Particle Arrangement

© 2007 Donald G. McGahan (aka soilman) All Rights Reserved

Distribution of size separates, often referred to as particle size, goes a long way to understanding many attributes possessed by soil. However, because the soil separates are rarely unattached and independent from each other, understanding the arrangement of a relatively homogenous mix of soil separates into larger stable units is necessary. The units form because cohesion is greater within the units than between the units resulting in ruptures at the weaker planes under stress. Collectively the relatively homogenous mix of soil separates into larger stable units is referred to as structure or when referring to an individual structural unit the term is an aggregate or ped.

In soil, the arrangement of primary particles into secondary units or peds. The units are characterized and classified on the basis of size, shape, and degree of distinctness. In crystals, the arrangement of atoms or molecules. In molecules, the arrangement and bonding of atoms. ()
A natural unit of soil structure. Another term for aggregate. ()

Some soils are, or have zones, that are structureless. The two types of structureless conditions are massive and single grained. Massive conditions are when the soil has no observable units in place, but there is cohesion amongst the particles. Single grain is a condition where insufficient cohesion exists between the size separates such that when a stress is applied the resulting rupture results in greater than half (50 percent of the particles in the volume of soil be assessed) of the size separates behave as individuals.

Soil structure

Principally we recognize first and foremost that the aggregation of soil separates into peds changes the size and distribution of the voids in the soil. This change in solid to void distribution has an impact on aeration, water movement, and soil water storage. Additionally, the stability of the aggregation impacts how resistant the soil is to air and water erosion. Structure also impacts nutrient delivery to the biological’s in the soil due to the redistribution of the voids in the soils. The void redistribution also influence the carbon cycling in soils allowing more gas exchange with the above soil atmosphere promoting increased rates of decomposition of the organic matter in more oxygenated zones of the soil.

Structure has an on influence:

  • water movement
  • heat transfer
  • aeration
  • porosity
  • erosion

Previously it was stated that it is difficult to change the mix, or distribution of, separate primary particles, but soil structure can be more readily changed by management practices.

Describing aggregation in the field

The structure, or absence thereof, can be described in the field as part of a soil morphology description. The structural units, or peds, are placed into classes by type, size, and strength. The shape of the ped is its type. A change of the type, size, and or strength of the structural units with soil depth is one way that soil may be separated into different horizons.

Aggregates Terms

Primary particles: Individual mineral particles; sand, silt, and clay are primary soil particles.

A structure whose units (aggregates) are nearly equidimensional, square, or rectangular and the corners are not rounded. ()
A soil structure type whose vertical and horizontal axes are nearly equal. See angular blocky and subangular blocky. ()
An aggregate shape that is longer than it is wide, with a rounded top and edges. ()
Spherically shaped; used to describe soil aggregates, fertilizer particles, and the like. ()
A soil structure type whose horizontal axis is much longer than its vertical axis. ()
A soil structure type whose vertical axis is much longer than its horizontal axis and that has angular edges, compared with a columnar structure, which has a similar shape but curved edges. ()
Subangular blocky
A structure type whose aggregates are nearly equidimensional, square, or block like with rounded edges. ()

Examples of soil structure types.

Examples of soil structure types
Figure –– Examples of soil structure types.

While no hard and fast rule can be applied to exclude a particular structure type from a particular morphological horizon master designation there are some trends.

Type of Structure (or name used to describe) Horizon of greatest occurrence
Structured soils
a. Granular A
b. Crumb (more porous than granular) A
c. Platy (wider than tall) E
d. Prismatic (taller than wide - without rounded tops) B
e. Columnar (taller than wide - with rounded tops - reduced pigmentation on tops) B
f. Blocky (roughly equivalent dimensions) B
g. Subangular blocky (less angular edges) B
h. Wedge (Approximately elliptical, interlocking lenses that terminate in sharp angles) B
Structureless soils (soils having no noticeable aggregates)
a. Single grained (beach sands; all individual particles) C
b. Massive (all particles stuck together but no aggregates) C

Aggregate Size

Table –– The size of the structure for each type also ranges.
Size class Diameter of granules Thickness of plates Diameter of blocks Diameter of prisms and columns
Very fine < 1 < 1 < 5 < 10
Fine 1–2 1–2 5–10 10–20
Medium 2–5 2–5 10–20 20–50
Coarse 5–10 5–10 20–50 50–100
Very course > 10 > 10 > 50 > 100
Sizes of structural types in millimeters.

Aggregate Grade

Aggregate grade is the structure strength.

This is sometimes termed structure Grade.

Structure grade
A grouping of soil structure based on the cohesion or stability of the structure. Grades are structureless, weak, moderate, and strong. ()
Structure strength
A term sometimes used in place of structure grade.
  1. how easy it is to see the structure in the pedon
  2. how many of the primary particles are aggregated
  3. how strongly the primary particles are held together in the aggregates

There are three structure grades: weak, moderate, and strong.

Each horizons structure is recorded separately and this is done in the field. Often structure differentiates one horizon from another.

Weak structure
is difficult to see in a horizon and when poked with a knife the particles that fall out are mostly not aggregated, and if aggregates do remain when they fall out, they fall apart with little handling.
Moderate structure
can be seen in the horizon and most to of the material removed from the horizon is aggregated.
Strong structure
an easily be seen in the pedon and most of the primary particles are aggregated. The aggregates do not readily fall apart when handled.

Mode of Formation

The soil separates (primary particles) are associated together, aggregated, by soil-stabilizing agents such as clay, humus, and iron oxides.

Some physical processes that contribute to formation of soil aggregates are repeated compression and contraction of soil particles which may be related to shrink/swell of clays, freeze-thaw cycles, root penetration, and soil animals (earth worm activity), etc.

Small aggregates may have a greater proportion of binding from electrostatic interactions among clay minerals and organic molecules.

Binding agents holding soil grains together: Destruction of aggregates:
Organic matter Sodium tends to disperse clays
Root exudates - polysaccharides lost from root Tillage of soils
Iron and aluminum oxides crushing and compaction
Clay minerals loss of organic matter
Carbonates Increased moisture and prolonged and repeated leaching away of the carbonates.