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What are soil aggregates? And why are they so important? by Nall I. Moonilall, The Ohio State University
These varied shapes allow for healthy soil to have pores spaces for air and water, needed for healthy plant growth.
Aggregate formation
Photo by Nall Moonilall
Soil aggregates retained on a 4.75 mm sieve after wet sieving experiment.
The ground beneath your feet might seem like a uniform material, but it’s really
UAC MAGAZINE | FALL 2019
a mixture of soil particles, organic matter, and other mineral/organic components. For a soil to be healthy, it must have good structure.
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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.” 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.
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.”
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.
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.
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.
