Soil Structure – Why is it Important?

To have good, productive plants, soil structure is a key factor.

With good soil structure, air and water are allowed to move freely. This will improve drainage and reduce soil erosion caused by excess surface runoff. Without good soil structure structure, soils will suffer from oxygen starvation, waterlogging, nutrient lock-up, and lack of gas exchange.

Soil Structure vs Soil Texture

It’s important to note the differences between soil structure and soil texture. Soil texture generally refers to only the makeup of the soils component parts. That is, the proportions of clay, silt, loam and sand in the soil.

Soil texture considers the proportions of soil type and the compaction or density of the soil. Soil structure is generally defined as how particles of soil are grouped together into aggregates.

This is not a small difference. Let’s take a simple example.

In both examples the soil texture is the same.

Most of us have seen dirt roads that have been compacted allowing for automotive traffic. It’s unlikely that much plant life would grow on the compacted dirt.

Now imagine that some of the soil was left over on the side of the road. This non compacted dirt would have the same soil texture, but the dirt structure would be much different.

The uncompacted soil would have pores that would allow both water flow and gas exchange. In time, the leftover soil would have a much more diverse plant population than the compacted soil of the road.

That’s the difference between soil structure and soil texture.

Together, soil texture and soil structure have the greatest influence on pore space in a soil, and how easily air, water, and roots can move through a soil.

How Does Soil Structure Develop?

Soils are cemented or bound together by three processes: physical, chemical, and biological.

Physical and Chemical processes are generally lumped together. So we’ll continue that here.

Main Physical and Chemical Processes

• Polyvalent cations like Calcium (Ca2+), magnesium (Mg2+), and aluminum (Al3+) bind together clay particles
• Soil particles are continuously changed by environmental factors. Such as being pushed closer together by freezing and thawing, wetting and drying, and by plant roots pushing through the soil as they grow in length and width.

Biological Processes that Affect Soil

• Soil particles are cemented together by humus, these are essentially organic glues. Humus is created in a number of ways.
  • Humus is created by mycorrhizal fungi and beneficial soil bacteria.
  • Soil microbes decomposing organic matter adds to the humus.
  • Plants add to humus as well by producing polymers and sugars that are excreted from roots.
• Fungal hyphae and fine roots stabilize aggregates
• Arbuscular mycorrhizal (AM) fungi produce a glycoprotein, glomalin, that is deposited on their outer hyphal walls and on adjacent soil particles. This process contributes to the formation of relatively stable aggregate structures. This is sometimes called a sticky-string bag and can sometimes be seen by the naked eye.

Taken together the biological, chemical and physical changes increase the organic matter in soil. Making a more fertile soil that is more compatible with plant growth.

How Soil Conditions Changes Over Time

Soil structure changes over time and this change can either be a benefit or a detriment to the organic nature of the soil.

Key Negative factors include: compaction, cultivation, removal of vegetation, excessive handling of soil, and excessive salt (sodium).

Ways to Preserve Desirable Soil Structure

The USDA explain it best when they say: “practices that provide soil cover, protect or result in the accumulation of organic matter, maintain healthy plants, and avoid compaction improve soil structure and increase macropores.”

I would add the other key practices such eliminating soil screening and minimizing handling, and avoiding use of sodium salts.

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Photo by SoilScience.info