Soil Profile
The soil profile is defined as a vertical section of the soil from the ground surface downwards to where the soil meets the underlying rock which posses’ different horizons. HORIZON: soil layer parallel to surface with characteristics produced by soil-forming processes. 3 basic soil units (scale)
Pedon: smallest volume called a soil
- Polypedon: soil body (2 or more pedons) in which soils are relatively uniformA group of similar pedons that are closely associated in the field are called polypedons, and are recognized as soil individuals.
- Soil Series :All soil individuals that have in common a suite of soil profile properties and horizons that fall within a particular range are said to be in the same soil series. groups of like polypedons.
- Master horizons: O, A, E, B, C and R
- Solum = “true soil”: O, A, E, B
Regolith: While regolith is a layer of loose, uncompacted dirt, dust and rocks sitting on top of bedrock, soil is the portion of the regolith that is able to support plant life. Soil contains organic matter, liquids and minerals, while most of the other layers of the regolith do not. Regolith appears not only on Earth but also on the Moon, Mars and Saturn’s moon Titan. Soil, however, has only been found on Earth, the only one of those bodies to support plant life.
O horizons
- They are dominated by organic material. Some O layers consist of undecomposed or partially decomposed litter, such as leaves, twigs, moss, and lichens, that has been decomposed on the surface; they may be on the top of either mineral or organic soils.
- Other O layers, are organic materials that were deposited in saturated environments and have undergone decomposition. The mineral fraction of these layers is small and generally less than half the weight of the total mass. In the case of organic soils (peat, muck) they may compose the entire soil profile.
- Oi slightly decomposed (litter)
- Oe intermediate decomposition
- Oa highly decomposed
A Horizons
Mineral horizons that formed at the surface or below an O layer, that exhibit obliteration of all or much of the original rock or depositional structure (in the case of transported materials). A Horizons show one or more of the following:
- An accumulation of humified organic matter intimately mixed with the mineral fraction and not dominated by characteristic properties of the E or B horizons or
- Properties resulting from cultivation, pasturing or other similar kinds of disturbance.
E horizons
- Mineral horizons in which the main feature is loss of silicate clay, iron, aluminum, or some combination of these, leaving a concentration of sand and silt particles and lighter colors.
The horizons exhibit obliteration of all of much of the original rock structure.
- LEACHING : removal of soluble minerals (Ca, Mg, Na) in solution
- ELUVIATION : loss (by water) of suspended materials (clay, humus, oxides)
- ”emigrating”
- ILLUVIATION : accumulation of suspended materials (clay, humus, oxides) and/or precipitated materials from solution (Ca, Mg, Na)
- “immigrating
B horizons
- ACCUMULATED (illuviated ) silicate clay, Fe & Al oxides, carbonates, gypsum, humus
- distinguished from A :
- stronger, redder, or darker color
- OR different texture
- An illuvial concentration of silicate clay, iron, aluminium, carbonates, gypsum, or humus
- A residual concentration of sesquioxides or silicate clays, alone or mixed, that has formed by means other than solution and removal of carbonates or more soluble salts
- Coatings of sesquioxides adequate to give darker, stronger, or redder colors than overlying and underlying horizons but without apparent illuviation of iron
C Horizons
- Mineral horizon (not bedrock) underlying A,E,B horizons
- not affected by soil-forming processes
- may be parent material
- The material designated as C may be like or unlike the material from the A, E, and B horizons are thought to have formed.
R Horizons
- R layers are cemented, and excavation difficulty exceeds high.
- The R layer is sufficiently coherent when moist to make hand-digging with a spade impractical, although it may be chipped or scraped. Some R layers can be ripped with heavy power equipment.
- The bedrock may contain cracks, but these are generally too few and too small to allow roots to penetrate. The cracks may be coated or filled with clay or other material.
Transitional Horizons
- These horizons are intermediary horizons between two master horizons (e.g. O, A, E, B and C). The may be dominated by properties of one horizon but have prominent characteristics of another. Both capital letter are used to designate the transition horizons (e.g. AE, EB, BE, and BC), the dominant horizons being written before the subordinate one.AB
- A B; A dominates
- BA B C B dominates
- AC A C; A dominates
- EB E B; E dominates
Mixed Horizons
- One horizon scattered within another horizon
- B/A mixed A&B; B is matrix for A
- E/B E is matrix
- The first letter designates the material of greatest volume in the transitional horizon. Sub horizons
- Lowercase letters symbolizing divisions within master horizons or characteristics of master horizons