Structure refers to the arrangement of soil particles. Soil structure is the product of processes that aggregate, cement, compact or unconsolidate soil material. In essence, soil structure is a physical condition that is distinct from that of the initial material from which it formed, and can be related to processes of soil formation. The peds are separated from the adjoining peds by surfaces of weakness. To describe structure in a soil profile it is best to examine the profile standing some meters apart to recognize larger structural units (e.g. prisms). The next step is to study the structure by removing soil material for more detailed inspection. It should be stressed that soil moisture affects the expression of soil structure. The classification of soil structure considers the grade, form, and size of particles.

The grade describes the distinctiveness of the peds (differential between cohesion within peds and adhesion between peds). It relates to the degree of aggregation or the develoment of soil structure. In the field a classification of grade is based on a finger test (durability of peds) or a crushing of a soil sample.

The form is classified on the basis of the shape of peds, such as spheroidal, platy, blocky, or prismatic. A granular or crumb structure is often found in A horizons, a platy structure in E horizons, and a blocky, prismatic or columnar structure in Bt horizons. Massive or single-grain structure occurs in very young soils, which are in an initial stage of soil development. Another example where massive or single-grain structure can be identified is on reconstruction sites. There may two or more structural arrangements occur in a given profile. This may be in the form of progressive change in size/type of structural units with depth (e.g. A horizons that exhibit a progressive increase in size of granular peds that grade into subangular blocks with increasing depth) or occurrence of larger structural entities (e.g. prisms) that are internally composed of smaller structural units (e.g. blocky peds). I such a case all discernible structures should be recorded (i.e. more rather than less detail).

The size of the particles have to be recorded as well, which is dependent on the form of the peds.

Table 9.6.1.1. Classification of soil structure considering grade, size, and form of particles.

 

Size	Angular and subangular blocky structure [mm] diameter	Granular and crumb structure [mm] diameter	Platy structure [mm] width	Prismatic and columnar structure [mm] diameter
Very fine	< 5	< 1	< 1 (very thin)	< 10
Fine	5 - 10	1 - 2	1 - 2 (thin)	10 - 20
Medium	10 - 20	2 - 5	2 - 5	20 - 50
Coarse	20 - 50	5 - 10	5 - 10 (thick)	50 - 100
Very coarse	> 50	> 10	> 10 (very thick)	> 100

Figure 9.6.1.1. Soil structures (Foth, 1984)

The three characteristics of soil structure are conventionally written in the order grade, size, and shape. For example, weak fine subangular blocky structure.  

 The distribution of different particle sizes in a soil influence the distribution of pores, which can be characterized by their abundance, size, and shape.

Table 9.6.1.2. Abundance, size, and shape of pores.

Significance of Soil Structure

Soil formation starts with a structureless condition, i.e., the structure is single-grained or massive. Soil development also means development of soil structure, which describes the formation of peds and aggregates. Soil structure forms due to the action of forces that push soil particles together. Subsurface structure tends to be composed of larger structural units than the surface structure. Subsoil structure also tend to have the binding agents on ped surfaces rather than mixed throughout the ped.

Climatically-driven physical processes that result in changes in the amount, distribution and phase (solid, liquid, vapor) of water exert a major influence on formation of soil structure. Phase changes (shrinking-swelling, freezing-thawing) result in volume changes in the soil, which over time produces distinct aggregations of soil materials.

Physico-chemical processes (e.g., freeze-thaw, wet-dry, clay translocation, formation/removal of pedogenic weathering products) influence soil structure formation through out the profile. However, the nature and intensity of these processes varies with depth below the ground surface. The structure and hydrological function of plant communities, texture, mineralogy, surface manipulation and topography all serve to modify local climatic effects through their influence on infiltration, storage and evapotranspiration of water.

Biological processes exert a particularly strong influence on formation of structure in surface horizons. The incorporation of soil organic matter is usually largest in surface horizons. Soil organic matter serves as an agent for building soil aggregates, particularly the polysaccharides appear to be responsible for the formation of peds. Plant roots exert compactive stresses on surrounding soil material, which promotes structure formation. Soil-dwelling animals (e.g., earth worms, gophers) also exert compactive forces, and in some cases (e.g., earth worms) further contribute to structure formation via ingestion/excretion of soil material that includes incorporated organic secretions.

Information provided by: http://www.soils.wisc.edu