What Are Soil Aggregates?
Excerpted from: Soils Matter, Get The Scoop!
The ground beneath your feet might seem like a uniform material, but it’s really a mixture of soil
particles, organic matter, and other mineral and organic components.
For a soil to be healthy, it must have good structure. Soil is made up of a combination of primary
particles – sand, silt and clay. These particles can be bound together into aggregates.
When you pick up a handful of soil, and it breaks apart into little pieces, you are looking at soil
aggregates. Each aggregate is made up of soil particles of different sizes held together by both
the attraction of soil particles and the binding of organic matter between soil particles.
Examples of different types of soil structure: a) blocky, b) columnar, c) massive, d) single grain, e) platy.
Aggregates are clumps of soil that range from the micro level (less than 0.25mm in diameter) to the
macro level (greater than 0.25mm in diameter). Furthermore, they can resemble various shapes:
granular, blocky, etc.
The varied shapes allow for healthy soil to have pores spaces for air and water, needed for healthy
Soil aggregates are formed through physical, chemical and biological activity below ground. They are
even influenced by human factors, like tilling, walking on the surface, or even how you fertilize your
Formation of aggregates begins with finer soil primary particles binding together. You may know that
clay particles have a negative charge. And, the fertilizers you use include salts that have positively
charged cations (things like potassium nitrate, etc.) The positively charged cations allow the negatively
charged clay particles to bind together creating floccules.
The type and amount of clay minerals in the soil often plays an influential role in aggregation formation.
The second part of aggregate formation deals with cementation (binding, not literally cementing).
Here, the clay floccules and other soil particles are bonded together by some type of cementing agent. Examples
of cementing agents include organic matter, and liming materials like calcium carbonate. Even
types of oxides, like iron and aluminum can help cement particles together.
In the case of organic matter, it is broken down by the soil microorganisms and soil fauna such as earthworms.
When breakdown occurs, these organisms secrete organic compounds that are the “glue” that makes
Plant roots also play a role in aggregate formation by secreting organic compounds called root
exudates. These help bind soil together near the root zone. Fungal hyphae also contribute to
aggregate formation by entangling and weaving around soil particles.
Soil bacteria and fungi.
Soil particles are arranged together to form aggregates which are held together by organic matter and
microbial agents. Soil microorganisms play an important role in the formation and stabilization of
aggregates. Bacteria and fungi produce a variety of mucilaginous polysaccharides which act like glue
and help them attach to clays, sands, and organic materials, resulting in the formation of new aggregates.
Fungal hyphal networks facilitate the formation of soil aggregates by physically binding soil particles
together, creating stable aggregates that help increase water infiltration and soil water holding capacity.
Soil aggregates play a major role in soil structure formation and soil health. In agriculture, the stability
of aggregates is critical to how well an agroecosystem will function. The pore spaces in soil influence
air and water storage, and gaseous exchange. They create habitat for soil microorganisms, and allow
for plant root development and penetration. They also assist in nutrient cycling and transport.
Soils that have high aggregate stability are less susceptible to erosion. They hold their shape when
exposed to disruptive forces, like water, and do not easily break apart.
Poorly aggregated soils disintegrate easily when exposed to erosive forces. They tend to break down
faster, leading to soil degradation. Poor stability can lead to pore spaces being filled in and can ultimately
result in the formation of soil crusts. This can lead to reduced infiltration and gaseous exchange. Poorly
aggregated soils can reduce plant success.
Soil management often influences aggregate size, shape, and stability. Favorable practices that promote
and maintain greater stability include:
Minimizing soil disturbance, like minimal tillage. Tillage destroys aggregates in two ways: (1) by
physically breaking the aggregates apart and (2) by stirring air into the soil, stimulating microbes
to increase the rate of organic matter decomposition. Soil organic matter is lost to the atmosphere
as carbon dioxide and, over time, this can result in less organic material to bind aggregates together
Adding organic matter enhances aggregate strength and stability. Adding organic materials, such
as manure or mulch residues, can provide the soil with both nutrients and organic matter, while
improving aggregate stability over time. The latter is a result of greater amounts of organic carbon
combined with greater microbial activity, enhancing the production of aggregate glues.
Keeping soil covered is essential to keeping soil intact. Vegetative cover on the soil reduces the
impact of erosive forces.
Systems that promote perennial plants or meadows have expansive rooting systems.
Once established, perennial crops cannot be tilled. In addition, these crops will develop
deep and extensive root systems. Thus, cover and sod crops in a rotation contribute to
organic matter buildup over time. This addition of organic matter promotes aggregate stability.
Grasses have strong root systems, but if animals graze too long, that can be disruptive to the forage
system. There are many ways to graze animals and preserve or enhance soil stability.
Main crop selection, crop rotation, and use of cover crops can also impact aggregate stability.
What Is Soil?
Improving Soil Health
The Wood Wide Web
Soil In Winter
Benefits of Mycorrhizae Fungi