Healthy soil and how to make it

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.

  What Are Aggregates?

Graphic showing types of soil aggregates in soil
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 plant growth.

  How Are Soil Aggregates Formed?

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.

  Cementation of Soil Particles

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.

  Role of Organic Material

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 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.

  Soil Microbes And Soil Aggregation

Soil bacteria and fungi.
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.

  How Do Soil Aggregates Contribute To Soil Health?

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.

  How To Improve Soil Aggregation

Soil health.

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.

Further Information

 What Is Soil?
 Improving Soil Health
 The Wood Wide Web
 Soil In Winter
 Benefits of Mycorrhizae Fungi

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