Soil structure

Amber Anderson

Learning Objectives
  • Identify soil structures and factors influencing their development
  • Predict what structure might be present given additional information such as soil conditions or horizon
  • Explain how structure may impact plant growth or other soil functions
  • Predict how management factors might impact structure
Keywords: Structure, aggregation, granular, platy, blocky, prismatic, columnar, massive

 

Structure

Two soil profiles sitting next to each other, taken from the same field/soil but two treatments. Pasture shows a much finer aggregation, more roots, and darker color while the field soil shows larger and less consistently sized aggregates, with lighter colors mixed into upper 20 cm.
Even in the same soil or area, management practices can influence structure. Photo Credit: Amber Anderson. Click to enlarge.

Soil structure is the shape in which soil particles group together and form aggregates. A soil aggregate, or conglomerate of sand, silt, clay, and sometimes organic material, may be a variety of different shapes.

Structure is important because it allows critical areas of open space, vital for water to move, roots to grow, and soil organisms. Consider a classroom or the space in which you are currently viewing this: when the materials are put together effectively, it allows space for interactions. If only a pile of building materials, the space doesn’t serve the same function. 

Factors influencing aggregation

A variety of factors influence how soil particles aggregate or group together. Biological activity, organic addition, wetting/drying cycles, freezing/thawing cycles would be expected to increase aggregation, whereas tillage, compaction, and chemical properties such as sodium would decrease aggregation.

 

Shapes

Granular

These rounded groups of particles don’t pack together well, allowing more space for water to move through. They are most commonly found in A horizons with higher levels of organic matter, healthy root growth, without significant compaction.

An A horizon sample with granular structure in the palms of someone's hands.
This forest A horizon has primarily granular structure, with a few small blocks. Photo credit: Amber Anderson. Click to enlarge

Platy

Commonly found in E horizons, the natural breaks in this soil are horizontal rather than vertical. These are easily destroyed by tillage. Note that this is different from ‘plates’ formed by operating equipment when a soil is wet. Although they look somewhat similar, this structure is naturally formed over time.

This soil, found just north of Ames in a more forested area, has some visible platy structure in the upper part of the soil. Cracks in the soil run horizontally rather than vertically.
Although fragile, platy structure can be seen here, especially around 7-8 cm as the lines in the soil run horizontally rather than vertically. Photo Credit: Amber Anderson. Click to enlarge

Blocky

Blocky structural units are common to find in a B horizon or cultivated A horizon. They can be grouped by either angular blocky having sharp angles likely found in higher-clay soils, or subangular blocky, the more rounded corners.

Two soil profiles sitting next to each other, taken from the same field/soil but two treatments. Pasture shows a much finer aggregation, more roots, and darker color while the field soil shows larger and less consistently sized aggregates, with lighter colors mixed into upper 20 cm.
The impact of tillage can be seen here, with more blocky structure found in the right, cultivated core, while more granular structure is found on the left core, taken from a permanent pasture. Photo credit Amber Anderson. Click to enlarge

Prismatic

These have longer natural breaks vertically in the soil rather than horizontally. As was the case with this large prism in the photo, they are generally found in B horizons. Water and roots in this soil will likely move preferentially through the breaks between these units.

Amber holding a large prism in NW Minnesota, approximately large soil knife in length, and the width is about half of the height.
A very large prismatic structural unit found in NW Minnesota. Photo credit: Amber Anderson. Click to enlarge.

Columnar

Columnar are a special type of structure created when sodium impacts a prismatic structure. A ‘muffin top’ or ‘popcorn’ looking appearance on the top of a prism develops from sodium dispersing particles. These are agronomically challenging soils to manage. Both water and roots will likely have problems moving through this soil easily.

A part of a soil profile that shows sodium accumulation due to a lack of excess precipitation meaning that soil structure is negatively impacted, resulting in a columnar structure. This soil feels very hard around 25 cm due to sodium interacting with soil particles.
Columnar structure on a sodium-impacted soil in South Dakota. Significant sodium accumulation above the orange nail, around 25 cm of depth. Photo Credit: Amber Anderson. Click to enlarge

Massive or Single grained

A soil face that depicts a massive structure indicating it is structureless.
This soil has been recently deposited, and has not had time for structure to develop, so would be classified as massive. Photo Credit: Amber Anderson. Click to enlarge

These units of ‘non structure’ indicates there has been limited changes to this soil since deposition. In glacial till materials, a large piece will likely break between the points of pressure applied, rather than falling apart on pre-determined lines. A midwestern soil at perhaps five feet of depth may not have developed structure because this takes something acting on it. Roots, freeze-thaw, wetting-drying and other factors are less active here, slowing down changes.

For single-grained soils, a lack of fine particles or organic matter means that there are not significant forces to hold sand grains together. This is an effect you may have seen in a sandbox or beach, as a small disturbance will cause the sand to fall apart to individual grains.

 

Management Impacts

Since plant growth tends to increase soil structure, more plant growth tends to lead to a better structure. In prairie ecosystems, a strong granular structure is expected.

Tillage can have negative impacts on soil structure, particularly when done in poor (generally too wet) conditions. Consider the building construction of the earlier example in this discussion. When being built from that pile of building materials, a wall is removed or damaged, so this must be rebuilt first, slowing down progress.

 

Key Takeaways
  • Structure is important as it indicates the arrangement of soil particles
  • Soil structure can change over time due to changing conditions or disturbance
  • A variety of shapes exist, these tell you where water and roots are likely to move along those natural breaks
  • Management has an impact on soil structure

 

License

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Introduction to Soil Science Copyright © 2023 by Amber Anderson is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.