What Are Soil Aggregates? How Soil Particles Clump Together

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Soil aggregates are the clumps that form when soil particles bind together. Penn State Extension puts it simply: “These groups of soil particles are called aggregates.” The way these clumps form and hold together affects how water moves into the ground, how air gets in, and how easily roots can grow. This guide explains what soil aggregates are, how they form, why their stability matters, what breaks them down, and how you can check and build them. Every claim below is tied to a quote from one of two university extension publications.

Quick answer

Soil aggregates are groups of soil particles that stick together. Penn State Extension calls them “groups of soil particles.”
Oregon State University says they are formed from “sand, silt, clay and organic material bound together with mineral and organic cements.”
Stable aggregates “do not dissolve when wet,” and that stability is “key to healthy soil,” according to Oregon State University.
Tillage can “break up soil aggregates,” which can reduce water infiltration, limit root penetration and cause the soil to crust.

What a soil aggregate is

A soil aggregate is a unit of soil in which particles are held together rather than loose. Oregon State University describes the materials that make up that unit: “The aggregates are formed from sand, silt, clay and organic material bound together with mineral and organic cements.” The cements are the part that holds the pieces together.

Penn State Extension separates aggregates from soil texture. It explains that “Whereas texture is the composition or relative proportion of three soil particle types (sand, silt, clay), soil structure is the arrangement or geometry of these soil particles.” In that framing, texture tells you what the particles are, while aggregates are part of the structure, meaning how those particles are arranged.

Penn State also points to the difference that management can make. It says that “While the texture of the soil is inherent and difficult, if not impossible, to change, we can influence the structure of the soil with our management practices.” Aggregation is the part of structure that gardeners and soil managers can most easily change, which is why it gets so much attention.

How soil aggregates form

Penn State Extension names living organisms as the builders of aggregates. “Bacteria and roots produce sticky substances that glue soil particles together.” The same publication adds that “Fungi and root hairs wrap soil particles into balls called aggregates.”

The summary sentence in that publication names three mechanisms: “Aggregates are formed in part by exudates from bacteria, entanglement of soil particles in fungal hyphae, and digestion by earthworms.” Each named mechanism involves living organisms. The sources do not say how much each mechanism contributes in a given garden, so this guide does not rank them.

Because roots and microbes are involved, the health of the living soil matters. Oregon State University notes that “Stable soil aggregates are the underpinnings of a healthy soil habitat.” It also says that “The crevices and spaces in and between the soil aggregates are important habitat for soil biota.” In other words, the aggregates themselves create the spaces where soil life lives.

Why aggregates matter for water, air and roots

Penn State Extension states the benefit in one sentence: “Important physical characteristics of soil-like structures and aggregation allow water and air to infiltrate, roots to explore, and biota to thrive.” Water and air need open paths through soil, and aggregates help create them.

Penn State also describes the pore spaces that structure creates. “Soil with good structure has a wide range of pore spaces or empty space between the soil particles.” The publication also says that good structured soils have “many crumbly pieces, large pores, clumps, roots, and decaying pieces of organic material.”

When aggregates are weak, the opposite can happen. Penn State says that “Soils with low aggregate stability can constrict crops because they form surface crusts.” It adds that crusts “can reduce both water infiltration and air exchange, make the soil more difficult to manage, and reduce its ability to dry off quickly.”

Oregon State University links stability to water movement at the surface. Its description of the crusting process is: “When raindrops impact the soil and break down aggregates, the clay and silt particles wash into soil pores and create a crust upon drying.”

What breaks soil aggregates down

The main cause named by both sources is disturbance. Oregon State University says that “The physical disturbance caused by tillage breaks up soil aggregates, which can reduce water infiltration, limit root penetration and cause the soil to crust.” Penn State Extension says that “Over the long term, repeated soil tillage can reduce soil tilth and break down stable soil aggregates.”

Tillage also affects the carbon stored in soil. Oregon State University explains that “Tillage breaks open soil aggregates, exposing organic matter to oxygen and microorganisms, which hastens decomposition and loss of carbon from soil.” In that account, breaking aggregates open is what exposes organic matter to oxygen and microorganisms.

Rain is a second cause. Penn State Extension describes aggregates as “water-stable” when they “resist falling apart when wetted and hit by raindrops.” Oregon State University describes a related process in which “The splash caused by raindrops or irrigation detaches soil particles from aggregates and allows them to be moved by water.”

Both sources point to the same practical point: the less a soil is disturbed, the more its aggregates are protected. Penn State says that “If you can reduce your tillage operations, you may reduce the disturbance to the soil biota that are essential for building aggregation.” The same publication notes that “Then bacteria and fungi work to help make aggregation happen.”

How to check aggregate stability

Oregon State University describes a simple test. “A slake test, where soil clods are saturated in water, will tell you about your soil’s structure and aggregate stability.” The publication includes a photo comparison and explains what it shows: “This slake test shows that the soil on the right has poor aggregate stability, which could lead to reduced water infiltration and root penetration.”

The sources describe the result only as stable or poor, and they do not give a numerical scale for the test, so this guide does not supply one.

Oregon State University ties the same idea to stability when it says that “Stable aggregates, those which do not dissolve when wet, are key to healthy soil.”

How to build soil aggregates

Penn State Extension gives the most direct guidance on building aggregates. It says that “As soil managers, we can help build soil aggregates by growing green manure cover crops or adding animal manure.”

Oregon State University recommends a related set of practices. It advises to “Implement organic no-till practices, using cover crops and a roller or crimper.” It also recommends “Planting deep-rooted cover crops such as daikon radish, and adding organic matter to alleviate compaction.”

For related reading on working organic material into garden soil, see how to make garden soil better. For a broader plan for keeping soil in good condition, see how to keep garden soil healthy.

What we could not verify

The two sources cover the basics well, but they do not answer every question a gardener might have. The gaps are listed below so you know where the guidance stops.

What the sources do not say

The sources do not give a numerical scale for the slake test, so no score can be assigned to a specific garden soil from this guide.
Penn State says the mechanisms are “in part” biological, and the sources do not rank bacteria, fungi and earthworms against each other.
The sources do not give a time frame for how long it takes aggregates to rebuild after tillage stops.
The sources do not give product or amendment rates for building aggregates, and this guide does not add any.

How we chose these sources

The two sources were chosen because both are university extension publications that discuss aggregate formation, stability and disturbance in plain language. Each returned HTTP status 200 when checked on 2026-10-27, and every quote in this guide is copied from the text of those fetched pages. No retail or product pages were used.

Frequently asked questions

What is a soil aggregate?

A soil aggregate is a group of soil particles that hold together. Penn State Extension says that “These groups of soil particles are called aggregates.” Oregon State University adds that aggregates are “formed from sand, silt, clay and organic material bound together with mineral and organic cements.”

Are soil aggregates the same as soil texture?

No. Penn State Extension states that texture is “the composition or relative proportion of three soil particle types (sand, silt, clay),” while structure is “the arrangement or geometry of these soil particles.” Aggregates are part of structure.

Does tillage destroy soil aggregates?

Tillage can break them down. Oregon State University says that “The physical disturbance caused by tillage breaks up soil aggregates.” Penn State adds that “repeated soil tillage can reduce soil tilth and break down stable soil aggregates.”

How can you check whether soil aggregates are stable?

Oregon State University describes a slake test in which “soil clods are saturated in water.” The sources describe the result as stable or poor, and they do not give a numerical score.

Do cover crops help build soil aggregates?

Penn State Extension says that “we can help build soil aggregates by growing green manure cover crops or adding animal manure.” Oregon State University recommends “cover crops and a roller or crimper” as part of organic no-till practice.

Sources

Penn State Extension, Soil Quality Information. https://extension.psu.edu/soil-quality-information (HTTP 200, checked 2026-10-27)
Oregon State University Extension, EM 9304, Soil: The Dirty Secrets of a Living Landscape. https://extension.oregonstate.edu/catalog/pub/em-9304-soil-dirty-secrets-living-landscape (HTTP 200, checked 2026-10-27)