Lecture 6: Soil Profiles: Diagnostic Horizons

Lecture 6: Soil Profiles: Diagnostic Horizons

Complexity in Soil Profiles

Soil Horizons Soils display distinct layering O Horizon: Partially decomposed organic matter (OM) A Horizon: Near surface, mineral soil rich in OM E Horizon: Leached layer below A horizon (sometimes absent) B Horizon: Accumulation of minerals, low in OM C Horizon: Weathered parent material

Subordinate Disctinctions Common distinctions: t: clay accumulation k: carbonate accumulation o: oxide accumulation g: gleying (grey/green color) n: sodium accumulation

Subdivisions and Transition Horizons Horizons may be subdivided if distinct layers are present with a horizon Example: B1-B2-B3 Lower horizons developed in different parent material than upper horizons are preceded with a 2 Example: O, A, and B horizons formed from loess (windblown silt), C formed in glacial till: O-A-B-2C Distinct layers may exists between horizons that have characteristics of both Double capital letters are used, with the dominant horizon listed 1 st

Effects of Humans on Soil Profiles Loss through erosion and compaction Plow Depth

Truncated Profiles

Buried Soil A = modern A horizon C1 = 1 st C horizon 2C2 = 2 nd parent material, 2 nd C horizon 3C3 = 3 rd parent material, 3 rd C horizon 4C4 = hopefully you get it by now! 5Ab = 5 th parent material, buried (b) A 5Btgb = 5 th parent material, buried (b) clay-rich (t) B horizon with gleying (g) The buried soil is ~400 years old and was likely an Alfisol

Urban Soil Profiles M = Asphalt Horizon

Urban Soil Profiles

Key Concepts in Soil Profiles Soil profiles show distinct horizons The horizons displayed by a soil reflect the soil forming factors and processes at the site There is a system used to describe soil horizons; this is a key component of the language of soil science Hint: You need to learn this, at least the basics! There are many complex ways in which horizons may be arranged in a soil

Diagnostic Soil Horizons

Diagnostic Soil Horizons Soils often contain horizons with distinctive properties These are indicators of a soil s ability to carry out important functions in natural and engineered systems Diagnostic horizons are also key features used in soil classification Diagnostic horizons are identified based on visual and textural properties as well as chemical and mineralogical features: Color Base saturation Texture Cation exchange capacity ph Clay mineralogy We will learn about many of these properties in the coming weeks!!!!!!

Concept of Base Saturation Soil materials (organic matter, clay minerals) have the ability to cation exchange A cation on an exchange site (X) can be swapped for a cation in solution X-Na + K + X-K + Na + Cation Exchange Capacity (CEC) is a quantitative measure of the ability of a soil to undergo cation exchange Base saturation describes the percentage of available exchange sites occupied by base cations: Ca 2+, Mg 2+, K +, Na + Base cations are those that are strong bases when in the form of a hydroxide solid (e.g., NaOH) Base cations are also called non-acid cations Acid cations (e.g., Al 3+, Fe 3+, H + ) produce acid (H + ) in water

Clays and Soil Classification The types of clay minerals present in a soil affect how soils are classified High-activity clays (e.g., smectite, vermiculite) have a high cation exchange capacity Some high activity clays swell when exposed to water (see Vertisols, which are rich in smectite) Low-activity clays (e.g., kaolinite) have a low cation exchange capacity In soils formed on volcanic ash, a poorly crystalline clay called allophane is common

Diagnostic Surface Horizons (Epipedons) * Only five of the eight Epipedons occurs naturally over substantial areas

Mollic Epipedon Mineral surface horizon Dark in color from OM Thick (>25cm if not eroded; at least 15 cm) Soft even when dry >50% base saturation Typically found under grasslands

Ap A2 Mollic Epipedon Bt1 Argillic Horizon Bt2 Bk

Ochric and Umbric Epipedons Ochric: Mineral horizon Not mollic or umbric: Too light in color Too thin Too low in organic matter Hard and massive when dry Umbric: Visually similar to Mollic Chemically different: Low base saturation Ochric Epipedon

Melanic and Histic Epipedons Melanic: Mineral horizon High OM content (>5%); black Contains minerals like allophane Forms in volcanic ash Histic: Organic soil horizon overlying a mineral soil Made of peat or muck Black or dark brown

Plaggen General not found in US Mostly common in northwestern Europe Formed by plowing sod/manure mixtures into soil for 100s of years during medieval times General found on sandy spodosols A mineral horizon with a few % organic matter

Diagnostic Subsurface Horizons

Argillic Horizon (Bt) Subsurface accumulation of silicate clays Forms by translocation from upper horizons Often contains shiny coatings of clays on natural soil blocks (peds) Clay skins or argillans Ap A2 Bt1 Bt2 Bk Mollic Epipedon Argillic Horizon

Clay Skins Films of clay that coat the surface of peds (natural soil blocks); often are shiny

Natric (Btn) Silicate clay accumulation with clay skins Clays have >15% exchangeable sodium Horizon contains columnar structural units Mostly found in arid and semiarid environments

Oxic (Bo) Rich in iron and aluminum oxides Low ability to hold and exchange cations Horizon shows diffuse boundaries Forms in humid tropical or equatorial regions with highly weathered soils Puerto Rico, Brazil, Hawaii, Central Africa

Spodic (Bh, Bs, Bhs) Illuvial accumulation of organic matter and aluminum oxides (sometime with iron oxides) Mostly found in leached forest soils in cool humid climates Conifer forests in Maine, northern Michigan Typically formed from sandy parent material

Albic (E) Albic Horizon Light-colored eluvial (leached) horizon Low in clays Low in Fe and Al oxides Low in OM Fe, Al, and clays typically found in lower horizons Found in some forest soils

Calcic, Gypsic, Salic Ap A2 Bt1 Bt2 Bk Mollic Epipedon Argillic Horizon Calcic Horizon Found in arid/semiarid soils Calcic (Bk): Accumulation of calcium carbonate (CaCO 3 ) Often occurs as nodules Gypsic (By): Accumulation of gypsum (CaSO 4 2H 2 O) Salic (Bz): Accumulation of soluble salts

Calcic Horizon

Pans Some subsurface horizons are cemented or densely packed These relatively impermeable layers are called pans Fragipan: Dense, brittle horizon Possibly formed by infilling of pore space by silt Duripan: Horizon cemented by silica Placic Horizons: Horizon cemented with Fe, Mn oxides Pans limit plant growth and promote water runoff and erosion Fragipans occur in humid regions, other pans typically occur in arid regions

Fragipans Looking Down on a Horizontal Slice through a Fragipan

Relict Argillic Horizons in Deserts Southern Idaho Southern New Mexico

Key Concepts in Diagnostic Soil Horizons Diagnostic soil horizons indicate important soil properties and are essential features needed for soil classification Both surface (epipedons) and subsurface horizons are common Multiple diagnostic horizons are often present in a soil; these can overlap Horizons indicate soil forming processes, either at present or in the past