Department of Geography and Environmental Studies Wilfrid Laurier University GG282 Laboratory Exercise Five Applied Geomorphology: Soil Properties and Distribution Introduction In this exercise, students will review: (i) the basics of soil horizons, (ii) the textural terms that are used to describe soil horizons, (iii) properties that can help us to differentiate between horizons, (iv) data on soil horizons and profiles, (v) a series of mapping resources available regarding the distribution of soils and their characteristics, and (vi) mapping data from some local soils. Reference Material The Canadian System of Soil Classification, Third Edition. (1998) Agriculture and Agri-Food Canada Publication 1646, 187 pp. Available online at: Soil Horizons A soil horizon is a laterally continuous layer in a soil profile that is approximately parallel to the surface. Within a given horizon the physical properties of texture, colour, consistency, organic content, and structure are relatively uniform. Horizons are classified based on their position in the soils profile, their composition and on their physical characteristics. Soil types or soil orders are determined based on the sequence of horizons that are present in a profile at a location. There are two tables that follow that present the codes that are used to designate soil horizons in Canada. Table 1 presents the major mineral and organic horizons of the Canadian System of Soil Classification. The major mineral horizons are designated as either A, B, or C depending upon their position and characteristics. There are also organic horizons that are designated as either O, L, F or H. The properties of the major mineral and organic horizons are described in Table 1. These major horizons can in turn be divided into subhorizons. The common subhorizons designations employed in the Canadian System of Soil Classification are presented in Table 2. The suffixes that are presented in Table 2 are used to modify the terms in Table 1. Horizon Properties Soil Texture One of the most important properties of a soil horizon is its texture. When we determine the texture of a soil horizon we exclude particles that are larger than sand sized. There are a series of common terms that are used to characterize soil texture. In a field setting it is relatively simple to approximate the texture of a sample by a hand test (recall Labs 1 and 2). The typical field categories and their general characteristics are outlined below, this is not an exhaustive list: 1) Sand – individual grains can be seen, when moist it will form a cast that crumbles when touched 2) Sandy Loams – sand dominated but with enough silt and clay to give some coherence, when moist it will form a cast that can be handled 3) Loam – a even mixture of sand, silt and clay, has a gritty feel but is somewhat plastic, will form a cast when dry, a moist cast can be handled without breaking 4) Silt Loam – more than 50% silt, with a moderate amount of (fine) sand and a small amount of clay, may form clods when dry, but when crushed feels floury, when moist can form into a rough, broken ribbon 5) Clay Loam – fine soil that has moderate amount of clay mixed with silt and ( fine) sand, forms hard clumps when dry, when moist it will form a ribbon, but will break with much handling 6) Clay – dominated by clay, forms hard clumps when dry, plastic and often sticky when wet, will form a thin flexible ribbon when wet (normally) From a full particle size analyses it is possible to determine the %’s of sand, silt and clay in a soil sample. The textural classification used by the Canadian System of Soil Classification is shown on the next page. The classifications are often shown as ternary diagrams (triangular graphs). The graph that follows allows the texture of the soil sample to be determined if the %’s of clay and sand are known. The % clay and % sand data are plotted as a point on the graph and the soil texture is read off. For example, a sample that is 30% clay, 30% silt and 40% sand would plot in the Clay Loam portion of the classification. Soil texture plays an important role as an influence on water movement and moisture storage, soil structure as well as a control on the distribution of nutrients. For the four samples below, plot the data on the textural triangle and determine the appropriate textural term to describe those samples. Soil Sample % Sand % Silt % Clay 1 40 40 20 2 30 50 20 3 10 30 60 4 80 15 5 Textural Term Horizon Properties Soil Colour The colour of each horizon, or sampling point in a profile, should be determined by using a Munsell Soil Colour Chart. To use the Munsell System for soil colour, a soil sample is held up against a series of coloured standards (chips) and the soil colour is determined by comparing the sample to the standards until a match is found. The Munsell System describes colour using three components: Hue (a specific colour in the visible spectrum), Value (refers to lightness or darkness), and Chroma (refers to the colour intensity or saturation). The standard colour chips are arranged by Hue in pages of colour chips. The soil Hue is first determined and then the Value and Chroma are determined. The Hue is denoted using a number and uppercase letter(s) such as 7.5YR (for Yellow Red). The Value is denoted by a number that runs vertically on the page, with lower values indicating darker colours and higher values lighter colours. The Chroma is arranged horizontally on the page with the lower Chroma indicating less colour intensity (more grey) and higher Chroma more intense colours. In the example circled in Yellow to the right, the For a complete description of colour it should be recorded under the following conditions: (i) moist soil colour, (ii) ped face colour, (iii) air dry colour (in lab), (iv) rubbed colour (rub soil onto a clean white surface). The Munsell System is described at: Example Munsell charts can be seen at: and with an interesting blog posting on soil colour at: Stoniness The textural terms described above can be modified if a soil horizon as a very coarse texture. For example, if the A horizon in a soil has a high gravel content, its texture might be described as a gravelly sand or gravelly loam. The gravel components of a soil may be referred to as stones. The term stoniness is applied to designate the proportions of gravel sized particles (stones) in a soil sample. An assessment of stone prevalence can be made in each soil horizon in a profile (or sample) by determine the percentage of the mass of a horizon (or sample) that is comprised of gravel sized clasts. The intent is to describe what fraction of the soil horizon or sample is comprised of gravel materials (anything coarser than sand). One of the commonly used classifications of stoniness is: Few stones (1-5%), Common Stones (6-15%), Many Stones (16-35%), Abundant Stones (36-70%), and Extremely Abundant Stones (>70%). Mottling Mottles are localized nodular (spot) or streaked shaped discolourations that have a substantially different colour than the surrounding dominant soil colour (colour of the soil matrix). Mottles can occur under a range of conditions but in Canadian soils they can most common in poorly drained soils such as Gleysols. The typical colour of a Gleysol is a blue grey. As Gleysols periodically dry out there maybe some localized oxidation which can produce pale yellow or reddish discolourations (mottles). Alternatively the mottle colours could be different such as a grey or blue colour and be found in a soil with a different dominant colour. The presence, abundance, size and boundary conditions of mottles should be noted if they are present. Organic Matter The distribution of organic matter should be noted, include information on state of decomposition, presence of organic coatings on stones, degree of mixing, and the presence of roots or other organics. Soil Structure Soil structure should be described based on the size, shape and development of aggregates of soil particles (peds). Common soil structures are: (i) Platy (sheet like peds), (ii) Columnar (or prismatic, long vertically developed peds), (iii) Granular (small equant peds), when a large granular structure is seen the term blocky is used. If no soil structure can be discerned, use the term Apedal. Soil Consistence Consistence can be described by measuring the: (i) strength, (ii) stickiness, (iii) cementation, and (iv) plasticity. Soil Orders and Horizons We determine the soil order by examining the sequence of horizons that are present. The characteristics of the major horizons are reviewed in lecture. Table 1 presents this information focusing on the mineral and organic horizons. Information on subhorizons is presented in Table 2. The suffixes that are presented in Table 2 are used to modify the terms in Table 1. Table 1: The major Mineral and Organic Horizons of the Canadian System of Soil Classification. Mineral Horizons (contain 17% or less organic Carbon (about 30% organic matter) by weight) A This mineral horizon forms at or near the surface. It is characterized by: leaching, eluviation, accumulation of organic matter or some combination of these. An accumulation of organic matter is usually seen by a darkening of the soil (Ah). Conversely, the removal of organic matter is usually expressed by a lightening of the soil colour (Ae). An Ae horizon may have a light ash grey appearance. The removal of clay (Ae) is expressed by a coarser soil texture relative to the underlying subsoil layers. The removal of iron is indicated by a paler or less red soil colour (Ae) relative to the lower part of the subsoil. B This mineral horizon is characterized by enrichment in organic matter, sesquioxides (iron and aluminum oxides), or clay; or by the development of soil structure; or by a change of colour denoting hydrolysis, reduction, or oxidation. B horizons enriched in iron compounds often have a red or reddish orange colour (Bf). In B horizons, accumulated organic matter (Bh) is evidenced usually by dark colours relative to the C horizon. Clay accumulation is indicated by finer soil textures and by clay coatings on particles and lining pores (Bt). Soil structure developed in B horizons includes prismatic or columnar units with coatings or stainings and significant amounts of exchangeable sodium (Bn) and other changes of structure (Bm) from that of the parent material. Colour changes include relatively uniform browning due to oxidation of iron (Bm), and mottling and gleying of structurally altered material associated with periodic reduction (Bg). C This mineral horizon is comparatively unaffected by the soil forming processes operating in A and B horizons, except the process of gleying (Cg), and the accumulation of calcium and magnesium carbonates (Cca) and more soluble salts (Cs, Csa). R Bedrock layer that is too hard to break with the hands or to dig with a spade when moist. It does not meet the requirements of a C horizon. Organic Horizons (contain >17% organic Carbon (about 30 % or more organic matter) by weight) O This peat rich organic horizon is developed from mosses, rushes, and woody materials L, F, H: These organic horizons are developed primarily from the accumulation of leaves, twigs, and woody materials with or without a minor component of mosses. They are subdivided into three types. L Original structures are easily discernible. F Partly decomposed organic matter. Some of the original structures difficult to recognize. H Characterized by an accumulation of decomposed organic matter in which the original structures are indiscernible. This horizon differs from F by having greater humification due chiefly to the action of organisms. Table 2: Subhorizons of the Canadian System of Soil Classification (this table is abbreviated and simplified, see CSCS for full table) Subhorizons : Lowercase suffixes b Buried soil horizon. c Cemented (irreversible) soil horizon (e.g. a layer cemented by calcium carbonate (CaCO3)). ca Horizon of secondary carbonate enrichment in which the concentration of lime exceeds that in the unenriched parent material. e Horizon characterized by the eluviation of clay, Fe, Al, or organic matter alone or in combination. It is used in combination with A. f Horizon enriched with amorphous material, principally Al and Fe combined with organic matter; reddish in colour near upper boundary becoming yellower with depth. g Horizon characterized by grey colours, or prominent mottling, or both, indicating permanent or periodic intense reduction (gleying). h Horizon enriched with organic matter. j A modifier of suffixes e, f, g, n, t, and v to denote an expression of, but failure to meet, the specified limits of the suffix it modifies. k Denotes the presence of carbonate as indicated by visible effervescence when dilute HCI is added m Horizon slightly altered by hydrolysis, oxidation, or solution, or all three to give a change in colour or structure, or both. n Horizon in which the ratio of exchangeable Ca to exchangeable Na is 10 or less. It must also have a prismatic or columnar structure, have dark coatings on ped surfaces, and have a hard to very hard consistence when dry. p Horizon disturbed by anthropogenic activities such as cultivation, logging, and habitation. s Horizon with salts, including gypsum, which may be detected as crystals, veins or surface crusts of salts. sa Horizon with secondary enrichment of salts more soluble than Ca and Mg carbonates; the concentration of salts exceeds that in the unenriched parent material. t An illuvial horizon enriched with silicate clay

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