Understanding Structure

By Jim Anderson, Ph.D., and David Gustafson, P.E.,

This story was published in Onsite Installer magazine and is used with permission from COLE Publishing.

The way soil particles aggregate into clusters has a great deal to do with how well the soil transmits water and how well it accepts wastewater

Soil structure is an important soil property because many jurisdictions use it as one of the soil attributes that determine the soil sizing factor for onsite systems. We’re including an example of a soil sizing table based on a relationship between structure and texture.

Soil structure refers to the aggregation of the soil separates (sand, silt and clay particles) into clusters, called peds. Structure describes how the soil is glued together.

The peds are separated by surfaces of weakness. In some soil horizons, simple structures occur where each ped is a single entity without small peds inside. In many horizons, one or more sets of peds are held together to form discrete bodies recognized as larger peds.

Knowing the units

Since structure can significantly alter water transmission in soils, it is important to recognize the different units and to have a more detailed description of structure. Field terminology for soil structure consists of separate sets of terms designating each of four properties. The four terms are combined in this order:

  • Grade
  • Size
  • Shape
  • Consistence

Grade describes the distinctness of the peds. It is a field determination of the ease with which the soil separates into discrete peds. Grade is designated as weak, moderate or strong.

Size is split into five size classes: very fine, fine, medium, coarse and very coarse.

Shape is described by four distinct types: granular, platy, blocky, and prismatic (or columnar). There is also a single-grain designation, which is the structure found in sandy soils where the particles are not held together.

Consistence refers to the resistance of the structure to deformation or rupture. Consistence depends heavily on the soil-water state. Therefore, moist samples should be used to assess this characteristic. To determine consistence, place a one-inch block-like sample between the thumb and forefinger and apply stress for one second. Moist consistence classes are:

  • Loose: No intact sample can be obtained.
  • Friable: Structure breaks down with slight force between the fingers.
  • Firm: Structure breaks down with moderate force between the fingers.
  • Extremely firm: Structure breaks down with moderate force between the hands or slight foot pressure.
  • Rigid: Structure breaks down only with foot pressure.

Between the peds

Between soil peds are voids, or pores. Pores between peds are often relatively large and continuous when compared with the spaces between individual sand, silt and clay particles. The type of structure determines the dominant direction of the pores and hence determines water movement in the soil.

Soils with strong structure have distinct pores between peds. Well-structured soils with large voids between peds will transmit water more rapidly than structureless soils of the same texture, especially if the soil has become dry before water is added. Small structural units create more pores in the soil than large structural units, but fine-textured, massive soils (soils with little structure) have very slow percolation rates.

You can expect faster percolation where soil structure is well developed. Strong structure can compensate for high clay content and produce suitable permeability. It also resists mechanical damage when an onsite treatment system is installed.

Small peds and single-grained structures have rapid percolation rates. Soils with granular, blocky, prismatic or columnar structures enhance flow both horizontally and vertically. Platy structures restrict downward movement of water because the ped faces are oriented horizontally, against the flow. Platy structures are often associated with lateral movement of water.

Easy to change

Structure is one soil characteristic that is easily altered or destroyed. When you avoid compaction by working when the soil is dry and working to keep the soil in its natural condition, you maintain the soil’s capacity to accept sewage effluent.

Structure is very dynamic and changes in response to moisture content, chemical composition of the soil solution, biological activity, and management practices. Soils that contain clay minerals that shrink and swell can show dramatic changes in permeability.

When soil peds swell, the pores become smaller, and water movement is reduced. This is why where perc rates are still used for sizing systems, there is a required time to wait to run the test so that the soil can swell and imitate the moisture conditions that will exist when sewage effluent is added to the soil.

We hope that now you see that by recognizing soil structure, you can anticipate how the soil will perform in accepting sewage effluent. To learn more about structure, texture and color, take a soils class. A good course will give you some hands-on experience identifying these properties. The time you spend pays dividends to your business down the road.