Diagnostic Horizons Quiz: Surface, Subsurface, and Soil Identity

Diagnostic horizons in Soil Taxonomy are strictly defined by quantitative criteria including minimum thickness, chemical composition, color, structure, and other measurable properties. They replace subjective descriptions with objective, reproducible classification criteria. The presence or absence of specific diagnostic horizons determines which soil order, suborder, and great group a soil belongs to, making them the foundation of the Soil Taxonomy classification system.

The ochric epipedon is defined by its failure to meet the criteria of any other epipedon. It is typically pale in color due to low organic matter content, or it may be thin or hard when dry. It is the default diagnostic surface horizon found in soils with limited vegetation input, including Aridisols, Entisols, and some Inceptisols where pedogenic development of a darker, richer surface layer has not occurred.

The mollic epipedon requires both dark color and a base saturation of at least 50 percent by ammonium acetate extraction throughout its thickness. This high base saturation reflects nutrient-rich conditions maintained by grassland vegetation cycling calcium, magnesium, and potassium back to the surface. Without meeting the base saturation requirement, a dark surface horizon is classified as an umbric epipedon rather than a mollic epipedon.

The umbric epipedon is morphologically similar to the mollic epipedon in color, thickness, and organic matter content, but it has base saturation below 50 percent. This low base saturation reflects intense leaching under humid forest vegetation that removes calcium, magnesium, and other cations. Umbric epipedons are characteristic of Inceptisols and Andisols in humid, leached environments where organic matter accumulates but bases do not.

The argillic B horizon is one of the most widely recognized diagnostic subsurface horizons in Soil Taxonomy. It forms through lessivage, the downward translocation of fine clay particles by percolating water. Its presence is identified by a minimum increase in clay content relative to the overlying eluvial horizon and by the occurrence of clay skins or argillans on ped faces and pore walls, visible both in hand specimen and under a polarizing microscope.

Recognized diagnostic subsurface horizons include the argillic, enriched with translocated clay, the spodic, enriched with iron, aluminum, and humus, and the natric, an argillic horizon additionally enriched with exchangeable sodium producing characteristic columnar structure. The albic or eluvial horizon is a diagnostic horizon defined by the loss rather than accumulation of materials, placing it in the category of diagnostic surface and near-surface horizons rather than illuvial subsurface horizons.

A natric horizon is a specialized form of argillic horizon that additionally requires high exchangeable sodium or high combined sodium and magnesium relative to calcium plus acidity. Elevated sodium disperses clay and produces the characteristic columnar or prismatic structure visible at the surface of the horizon. Natric horizons develop in soils subject to alternating wetting and drying in sodium-rich parent materials and are diagnostic for Aridisols with sodic properties.

The cambic horizon is defined by evidence of pedogenic alteration including color change, structure development, or removal of carbonates, without meeting the quantitative criteria for more strongly developed horizons such as the argillic or spodic. It indicates that pedogenesis has begun transforming the parent material but has not produced sufficient clay translocation or mineral illuviation to qualify as a more diagnostic horizon. Inceptisols are the primary order defined by cambic horizons.

The albic horizon is defined by very pale color resulting from the loss of iron oxide coatings and clay from individual grains by eluviation. It typically overlies a more accumulative horizon such as an argillic or spodic B horizon into which the eluviated materials have been translocated. The albic horizon is most strongly developed in Spodosols undergoing podzolization and in Alfisols and Ultisols with strongly leached profiles in humid climates.

Diagnostic surface horizons include the histic epipedon formed in waterlogged organic conditions, the anthropic epipedon modified by prolonged human activity such as addition of manure and cultivation, and the melanic epipedon, a very dark organic-rich horizon with specific optical properties associated with Andisols derived from volcanic material. The spodic horizon is a diagnostic subsurface horizon, not a surface horizon, forming in the subsoil below the albic eluvial zone.

The oxic horizon is a diagnostic subsurface horizon defined by dominance of low-activity 1:1 clay minerals such as kaolinite, and hydrated iron and aluminum oxides such as goethite and gibbsite. It has very low cation exchange capacity, typically below 16 centimoles per kilogram of clay, reflecting extreme weathering that has removed nearly all primary minerals and high-activity clays. It is the defining diagnostic horizon of Oxisols and reflects millions of years of pedogenesis under tropical humid conditions.

The petrocalcic horizon is defined as a cemented or indurated calcic horizon that is so strongly consolidated by calcium carbonate that it is brittle when dry and does not slake in water. It represents an advanced stage of carbonate accumulation beyond the powdery or nodular calcic horizon. Petrocalcic horizons restrict root penetration and drainage, posing significant challenges for agriculture and engineering in Aridisol landscapes where they commonly occur.

A fundamental principle of Soil Taxonomy is that diagnostic horizons are defined by specific, quantitative, measurable criteria including minimum thickness, chemical composition thresholds, color values using Munsell notation, and texture requirements. This quantitative approach allows different soil scientists to classify the same soil consistently regardless of their subjective interpretation, making the system reproducible, comparable across regions, and suitable for use in international research.

Chemical accumulation defines several subsurface horizons. The spodic horizon accumulates amorphous iron, aluminum, and organic carbon complexes through podzolization. The calcic horizon accumulates secondary calcium carbonate by partial leaching. The gypsic horizon accumulates gypsum in hyper-arid conditions. The argillic horizon is defined by physical translocation and accumulation of silicate clay particles through lessivage, which is a mechanical rather than purely chemical process.

The kandic horizon satisfies the clay content requirements of the argillic horizon but differs fundamentally in clay mineralogy and activity. Its cation exchange capacity is below 16 centimoles per kilogram of clay, reflecting dominance of low-activity 1:1 kaolinite and iron oxides produced by advanced tropical weathering. This low activity distinguishes deeply weathered tropical soils such as Ultisols in humid subtropical regions from the less weathered argillic soils of temperate climates.