Updated: Jul 23, 2021
Soil structure is defined by the way individual particles of sand, silt, and clay are assembled. Single particles may be assembled together, appearing as a larger particle. The assembled groups of particles are called aggregates. The aggregation of soil particles is what constitutes soil structure. Aggregation can occur in different patterns, which will result in different soil structures.
Soil structure is described in terms of:
Grade (degree of aggregation)
Class (average size of soil particles)
Types of aggregates (form of particles). Occasionally different types of aggregates will be found together and can be separately described.
Image: USDA Soil Texture Compass, used to define the overall composition of soil by its constituent parts. Source.
The physical structure of a soil will be easiest to recognize when the soil is mostly dry and only slightly moist. To properly examine a soil profile, a fresh soil profile must be used to accurately examine the structure.
Soil texture, on the other hand, is a measure of the proportion of soil particles, such as the amount of sand relative to the amount of clay, as well as the fineness or coarseness of a soil. Soil structure is directly related to the arrangement of soil particles into aggregates of varying sizes. Soil texture will affect plant rooting, soil structure, and organic matter content. Soil texture and structure will determine the pore-size distribution, soil water holding capacity, and the amount of water to air-filled pore space that will be present in soil aggregates, providing habitat for soil organisms.
Grades of Soil Structure
The grade of soil structure directly relates to the level of cohesion within soil aggregates, and between soil aggregates. The level of cohesion will vary depending on soil moisture content. There are four major grades of soil structure, from 0-3
No observable aggregation or no orderly arrangement
Massive or Single Grain Structure
Massive: entire soil horizon appears to be cemented in one large mass
Single Grain: soil particles show no tendency to cling together, like pure sand on a beach
Poorly formed from indistinct aggregates that are barely observable
When removed from the profile, soil material breaks down into mixture of few aggregates, many broken aggregates and loose soil material
Well formed from distinct aggregates that are moderately durable and evident but not distinct in undisturbed soil
When removed from profile, soil material breaks down into many distinct and broken aggregates
Well formed from distinct aggregates that are durable and very evident in undisturbed soil
When removed from the profile, soil material consists primarily of entire aggregates with few broken aggregates and little to no loose soil
Microbes and their Influence on Soil Aggregation
An aggregate is a naturally formed assemblage of sand, silt, clay, organic matter, plant root hairs, microorganisms, the glue-like secretions that microorganisms produce, and the hyphae of fungi (branching structures of certain fungus in the soil). Soil aggregates often have fine plant roots that grow into soil pores, which then pulls soil aggregates into the root zone of plants, or the rhizosphere. More persistent binding agents like organic matter and metals can stabilize smaller microaggregates. Temporary binding agents, like fungal hyphae or extracellular polysaccharides (complex sugars), are produced by soil organisms and aid in the formation of larger macroaggregates. The larger macroaggregates function as small ecosystems, or arenas of activity.
Image: All four quadrants show some examples of how fungal hyphae branch out through soil and “connect” with soil particles, pulling them together to form aggregates. In quadrant D, the fungal hyphae are dyed for the ease of visualization. Source.
Biological processes can improve soil structure. Some bacteria and fungi can produce substances during the decomposition of organic matter that will chemically or physically bind soil particles into microaggregates. As mentioned earlier, hyphal strands of fungi can link between soil particles which helps to form aggregates. One gram of soil will contain over a billion microorganisms, and several kilometres of fungal hyphae. Additionally, soil animals will increase pore space by tunneling through the soil and increasing aggregation by ingesting and metabolizing soil.
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Image: Diagram illustrating how plant roots and fungal hyphae intermingle with soil aggregates and pull them together. This pores in the soil space provide habitat for soil organisms to further improve structure.