Field Analysis:Is this a buried soil
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[edit] Is this a buried soil?
Buried soils are important repositories of information concerning the pre-settlement environment and environmental change, land management practices, and phases of stability (non-deposition).
In the field the identification of buried soils usually refers to the identification of the organic surface horizon. However, in practice it is often the presence of expected soil horizonation and soil development indicators below this horizon that form the basis of interpretation.
[edit] Indicators of buried soils
When attempting to identify a buried soil in the field a checklist of features that can be looked for/considered are:
- The presence of a homogeneous, slightly darker or browner, organo-mineral layer. Possibly retaining a granular structure, but devoid of any sedimentary structures (bedding), evidence of rooting patterns and/or a thin layer of pebbles 20-30 cm below the surface, and at least some of the following...
- Frequency of root traces decreasing with depth.
- A change in structure, where preserved, to blocky, prismatic or columnar below the organic horizon becoming apedal (structureless) with depth.
- Presence of soil features in underlying deposits (coatings, pans, crystals, nodules and concentrations) consistent with those expected in local soils.
- Change in properties towards that of the ‘soil’ parent material, whether that is the local solid or drift geology, natural sediments or anthropogenic sediments.
- Clear plough marks in the sub-soil.
If the soil has been truncated the organic topsoil may be missing, in which case identification is very much harder and uncertainties are very much greater. Identification relies on the presence of clearly distinct sub-soil horizons, for example an iron an enriched spodic, Bs horizon in a buried podzol soil. More information on soil horizon sequences can be found here.
[edit] Uncertainties in identifying buried soils
Uncertainties in identifying buried soils often arise from:
- The presence of organic rich anthopogenic deposits that can be mistaken for buried soils – particularly if these deposits are themselves formed from soil materials.
- The presence of inwashed organic layers in alluvial/estuarine deposits - post-depositional gleying can further complicate interpretation in these situations.
- Truncation of buried soils and loss of the organic rich topsoil.
- Incipient soil formation in ‘unstable’ landscapes (see standstill phases).
- Soils formed in archaeological deposits where horizonation is imprinted over complex stratigraphies with inherently different soil/sediment properties.
- Imprinting of post-burial features from gleying, iron panning, clay translocation etc. over the pre-existing soil profile.
- Collapse of soil structure due to compression and/or waterlogging, decomposition of organic matter etc.
- ’Welding’ of buried soil profile within later soil profiles where accumulation has been slow or the depth of burial has been insufficient to isolate the buried soil from the effects of soil processes operating at the new unsealed land surface.
[edit] How does the SASSA Field Tool make this interpretation?
A printable recording sheet for SASSA's 'Is this a buried soil?' interpretation tool can be found here
A high score tends to confirm that this context / deposit is part of a buried soil. To score highly you need:
- An organic or organo-mineral horizon relating to the topsoil. This question accounts for 20% of the maximum total score.
- The organic or organo-mineral topsoil would have a granular structure. In buried soils compression can overprint blocky or other structures, if you gently tap these larger structures they may fall open to reveal the former granular structure. This question accounts for 30% of the maximum total score.
- The underlying deposit / context will contain evidence of soil forming processes (soil features) including pans, concentrations, or coatings of clay or organic matter on pore walls and aggregate surfaces. This question accounts for 20% of the maximum total score.
The lower context boundary should be examined very carefully and you should find that:
- The soil colour changes over the boundary. This question accounts for 3% of the maximum total score.
- The soil composition should become less organic. This question accounts for 6% of the maximum total score.
- The soil structure changes over the boundary, from granular to blocky. This question accounts for 6% of the maximum total score.
- The frequency of roots and root traces decreases. This question accounts for 6% of the maximum total score.
- The frequency of stones increases. This question accounts for 6% of the maximum total score.
- The presence and / or frequency of soil features changes over the boundary. i.e. clay and organic coatings are found only in the underlying deposit and not in this. This question accounts for 3% of the maximum total score.
Truncated buried soils where the former topsoiil has been lost are often very difficult to identify, hence if you have a mineral soil the maximum total score will be only 85.
Key to identifying buried soils is the sequence of soil 'horizons', because of the potential diversity of processes and soils SASSA cannot attempt this. However using some of the other interpreation tools related to specific soil processes (podzolisation, calcification, clay and organic matter translocation) may add to your interpretation. Gleying (waterlogging) is a less good indicator as this process is also common in the burial environment.
[edit] Follow-on analysis
The certainty of buried soil identification can be increased in the field using:
Post-excavation laboratory analysis may also help to confirm the presence of a buried soil and add to the archaeological and palaeoenvironmental interpretation.
- Magnetic susceptibility,
- Organic matter content,
- Phosphate analysis,
- Micromorphology
- Pedogenic Fe and Al
