Field Analysis:Why Pores

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Why record porosity and roots?
The porosity of the soil is affected by the compaction, soil texture, soil structure, plant roots and soil fauna. The porosity influences the bulk density of the soil and its drainage characteristics. Soil with low porosity, or with closed non-continuous pores, such as rounded vesicles, will be slow to drain and hence may be prone to waterlogging. Whilst, very porous soils will drain rapidly and as long as they above the ground water table will rarely experience waterlogging. Loss of porosity is one of the effects of compaction following burial, which can exacerbate burial gleying (waterlogging).

The presence of channel pores indicate root or earthworm activity and can give an indication of bioturbation, particularly if former channels are filled with soil materials from adjoining contexts. This indicates mixing of deposits and potential loss of stratigraphy. Vesicular pores are formed by gas bubbles in the sol and can indicate burning.

Roots
Rooting patterns are important for identifying buried soils, vegetated surface in deposition standstill phases, and understanding post-burial modification of deposits. In aerobic sites, roots from previously buried land surfaces may have decomposed, but often their trace can be seen on the surfaces of blocky soil aggregates, or in larger channels sometimes ringed by iron concretions. Rooting density is highest immediately below the soil surface, where fine and medium roots of grasses, herbs and shrubs are dominant. It is this zone that denotes buried soil surface, Interpretation of vertical tap roots and in some cases coarser tree roots should be undertaken with care as these can penetrate to very great depths. For example, the roots of some Retama species can penetrate to depths of more than 25 m.

Soil porosity and rooting frequency can be a useful addition to the context record when the identification of buried soils and potential standstill phases is an issue, to assess the loss of stratigraphy due to bioturbation, and to identify the effects of burning and compaction in the soil.

More information on recording soil porosity and roots can be found here.

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