Field Analysis:How Structure

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Soil structure and consistence
Structure refers to the shape and nature of the aggregates formed when soil particles clump together.

Record the type of structure, it’s development, and strength. The basic structural units are ‘peds’, which are natural aggregates. The best way to observe peds is either in a moderately dry section where clays pull apart revealing the soil peds, or by taking a block of soil and tapping it gently with the trowel to see the shape of the aggregates as it falls apart.

Structure type
The images below describe the main types of soil structure.
 * Granular structures are small (usually no more than a centimeter across) crumbs of soil. If a soil contains a lot of coarse inclusions it may appear granular as the soil particles coat the coarse inclusions.
 * Blocky structures tend to be about 1 and 5 cm across and its sides are roughly equal in size. They are often pictured as being cubes of soil but in practice tend to be more irregularly shaped.
 * Platy structures are thin (usually less than a centimeter thick) plate like aggregates that have their longest axis in a horizontal direction.
 * Columnar and prismatic peds can be 10 or more centimeters across and may be considerably longer vertically. They tend to have well defined angular faces.

Structureless soils show no observable aggregation. Structureless soils can be divided into:
 * Single grain – usually associated with sandy materials where the soil separates into individual particles.
 * Massive – usually associated with materials containing clay where the soil particles aggregate together as one coherent whole and don't break down into smaller units.

Structural development
Structural development refers to how well developed the individual peds (soil aggregates) are.
 * Weakly developed peds barely observable in section. When disturbed, the soil breaks into a few entire peds as well as broken peds and/or unaggregated material.
 * Moderately developed Evident but not distinct in undisturbed soil. When disturbed the soil breaks into many entire peds, and a few broken peds and/or unaggregated material.
 * Strongly developed Peds distinct in undisturbed soil, and disturbed soil breaks almost entirely into entire peds.

Strength
The soil strength is the crushing force required to break the soil aggregates apart. In clay or silt soils it is possible that the individual peds are easily compressed but don't fracture and fail. This is known as being deformable.

Strength refers to individual peds, or if the soil is structureless to a 3 cm cube of soil.

Cementation
Cementation is caused by substances other than clay, for example iron and aluminium oxides, or calcium carbonate.

To test for cementation place an air-dry cube of soil in water for 1 hour. If it is *uncemented* the soil will fall apart (slake).

If the cube holds together remove the cube from the soil and try crushing it.
 * Very weakly cemented Can be crushed between thumb and forefinger.
 * Weakly cemented Can be crushed under foot on a hard surface.
 * Strongly cemented Resists being crushed under foot.

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