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.Source.
Soil microorganisms will play an essential role in decomposing organic matter, cycling nutrients, and fertilizing the soil. The reactions that cycle nutrients are often chemical in nature, but biochemical reactions facilitated by soil microorganisms are of prime importance to this process.
Soil Microbes are important for the development of healthy soil structure. These microorganisms produce a great amount of “gummy” substances like extracellular polysaccharides and mucilages that can cement soil particles together into aggregates. The gummy substances make soil aggregates less likely to break apart when exposed to water. Fungal filaments will also stabilize soil structure as the threadlike structures will branch out through the soil and surround soil particles like a fishing net. Fungi should be thought of as the threads of soil fabric. Please keep in mind that soil microorganisms themselves will have little influence on the physical structure of the soil, as that is mainly influenced by larger soil animals, but the microorganisms are responsible for the biochemical processes that bind soil particles.
Microorganisms are actively going to try to create a habitat that’s best suited for them, which is almost always a soil with strong structure. If their habitat is more conducive to cycling nutrients, they will actively try to recreate the conditions of that habitat in the soil through the production of polysaccharides and the extension of fungal hyphae throughout the soil.
Nurture Growth Bio-Fertilizer and Soil Structure Improvement
Nurture Growth Bio-fertilizer is a fertilizer teeming with millions of active ingredients: beneficial soil microorganisms. Bacteria and Fungi are the primary microorganisms that will bind soil particles and several species of each are included in our proprietary formula. Growers who have used Nurture Growth Bio-fertilizer have seen dramatically increased plant root size, increased quantity of root hairs, and darker, richer soil. The bacteria and fungi in our fertilizer will actively work to improve their habitat in the soil, which also benefits the grower by allowing plant roots to proliferate and for root hairs to bind more soil aggregates.
Sometimes you need to see it to believe it. We invite you to try out a bottle of Nurture Growth Bio-Fertilizer this year and tell us about the soil improvements you see. You can review us online by clicking through the following links:
I would also like to invite you to join our monthly webinars held on every third Wednesday and Thursday of the month.
Click here to view our upcoming events and register for the webinar series.
Eric is a gradate of the Environmental Science program at the University of Toronto. Coming from the green roof and landscaping industries, he does not hesitate to get his hands in the soil. He is actively searching for new ways to learn about our agricultural systems and get involved with his local agricultural community. Eric is an avid birdwatcher and advocate for environmental responsibility.