Soil Structure

There are a number of definitions and classifications for soil structure. In general terms soil structure describes how the soil particles and aggregates are arranged.

A soil that has a good soil structure will contain adequate pore spaces for root growth and will have aggregates that are of a size and shape that promote a relatively persistent state of the pores and aggregates in the soil.

A poorly structured soil is one that has minimal pore spaces, with soil particles and small aggregates filling the spaces between larger aggregates. A compacted sportspitch is a good example of a structureless soil.

Within a soil profile there are several types of soil ped (individual aggregates) that are recognised. These are as follows:

Soil ped name	Sub-division	Description	Diagrammatic representation of ped
Blocky	Angular	Vertical and Horizontal cracking are fairly evenly balanced. The edges of the aggregates are relatively sharp. When the soil is wet, the aggregates swell and fit together like a jigsaw.
	Sub-angular	Vertical and Horizontal cracking are fairly evenly balanced. The edges of the aggregates are more rounded. When the soil is wet, the aggregates swell and fit together like a jigsaw.
Laminar (or Platy)	Not applicable	Cracking of the aggregates is most significant along the Horizontal plane.
Prism - like	Columnar	Cracking of the aggregates is significantly along the Vertical. The tops of the aggregates have a generally rounded feature.
	Prismatic	Cracking of the aggregates is significantly along the Vertical. The tops of the aggregates are more angular and sharp edged.
Spheriodal	Crumb	These aggregates are more rounded than Blocky aggregates. Noticeable pore spaces remain between the aggregates when the soil swells. There is good porosity within the soil profile.	s
	Granular	These peds are similar to the crumb ped, however, they have fine particles in between the aggregates, resulting in low porosity.

The soil peds are typically made up of smaller aggregates (also called structural units). Four main structural units are recognised.

Structural Units