Field Analysis:How Soil Features
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[edit] Soil features
Soil features are distinct features within the soil that differ from the soil matrix in composition and which have been formed by the actions of soil forming processes such as gleying (seasonal waterlogging), podzolisation, or translocation. These features include:
- crystals
- nodules and concentrations of iron, manganese, gypsum, calcium carbonate.
- iron pans
- coatings of clay, silt of organic matter on stones, pore walls and soil aggregate (ped) surfaces.
- infillings in pores with a contrasting fabric to the surrounding soil.
Record the presence, frequency, distribution, and other appropriate properties for each of the following classes of soil feature.
[edit] Crystals
The commonest crystals found are calcite, gypsum, halite, and vivianite, though there are many more, particularly in cave environments. Crystals can occur randomly or clustered throughout the soil matrix, lining or infilling pores, on ped surfaces, or associated with organic matter. Record crystal type, frequency and distribution.
Field recognition of minerals and crystals (adapted from Soil Survey England and Wales, 1976)
| Calcite/Calcium carbonate | Can occur as crystals, small nodules, flowstone, or a fine powder, calcite crystals can’t be scratched with a fingernail but can be scratched with a penny. Reacts vigorously with acid. |
| Gypsum | Occurs as crystals, small nodules or a fine powder on surfaces. It does not effervesce with acid and gypsum crystals can be crushed between the fingernails and splits into thin flakes. |
| Halite | Usually colourless or white but can also be blue or pink, it can form as evaporite in arid environments, readily soluble in water. |
| Manganese compounds | Manganese can be difficult to distinguish from carbon and humus. However, manganese is difficult to crush between the fingernails, whereas carbon is readily crushed to form a black powder. Manganese will also react more violently with 20% hydrogen peroxide than carbon or humus. |
| Sulphides | Bluish or black iron sulphides, chiefly pyrite, are common in waterlogged and/or organic soils. They cause waterlogged/reduced soils to smell strongly of rotten eggs. |
| Vivianite | White patches or filaments, which turn bright blue after only a few hours exposure to air. Sometimes associated with bone, shell and organic matter. |
[edit] Soft concentrations
Soft concentrations are areas of material contrasting with the deposit matrix in colour or composition but which cannot be readily separated from the matrix.
The type (calcite, gypsum, iron/manganese etc.), frequency, distribution, and where appropriate colour should be recorded.
[edit] Nodules
Discrete bodies, easy to separate from the deposit matrix, with sharp boundaries, they are usually cemented in some way.
Record type (calcium carbonate, iron/manganese etc.), frequency and distribution, and also whether they are formed entirely of this one mineral or whether they are impregnating the deposit matrix.
[edit] Pans
These are bands of material cemented by organic matter and/or iron and manganese.
The number of pans in a deposit should be recorded (single, double or multiple), as should disturbance (intact, broken but undisturbed, or broken and disoriented), and continuity (discontinuous, continuous). The location of the pan should be closely observed, particularly with reference to the deposit boundaries, organic matter, pores, and sedimentary bedding.
Burning can produce a reddish brown or black cemented crust st the ground surface. More information on distinguising between an iron pan and a burning crust can be found here.
[edit] Coatings
Coatings can occur on ped faces, lining pores, and coating sand grains, stones and nodules. Record the type of coating, their distribution and frequency. It can also be useful to record their presence in underlying deposits.
The commonest types of coating are:
- Textural coatings clay, sand, and silt. clay coatings can give pore surfaces a slight metallic sheen.
- Organic coatings are usually dark coloured and may be mixed with clay.
- Iron/Manganese coatings are usually reddish or very dark brown or black.
- Carbonate coatings may be powdery or consist of larger crystals.
- Stress oriented coatings are formed in-situ usually in clay soils. They can be difficult to distinguish from clay coatings but may contain grooves. Also sometimes called slickensides.
[edit] Pore infillings
These are inclusions within a deposit filling voids, cracks or animal burrows. Record their frequency and the relationship between the infilling fabric and that of the described deposit and adjacent deposits.
[edit] Soil feature frequency
Soil feature frequency is determined by estimating the percentage surface area occupied by the feature of interest. Use the frequency diagram below to ascertain the correct class.
