Soil Structure
Soil structure refers to the arrangement of soil particles separated into units called soil aggregates.
An aggregate is made of solids and pore spaces.
The arrangement of soil aggregates into different forms gives a soil its structure.
The natural processes that aid in forming aggregates are:
1) wetting and drying,
2) freezing and thawing,
3) microbial activity that aids in the decay of organic matter,
4) activity of roots and soil animals, and
5) adsorbed cations.
The wetting/drying and freezing/thawing action as well as root or animal activity push particles back and forth to form aggregates.
Decaying plant residues and microbial byproducts coat soil particles and bind particles into aggregates.
Aggregates are described by their shape, size and stability.
Balance of large and small pores provides for good soil aeration, permeability and water-holding capacity.
Tillage, falling raindrops and compaction are primarily responsible for destroying aggregates
SOIL HORIZONS
Structure is one of the defining characteristics of a soil horizon.
A soil exhibits only one structure per soil horizon, but different horizons within a soil may exhibit different structures.
All of the soil-forming factors, especially climate, influence the type of structure that develops at each depth.
Granular and crumb structure are usually located at the soil surface in the A horizon.
The subsoil, predominantly the B horizon, has subangular blocky, blocky, columnar or prismatic structure.
Platy structure can be found in the surface or subsoil while single grain and structureless structure are most often associated with the C horizon.
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