The impact of soil structure on system installation

July 19, 2021

By Sara Heger, Ph.D.

While soil texture describes the percentage of each size class (sand, silt and clay), soil structure is the organization of these individual soil particles into a larger arrangement. The size and orientation of the aligned particles means that structure affects acceptance (and thus water treatment) in a similar way as texture.

Structure is described in three parts: grade, size and type. Grade refers to the expression of the structure; size refers to the size of the individual structural units (peds); type refers to the shape of the ped. All these parts of the description can be related to both water movement and treatment. 

Soil structure develops over time (many hundreds to thousands of years) through physical and chemical weathering. Examples of forces forming soil structure include freeze/thaw cycles, wet/dry cycles, plant rooting, earth-inhabiting invertebrate activity, etc. These forces are concentrated in the upper portion of the soil (within 3 to 5 feet of the soil surface) leaving virtually no soil structure at greater depths. If enough force is used, any body of soil material can be broken into smaller pieces. 

There are four grade levels of structure. Grade 3 is strong or well expressed; grade 2 is moderately expressed; grade 1 is weakly expressed; grade 0 is structureless. A well-structured soil (grade 2 or 3) accepts effluent more quickly because of stable well-expressed voids or macroporosity. However, excessively large voids may allow effluent to flow too quickly without proper treatment. Often this is referred to as preferential flow and may also occur through worm and root channels. A weakly structured soil has few well-defined structural pores, resulting in flow being controlled more by the soil texture than by soil structure. A structureless soil (grade 0) has no structural porosity so all water flow is controlled by texture. Loose sand is considered to have a grade 0 and is further defined as single grained. Because the inherent porosity of the sand is large, water will flow though it readily. 

On the other hand, if the soil is described as massive (coherent), water will flow through it more slowly. This grade is often associated with unstructured clays or more compacted soils. Grade influences the flow and the exchange of air. Well-structured soils promote greater air exchange. The greater air exchange results in more aerobic soil conditions and improved treatment. 

Structure is broken into sizes, ranging from very fine to very coarse. Soils with a finer structure have a greater amount of macroporosity. Soils with more macroporosity tend to conduct water more rapidly as well as allow for more air exchange. Finer soil structure may result in better air exchange and improved wastewater treatment.

Finally, structure is described in terms of its shape or type. Granular structure, common at the soil surface, looks like granola or Grape-Nuts. It may act more like a sand texture and allows for rapid water movement. Blocky structure is composed of peds that are roughly the same size. When these are rounded (subangular), they do not fit well together. Thus, voids (pores) tend to be more conductive than if they are angular. Prismatic structure has most pores oriented vertically. While this may be good for water movement, it may also promote preferential flow. Platy structure has horizontally oriented macropores which limit downward water movement. As the soil goes from granular to platy and then to massive, the structural porosity decreases, and the infiltration rate or ability to move water decreases.

Large pores develop between soil structural units. These pores allow a soil to accept and transmit water more efficiently than soils without soil structure. Understanding soil structure is key to the proper sizing of a soil treatment system.

Soil structure can be destroyed due to excavation and compaction during installation. Once the structure is destroyed, it takes decades for it to re-form in the soil. There is little that can be done to artificially recreate good structure. Therefore, a careless installation can turn a good site into an unusable one simply by destroying soil structure.