Soil Structure and What it Tells Us

1. Soil Structure and What it Tells Us Soils Training

2. Grade of Soil Structure Grade describes the distinctness of units. Structureless (0): Structural units are absent. Structureless soil material may be either single grain or massive. Single grain: (sg): limited to sands and loamy sand with no adhesion Massive (m): particles adhere together Weak (1): Structural units are barely observable in place. When gently rubbed, the soil material parts into a mixture of whole and broken units. Most material exhibits no structural surfaces. Moderate (2): Structural units are well formed and evident in undisturbed soil. When rubbed, the soil material parts into a mixture of many whole units, some broken units, and material that is not in units. Aggregate surfaces have properties distinct from those of fractured surfaces. Strong (3): Structural units are distinct in undisturbed soil. They separate cleanly when the soil is rubbed. When removed, the soil material separates mainly into whole units. Aggregate surfaces have properties distinct from those of fractured surfaces.

3. Granular Shape The units are approximately spherical or polyhedral and are bounded by curved or very irregular faces that are not casts of adjoining peds. Found only in A horizons. Formed by organic carbon, some clay, and organisms.

4. Platy Shape The units are flat and platelike. They are oriented horizontally. Found mainly in E and B horizons, unless the surface is compacted. Formed by freeze expansion or compaction (vertical pressure), or remain from bedding planes.

5. Virtual Prisms in a Fragipan

6. Prismatic Shape The individual units are bounded by flat to rounded vertical faces. Units are distinctly longer vertically, and the faces are casts or molds of adjoining units. Form by wetting and drying of adjacent aggregates, and always contain some substructure. There are physical prisms that can be extracted from the soil intact, and those in some fragipans that can be seen but not extracted.

7. Blocky Shape The units are blocklike or polyhedral. They are bounded by flat or slightly rounded surfaces that are casts of the faces of surrounding peds. The structure is described as angular blocky if the faces intersect at relatively sharp angles; as subangular blocky if the faces are a mixture of rounded and plane faces and the corners are mostly rounded. Form by wetting and drying of adjacent aggregates.

8. Slickensides

9. Size names and symbols for structure classes Platy structure symbol Other shapes Very thin 1 Very fine Thin 2 Fine Medium 3 Medium Thick 4 Coarse Very thick 5 Very coarse

10. Significance of Soil Structure • Water moves rapidly into open macropores (cracks, channels, and pores). Shrink-swell soils may form slickensides. They swell shut when wetting. • Water moves more slowly into micropores (pores between soil particles). As clay increases, so does the total porosity, but also the adhesion because pore size decreases. Soils that have a mixture of particle sizes and little porosity have restricted water movement.

11. Significance of Soil Structure • Strong, very fine or fine angular blocky structure with a non-expansive clay type promotes water movement because of the very high amount of vertical macropores. Thus as nonexpansive clay content in a soil increases, the smaller the aggregates, the better for water movement. Nonexpansive clays include Kaolinite and oxides of Al and Fe.

12. Important properties to observe with a hand lens • Connectivity of pores (vesicular or connected) • Openness of connected pores (look for clay plugging) • The number of horizontal planes (macropores) • The contrast in particle size between horizontal planes or soil horizons (clay increase, sand size and uniformity in sandy textures)

13. Important Properties to Observe • Look for vertical ribbons as a sign of restricted water flow through macro and micropores. • Look for dense horizons (hard to dig when slightly dry, even though the clay content is not high). • Look for pockets or vertical zones of stripped soil (E material) that indicates preferential water movement. • Look for horizons that have both stripping and accumulation. • Look for a frosting of E material above and beside prismatic structure.