Programme: M.Sc.(Environmental Science) Course: Soil Science Semester: IV Code: MSESC4007E04 Topic: Soil Classification Prof. Umesh Kumar Singh Department of Environmental Science School of Earth, Environmental and Biological Sciences Central University of South Bihar, Gaya Note: These materials are only for classroom teaching purpose at Central University of South Bihar. All the data/figures/materials are taken from several research articles/e-books/text books including Wikipedia and other online resources. 1 • Pedology: The origin of the soil , its classification, and its description are examined in pedology (pedon-soil or earth in greek). Pedology is the study of the soil as a natural body and does not focus primarily on the soil’s immediate practical use. A pedologist studies, examines, and classifies soils as they occur in their natural environment. • Edaphology (concerned with the influence of soils on living things, particularly plants ) is the study of soil from the stand point of higher plants. Edaphologist considers the various properties of soil in relation to plant production. • Soil Profile: specific series of layers of soil called soil horizons from soil surface down to the unaltered parent material. 2 • By area Soil – can be small or few hectares. • Smallest representative unit – k.a. Pedon • Polypedon • Bordered by its side by the vertical section of soil …the soil profile. • Soil profile – characterize the pedon. So it defines the soil. • Horizon tell- soil properties- colour, texture, structure, permeability, drainage, bio-activity etc. • 6 groups of horizons k.a. master horizons. O,A,E,B,C &R. 3 Soil Sampling and Mapping Units 4 Typical soil profile 5 O • OM deposits (decomposed, partially decomposed) • Lie above mineral horizon • Histic epipedon (Histos Gr. – tissue) • Oi (non to slightly decomposed) • Oe (intermediately decomposed) 6 A • Top most mineral horizons • Inorganic – OM • Usually dark in colour • If O horizon not there – k.a. – Ap (p= plow) • A horizon- used to classify soil (a.k.a diagonsistic epipedons) (Gr. Epi= over; pedon = soil) • Seven major epipedons are there. 7 Diagnostic Surface Horizons 8 E horizons • Located under A • Zone of eluviation (Clay, humus, Fe, Al) • Deposit (eluvial deposit) is the remaining material. • Usually white and pale (k.a. albic horizon) • AE • EB 9 B horizons • Illuviation • The B horizon is a zone of illuviation where downward moving, especially fine material, is accumulated. The accumulation of fine material leads to the creation of a dense layer in the soil. • Accumulation from A, E (Clay, Fe, Al, Humus) • Transition zone BC • Diagnostic subsurface horizons. 10 DIAGNOSTIC SUBSURFACE HORIZONS • Cambic Horizon (Bw horizon)- recently young • Argillic (Bt horizons) – enriched with clay • Natric Horizons - sodium • Albic Horizon- white in colour • Oxic Horizon – highly weathered horizon • Calcic Horizons, • Gypsic Horizons, and • Salic Horizons 11 DIAGNOSTIC SUBSURFACE HORIZONS: • Cambic Horizon (Bw horizon) • A young B Horizon • The cambic horizon can form quickly, relatively speaking, because changes in the color and structure, and some leaching will convert the subsoil parent material into a cambic horizon. • Cambic horizons are not illuvial horizons and, generally, they are not extremely weathered. A cambic horizon is a Bw horizon. 12 DIAGNOSTIC SUBSURFACE HORIZONS • Argillic (Bt horizons) and Natric Horizons (Btn horizons) • The gradual illuvial accumulation of clay in a cambic horizon converts a cambic horizon (Bw horizon) into a Bt horizon. Typically, clay skins occur on the ped surfaces of Bt horizons • A natric horizon is a special kind of argillic horizon. These horizons have typically been affected by soluble salts. The natric horizon is a Btn horizon; the n indicates the accumulation of exchangeable sodium. 13 DIAGNOSTIC SUBSURFACE HORIZONS • Albic Horizon • The loss by eluviation of sesquioxides • (A sesquioxide is an oxide containing three atoms of oxygen with two atoms (or radicals) of another element. For example, aluminium oxide (Al2O3) is a sesquioxide),and clay, during the formation of spodic and argillic horizons, tends to leave behind a light-colored overlying eluvial horizon called the albic horizon. • Albic is derived from the word white. The horizon is eluviated and is labeled an E horizon. The color of the albic horizon is due to the color of the primary sand and silt particles rather than to the particle coatings. Albic horizons are commonly underlain by spodic, argillic, or kandic horizons. 14 DIAGNOSTIC SUBSURFACE HORIZONS • Oxic Horizon • The oxic horizon (Bo horizon) is a subsurface horizon at least 30 centimeters thick, that is in an advanced stage of weathering. It is not dependent on a difference in the clay content of subsoil versus the topsoil horizons. • Oxic is derived from the word oxide. • Oxic horizons consist of a mixture of iron and/or aluminum oxides with variable amounts of kaolinite and highly insoluble accessory minerals such as quartz sand. • Soils with oxic horizons have essentially reached the end point of weathering. 15 DIAGNOSTIC SUBSURFACE HORIZONS • Calcic, Gypsic, and Salic Horizons • The calcic horizon is a horizon of calcium carbonate or calcium and magnesium carbonate accumulation. • Calcic horizons develop in soils in which there is limited leaching and the carbonates are translocated downward. However, the carbonates are deposited within the soil profile because there is insufficient water to leach the carbonates to the water table. The symbol k indicates an accumulation of carbonates, as in Bwk or Ck. • Gypsic horizons have an accumulation of secondary sulfates, and that is indicated with the symbol y. Cemented calcic and gypsic horizons are petrocalcic and petrogypsic horizons, respectively. • Salic horizons contain a secondary enrichment of salts more soluble than gypsic and are indicated with the symbol z. 16 Soil Profile = layers in soil observed with depth; Individual layers are horizons O horizon – contains litter on surface, humus + OM beneath A horizon – various stages of breakdown of organic matter B horizon – mineral soil in which organic compounds have been converted into inorganic C horizon – unmodified parent material Bedrock 17 Zonation in Soil Profile Rain, Leaching A horizon – may show color gradient as OM decreases – eluvial horizons = leaching B horizon – inorganic compounds leached from A horizon accumulate here – illuvial horizons = accumulation Bedrock 18 Eluviation: It is the mobilization and translocation of certain constituent’s viz. Clay, Fe2O3, Al2O3, SiO2, humus, CaCO3, other salts etc. from one point of soil body to another. Eluviation means washing out. It is the process of removal of constituents in suspension or solution by the percolating water from the upper to lower layers. The eluviation encompasses mobilization and translocation of mobile constituents resulting in textural differences. The horizon formed by the process of eluviation is termed as eluvial horizon (A2 or E horizon). Illuviation: The process of deposition of soil materials (removed from the eluvial horizon) in the lower layer (or horizon of gains having the property of stabilizing translocated clay materials) is termed as Illuviation. The horizons formed by this process are termed as illuvial horizons (B-horizons, especially Bt) The process leads to textural contrast between E and Bt horizons, and higher fine: total clay ratio in the Bt horizon. 19 Soil Profile is determined by • Vegetation • Climate ---- rainfall and leaching • Weathering ---- geological history, age • Topography • Parent Material • Etc. 20 Soil Classification • Profile • Texture • Origin • Vegetation • Minerals • Climate • Age • Etc. 21 Soil Taxonomy Hierarchy Kingdom Phylum Order Class 12 Order Family Suborder 63 Genus Species Great group 250 Sub group 1400 Family 8000 Series 19,000 22 Soil Orders 12 recognized, 10 used in agriculture 1. Histosols 2. Vertisols 3. Entisols 4. Inceptisols 5. Aridisols 6. Mollisols 7. Alfisols 8. Spodosols 9. Oxisols 10. Ultisols 11. Andisols 12. Gelisols 23 Soil Orders Use in Agriculture • Histosol • Vertisol Best • Entisol • Inceptisol Many types respond to • Aridisol management (fertilizer, irrigation) • Mollisol • Alfisol • Spodosol • Oxisol Poor • Ultisol 24 Formative Syllables, Derivations, and Meanings of Soil Orders Gelisols el Gk. Gelid very cold, pergelic soil temp. regime 25 Diagnostic Subsurface Horizons Formation Translocation Transformation Clays Organic Matter Oxides 26 Mollic Horizon • The word mollic is derived from mollify, which means to soften. The mollic horizon is a surface horizon that is soft rather than hard and massive when dry. • Mollic epipedon formation is favored where numerous grass roots permeate the soil and a moderate to strong grade of structure is created. • Except for special cases, mollic horizons are dark-colored, contain at least 1 percent organic matter, and are at least 18 centimeters thick. • They are only minimally or moderately weathered and leached. • The cation exchange capacity is 50 percent or more saturated with calcium, magnesium, potassium, and sodium when the cation exchange capacity is determined at pH 7. • The major grassland soils of the world have mollic epipedons. 27 Umbric and Ochric Horizons • The umbric epipedon is similar to the mollic in overall appearance of thickness and color; however, it is more leached and has a saturation with basic cations calcium, magnesium, potassium, and sodium that is less than 50 percent when the cation exchange