in too permeable soils). Table 3 gives a good correlation between some of these parameters. It shows that loamy sand can hold only half of plant-available water and almost three times less than a loam and clay soil, respectively. These correlations can be used to certain extent (because it disregards the impact of structure) as a rule of thumb in projects where no analytical facilities are available and rapid decisions need to be taken. Soil Texture Permeability (cm/h) Loamy sand 5 (2.5-25) Loam 1.3 ( ) Clay loam 0.8 ( ) Silty clay 0.25 ( ) Clay 0.05 ( ) Total pore space (%) 38 (32-42) 47 (43-49) 49 (47-51) 51 (49-53) 53 (51-55) Bulk density (g/cm3) 1.65 ( ) 1.40 ( ) 1.35 ( ) 1.30 ( ) 1.25 ( ) Field capacity (%) 9 (6-12) 22 (18-26) 27 (23-31) 31 (27-35) 35 (31-39) Permanent wilting point (%) 4 (2-6) 10 (8-12) 13 (11-15) 15 (13-17) 17 (15-19) Water content mm/m soil 8 (**) (7-10) 17 (14-19) 19 (17-22) 21 (15-23) 23 (20-25) * Average figures and range (between brackets). ** The moisture content on a volume basis (mm water per meter of soil) is obtained by subtracting weight percentages at field capacity and permanent wilting point, multiplied by bulk density. Table 3: Correlation between texture and a number of other soil properties: bulk density, permeability, water retention capacity (*) The presence of a clay accumulation zone in the profile can sometimes affect the internal moisture and nutrient uptake status for plants and crops. Up to a certain point, a clay increase in the subsoil is desirable as it can increase the amount of water and nutrients stored in that zone. By slightly reducing the rate of water movement through the soil, it will reduce the rate of nutrient loss through leaching. However, if the accumulation of clay is too drastic so as to form a clay-pan, for example it will severely restrict the movement of water and air, as well as the penetration of roots in the Bt horizon. It will also tend to increase the amount of water from rainfall or irrigation that will occur as runoff on sloping land. Finally, there is also a close correlation between clay content and cation exchange capacity (CE) through the specific surface area and charge properties. Hence, 100% clay soils should have a CE of me/ 100 g soil when the dominant clay type is kaolinte, me when it is illite-vermiculite, me when it is montmorillonite and >100

18 me when it is allophane (see: Soil Mineralogy) Soil Structure While texture concerns primary the size of soil particles, soil structure refers to their arrangement and coagulation. Structure is, therefore, the combination or arrangement of single soil particles (sand, silt, clay) into secondary compounds or peds. These peds are formed as a result of the coagulation of primary minerals bound by a cement like microbial gum, iron oxide, organic carbon compounds (in particular polysaccharides) or clay. Peds are separated from adjoining peds by surfaces of weakness. Structure is a descriptive term and cannot be adequately quantified. Though there is no formal agreement between researchers on the mechanisms that affect structural development, it is believed that the following factors play a role: (1) wetting and drying; (2) freezing and thawing; (3) physical activity of roots and soil animals; (4) the influence of decaying organic matter and rest products of microorganisms; (5) modifying effects of absorbed cations (examples are Ca and Na); and (6) soil tillage. Figure 4: Types of structure: A: prismatic; B: columnar; C: angular blocky; D: subangular blocky; E: platy; F: granular (USDA Soil Survey Manual, 1951) Measurement Structure is directly defined in the field by observing the nature and form of the soil peds in the profile. It is characterized by three criteria: type, class, and grade or degree of development. The type corresponds to the macroscopic appearance, shape and arrangement of the peds. Class indicates ped size, i.e. the width or thickness of the structural aggregate. The grade is the degree of aggregation or structure development; it expresses the differential between cohesion within peds and adhesion

19 between peds. Grade of structure varies with soil moisture contents and tends to be stronger as the soil dries; it is qualified as: structureless, weak, moderate and strong. There are 4 principal types of soil structure: spheroidal, including granular and crumb structures; platy-like; prism-like, including prismatic and columnar structures; and block-like, including subangular blocky and cube-like structures (Figure 4). Impact on other soil properties Structure modifies the influence of texture with regard to moisture, heat and air relationships in the profile. The macroscopic size of most peds results in the existence of inter-ped spaces much larger than those that can exist between adjacent sand, silt and clay particles within peds. It is this structural effect on the pore space relationships that makes structure so important (see: Soil Physics). Structure is important for the movement of water through the soil and for surface erosion. The structure of the surface layers, though it varies from soil to soil, tends to produce larger pores than would be the case if the soil had no structure. These pores allow the soil to take up large amounts of rain water, and thus to minimize runoff and surface erosion. The fact that many structural aggregates are water stable is important because it means that the percolating waters are fairly free of clay particles. However, such aggregates may become unstable in water and lead to a compact soil, as is the case in soils saturated with Na. The presence of free CaCO 3 in the soil has an opposite effect: it improves structural stability Soil Consistence Soil consistence is a term used to describe the resistance of a soil to mechanical stresses or manipulations at various moisture contents. It is a composite expression of those cohesive and adhesive forces that determine the ease with which a soil can be reshaped. Whereas structure deals mainly with the shape, size and distinctness of natural soil aggregates, consistence refers to the strength and nature of the faces between particles. This physical soil property affects mainly traffic and tillage conditions of the land. Like structure it can not be properly quantified and is therefore commonly estimated by feeling and manipulating the soil by hand, or by pulling a tillage instrument through it. The consistence of soils is generally described at 3 moisture levels: dry, moist and wet. If the soil is dry, the terms soft or hard are used; when moist, the terms loose, friable or firm are used; if wet the terms sticky or plastic are used Soil Color Soil color is easily determined in the field in a semi-quantitative way by comparing the field color to a standard Munsell Color chart. This consists of 175 color chips arranged systematically on seven charts according to hue, value and chroma, e.g. the three variables that combine to give the colors. Figure 5 is an example of chart 7.5 YR. The hue (on the top right of the chart) refers to the dominant wave-length or color of light between red, yellow and blue. Value (on the ordinate) or brilliance refers to the total quantity of light, obtained by adding a black or white fraction; it increases from dark to light colors. Chroma (on the abscissa) is the relative purity or intensity of the dominant

20 wavelength of light; it increases with decreasing proportions of white light. Figure 5: Example of a Munsell Color Chart (Munsell Color, 1975) The Munsell notation of color is a systematic numerical and letter designation of each of the three variable color properties described above left side of Figure 5). These properties are always given in the order: hue, value and chroma. Hence, a color is marked as, for example, 7.5 YR 6/4, whereby 7.5 YR stands for the hue, 6 for the value and 4 for the chroma. This corresponds to a light brown color, which is directly visualized in the Munsell Color book (right side of Figure 5). In most uniform soils there is normally only one color which defines the soil matrix. This is normally expressed by indicating the exact correspondence with the Munsell color (example 7.5 YR 6/4) or by referring to the nearest-by color (example: 7.5 YR 6/5 indicates that the soil color is intermediate between 7.5 YR 6/4 and 6/6). In a less uniform horizon or in layers with a spotted pattern (as in gley soils subject to a fluctuating water table), the color of both the dominant matrix and of all the mottles is marked. Impact on other soil properties Soil color is an indirect expression of a number of important soil qualities which are otherwise not easily quantified. A black color is