Soil Aggregates
What are soil aggregates?
Soil
is made up of a combination of primary particles – sand, silt and clay.These
particles can be bound together into what soil scientists call “aggregates.”
.
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 crop productivity. Soil management often influences
aggregate size, shape, and stability.
Soil Science Society of America
by Nall I. Moonilall,
Ohio State University
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/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.
 |
| Soil aggregates retained on a 4.75 mm sieve after wet sieving experiment. |
These particles can be bound together into what soil scientists call “aggregates.”
These 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.
These varied shapes allow
for healthy soil to have pores spaces for air and water, needed for healthy
plant growth.
Aggregate formation is a
complex process.
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 garden.
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.”
 |
Soil crust formation on a soil exposed to simulated
rainfall. You can see the crust formation on the surface of the soil as
well as how deep the crust extends. (This really is soil – not
cement!) |
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.
Here, the clay floccules
and other soil particles are bonded together by some type of cementing agent.
(Here we mean “binding” – not cement like in concrete!)
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
(earthworms, etc.)
When breakdown occurs,
these organisms secrete organic compounds that are the “glue” that makes
cementation occur.
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.
As you can see, aggregate
formation is the result of many interactions and feedback loops occurring below
ground.
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.
 |
| Keep soil covered! Crop residues on the soil surface help to protect soil from erosive forces. |
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 crop
productivity.
Soil management often
influences aggregate size, shape, and stability. Favorable practices that
promote and maintain greater stability include:
· Minimizing soil disturbance, like minimal tillage. This reduces
aggregate destruction because they are not physically or mechanically broken
apart;
· Adding organic matter enhances aggregate strength and stability;
· Keeping soil covered is essential to keeping soil intact.
Vegetative cover on the soil reduces the impact of erosive forces;
· Promoting a diverse cropping system. Systems that promote
perennial plants or meadows have expansive rooting systems and require no
tillage. Promoting this kind of diversity within a system will ensure that
soil’s function is not reduced;
· Managing for grazing. 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; and,
· Managing for pest control. The choice of plants and how they are
managed (e.g., annual vs. perennial, cover crops, rotation) are highly influential.
To recap – soil
aggregates are the building blocks that make up soil and their stability is
extremely important in the long-term.
Soils that are well
aggregated exhibit greater soil health, ensure greater agronomic productivity,
are less susceptible to soil erosion, and can play a role in carbon
sequestration.
Answered by Nall I.
Moonilall, Ohio State University
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