LECTURE 17 Soil Classification

Recap from yesterday… Soil classification: “The ordering of soils into a hierarchy of classes. The product is an arrangement or system of classification designed to express interrelationships of soils and to serve as a filing system. Broad groupings are made on the basis of general characteristics; subdivisions on the basis of more detailed differences in specific properties.” – Soil Science Society of South Africa

Why would we want to classify? To enable communication between specialists In theory construction Advancement in science = the ability to make generalizations and predictive statements For mapping purposes

There are many different classification systems worldwide. Basic units of classification: In many systems, this is the soil profile which is defined as being a 3-dimensional soil body with no significant lateral variation. In the USA, it is the soil pedon or polypedon.

Pedon: “The smallest 3-dimensional portion of the soil mantle needed to describe and sample soil in order to represent the nature and arrangement of its horizons. Rock or material that is too deep to be of interest to agricultural soil users mark the lower limits of the pedon. A group of contiguous, similar pedons is called a polypedon and is the soil individual for the purpose of classification.” –Soil Science Society of South Africa

Taxonomy: “Classification, especially according to natural relationships. The systematic distinguishing, ordering and naming of type groups within a subject field.” – Soil Science Society of South Africa

Soil Taxonomic System for South Africa… Has two levels Form Each form can contain various families To date, 73 forms and 400 families identified Each form has a defined set of Master Horizons These differentiated on grounds of position, as well as physical and chemical properties Other differences within these forms distinguish soil families e.g. signs of wetness, calcareous horizons and layers

You should understand the concept behind the SA system (i.e. the theory of it) and be able to describe at least 3 of the soil forms in SA along with their families, and know how each of the horizons is defined. Reference = Soil Classification – a Taxonomic System for SA (blue book) in the Geography library.

The United States “Soil Taxonomy” (USDA, 1975) Widely used comprising a hierarchy of 6 levels Order Suborder Great group Subgroup Family Series

Important concepts: Diagnostic surface horizons (epipedons) 7 recognized Diagnostic subsurface horizons 18 recognized Soil moisture regimes Soil temperature regimes

Nomenclature (names) are largely logical 12 orders: Alfisols Andisols (ando – blacksoil) Aridisols (aridus – dry) Entisols (recent) Gelisols (gelid – very cold) Histosols (histos – tissue) Inceptisols (inceptum – beginning) Mollisols (mollis – soft) Oxisols (oxide) Spodosols (spodos – wood ash) Ultisols (ultimus – last) Vertisols (verto – turn)

Suborders and great groups – see handout of Tables 3.6, 3.7 and 3.8 from Brady & Weil. Subgroups More than 1300 recognised “Typic” = properties typical to a particular great group. Other subgroups indicate slight deviations from the typic subgroup

Families About 8000 recognised. Within a subgroup, soils fall into a particular family if they have similar physical and chemical properties affecting growth of plant roots at a specified depth, e.g. CEC, texture, mineralogy. Series recognized in the US. Specific range of soil properties involving kind, thickness and arrangement of horizons.