Field Analysis:Why Structure

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Why record soil structure and consistence?
Structural development is part of the soil forming process. Soil structure refers to the way in which individual particles clump together into aggregates known as peds. Clay particles, organic matter, plant roots, soil fauna and fungi are necessary to the formation of ped structures, whilst compression can also modify soil structure. Some soils - including sediments where there has been very little soil formation, very sandy and very clayey soils – may be structureless (apedal). Other types of soil structure are granular, blocky, platy, prismatic and columnar. Strong structural development gives the soil some level of resilience to compression and erosion.

In a typical soil profile the topsoil has a granular structure that develops as a result of earthworm activity – the individual peds are formed from worm excrement. The B and C horizons will have a blocky or prismatic structure that with increasing depth become massive (structureless). Platy (plate-like) structures may be found below the A horizon in compacted and some cultivated soils (plough pan). This sequence of horizon structures is useful for:
 * Identifying buried soils.
 * Assessing the degree of soil formation, e.g. as an indication of stability in standstill phases.
 * Identifying truncated profiles.
 * Assessing the degree of bioturbation (granular structure) and compaction.
 * Identifying former cultivation.

In buried soils, compression and waterlogging can result in the collapse of structure or overprinting of granular structures by blocky or prismatic structures. Careful examination of the deposit may be needed to identify former ped structures.

Soil consistency
Ped strength and cementation affect the consistence of soil – how easily the soil falls apart. Firm and friable can be used to describe the consistence of moist soil, but recording ped strength and cementation separately gives more information about the cause of soil strength. This information can be used to assess the likely response of a soil to burial, and to identify soil processes such as calcification and freeze-thaw.

A record of soil structure should be made if identification of former soil development, cultivation, profile completeness (truncation) and degree of bioturbation are of interest.

More information on how to record soil structure can be found here.

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