Ngogo Mn. Ge:140 @2019 SOIL CLASSIFICATION An Introduction SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 Introduction • Classification of soil is the separation of soil into classes or groups each having similar characteristics and potentially similar behaviour. • The criteria for grouping soils depend on the purposes of classification. • Soil is not uniformly distributed. Properties of soil vary due to several soil forming factors. properties vary very widely. Soil types are many. To identify, understand, and manage soils, soil scientists have developed a set of soil classification or taxonomy systems. SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 Methods of Soil Classification Systems • Systems which use the properties of the soils are called TAXONOMIC SYSTEMS OF CLASSIFICATION. • Systems that employ soil usage are called CAPABILITY CLASSIFICATION. • Though these systems are considered to be subjective or utilitarian system of classification. • International soil classification are primarily for use by soil specialists. SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 Soil Classification Used in the Tropics I. The FAO/UNESCO soil map of the world (FAO/UNESCO, 1977) II. The New Comprehensive Classification System or US Soil Taxonomy Classification (Soil-Survey Staff, 1975) III. The French Classification System (Aubert (1964) SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 FAO Soil Classification System • In 1977 FAO/UNESCO published the first soil map with a uniform classification throughout. • The World Soil Map is not based on any one system, but rather, a collection of national soil maps influenced by US System. • In the FAO system, the soils that occur in a particular region and exhibit a certain relationship to one another are grouped in association or “ major soil Units” SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 Terms used in the FAO nomenclature Term Connotation Acric Extremely poor in nutrients Argillic Horizon with clay accumulation Chromic Soils with deep brown or red-brown B- horizon Dystric Base saturation less than 50% (relatively poor in nutrients) SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 Term Connotation Eutric Base saturation greatter than 50% (relatively rich in nutrients) Ferralic Soil with Ferralsol properties; low CEC, high in Fe, and Al-oxide Ferric Soil with iron concretions Gleyic Soils with hydromorphic properties at 0-50 cm depth Hapiic Soils with less strongly developed typical characteristics SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 Term Connotation Humic Soil with “umbric” horizon, i.e. relatively humus Mollic A horizon with high base saturation Ochric Relatively weak defined A horizon without “mollic and ‘umbric x-tics Orthic Normal soil formation no special featurs Oxic Oxide, complete mineral weathering; porous, permeable SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 Term Connotation Plinthic With plinthite from 0-125 cm(laterite develops from plinthite) Rhodic Bright red Vertic Vertisol characteristics (swlling, Shrinking) Xanthic Bright yellow Source: FAO/UNESCO (1977) SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 1. Histosols (H) • Organic or peat soils with an organic surface layer of greater or equal to 40 cm. • They contain at least 20 -30% organic matter depending on clay content. • Have limitations for agricultural use because they are usually wet. SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 2. Vertisols (V) • Very heavy clay soils which develop wide deep cracks in the dry season. • Their topsoil has a x-tics micro-relief (gilgai). • They are found in basins with poor drainage or broad plains with variably moist climates. • Have poor physical properties. • Due to poor drainage and their tendency to swell make them prone to water logging SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 3. Fluvisol • Are young deposits in river valleys, estuaries and coastal regions. • They do not form any distinct horizons other than ochric, histic and sulfic horizons • They are cultivated for dryland crops or rice and are used for grazing in the dry season. They occupy about 2.8 percent of the continental land area on Earth, mainly in the great river basins and deltas of the world (e.g., the Amazon basin and the Nile delta). • SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 4. Solonchanks (Z) • Are structure-less saline soils with free salts often with crystallisation of salts on the surface. • Thy occur in arid and semi-arid and semi-arid regions and generally have a high ground water level with a high salt content (Chlorides and sulphates). SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 5. Andosols (T) • Are soils developed from volcanic ash. • They frequently form heavy topsoil with dark humus. Most andosols are well supplied with water and have a high water holding capacity. • Andosols occur mostly in volcanic mountainous of the humid tropics SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 6. Gleysols (G) • Are hydromorphic soil (soil developed in the presence of water) • Are water logged throughout the year. • They exhibit a friable, light coloured layer of clay that graduates into a water-impermeable stratum where typical manganese concretions are found. • Occur in depressions and plains of the humid and sub-humid tropics and vary widely in character (mollic, plinthic etc.) SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 7. Lithosols (I) • Are very shallow soils, less than 10 cm deep on bedrock. • Have little available volume of soil for root extension and offer crop plants scant growth potential due to lack of water and nutrients. • They dry out quickly SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 8. Arenosols (V) • Are sandy soils with various B-Horizon which fall below the treshold of the definition “moderately pronounced” • Include soils of several groups. • Lower clay content and a higher proportion of sand (more than 50%). • They possess good roots and water permeability but have low water holding capacity SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 9. Ferralsos (F) • Are deeply weathered, uniformly red, yellow- red or yellow soils composed mainly of kaolinite clay (cemented through iron oxide). • Are typical soils of constantly humid tropics. • They are poor in nutrients due to heavy leaching and deep weathering. SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 10. Aerisols (A) • Are old, acidic clay soils with low base saturtion (<5%) and a pronounced clay enriched horizon (Bt) derived from clay deposits and clay accumulation. • The name is derived from the latin acris (sour), developed from base-poor, quartz-rich parent rock (granite, sand stone). • They are typical of the subhumid tropics • They are deficient in nitrogen and trace elements. SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 11. Luvisols (L) • Have a typical clay-enriched horizon, but also higher base saturation (over 50%) • Better siuted for crop growing than acrisol. • Posses vertisol-like x-tics or “ferric” properties with iron oxide concretions. • The soils are suitable for crop production hoever, it have some limitations similar to those encountered in acrisol. SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 12. Cambisols (B) • Are broen soils, which have undergone little or no ferrallisation or podzolisation. • There is almost no accumulation of the products of weathering. • Are often found in mountain areas in the tropics • Usally possess good cropping X-tics with good root permeability and little tendency to compaction SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 • In Tanzania the National Soil Service has adopted the FAO/UNESCO classification as one of the framework for a national soil classification. • In order to provide information useful at a national level, soils have been correlated with the FAO/UNESCO systemes and another major international system such as US SOI Taxonomy. SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 THE NEW COMPREHENSIVE CLASSIFICATION SYSTEM (US Soil Taxonomy) • NCCS is the mos comprehensive and logical system presently existing for the worldwide classification of soil. • The basic feature of the system is that it maintains the natural body concept and has two other major features. – Based on soil properties that are easily verified by others – It employs a unique nomenclature, which gives a definite connotation of the major characteristics of the soils in question SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 Soil Taxonomy categories: • Soil Taxonomy has six categories (From top to bottom) –order, –suborder, –great group, – subgroup, – family and –series SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 • Ten classes are in the order level. • Criteria used to differentiate orders are highly generalized and based more or less on the kinds and degrees of soil-forming processes. • Mostly these criteria include properties that reflect major differences in the genesis of soils. SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 A suborder • is a subdivision of an order within which genetic homogeneity is emphasized. • Soil characteristics used to distinguish suborders within an order vary from order to order. • The great group category is a subdivision of a suborder. • They are distinguished one from another by kind and sequence of soil horizons. • All soils belonging to one of the suborders will have more horizons. SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 Great group & sub goups • Are divided into three kinds of subgroups – typic, intergrade and extragrade. • A typic subgroup represents the basic concept of the great group from which it derives. • An intergrade subgroup contains soils of one great group, but have some properties characteristic of soils in another great group or class. These properties are not developed or expressed well enough to include the soils within the great group toward which they grade. SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 • Extragrade subgroup soils have aberrant properties that do not intergrade to any known soil. SAUT; Geography Dept Ngogo Mn. Ge:140 @2019 A soil family category • Is a group of soils within a subgroup that has similar physical and chemical properties that affect response to management and manipulation.