What Is Soil Made Of?
The ecological role of soil structure and soil organisms
∘ The soil beneath our feet
∘ Soil components
∘ Soil aggregates
∘ The ecological role of soil organisms
The soil beneath our feet
The term “soil”, depending on the context, can refer to different things.
When we use solely the word “soil”, it refers to a substratum or material beneath our feet, that we can touch and use to produce the vegetables we eat.
This substrate is also frequently misnamed “dirt” by many people (“not soil scientists!” would say Dr. Nyle C. Brady, an important soil scientist [1]), who associate it with something bad or undesirable.
When we are talking about “a soil”, it can also refer to a three-dimensional natural body (in the same sense that a lake or a tree).
This concept recognizes the existence of individual entities or units that vary in space and can be classified based on their characteristics.
To characterize a soil, soil scientists often dig a large hole several meters
deep and about a meter wide. The vertical section exposing a set of horizons in the wall of such a pit is named “soil profile” [1].
These horizons are layers of a few centimeters that look like layers of a cake and vary from one soil to another.
Soil components
Soil as a material is made of not uniform particles of different sizes, which include rocks, sediments, minerals, and organic matter.
Most material is not organic. Generally, only about 5% of soil volume consists of organic matter, which accounts for only about 2% of the weight [1].
Soil can be classified based on the relative content of different particle sizes (percent of sand, silt, and clay) using the soil texture triangle chart.
These components are mixed in complex patterns and shape a physical structure.
You can visualize it as a ball pit pool with different size balls.
Usually, only about half the soil volume consists of solid material, while the other half consists of pore spaces filled with air or water.
The spaces between the particles are just as important as the particles themselves. Soils with more than 50% of their volume in solids are likely to be too compacted for plant growth [1].
It is in these pore spaces that air and water circulate, roots grow, and different organisms inhabit.
Soil aggregates
Soil components such as clay, calcium carbonates, organic matter, and organic compounds produced by living beings, bind particles to form structural units called aggregates [2].
Aggregates can be variable in size and shape. They can be as large as a fist or look like tiny rocks on the soil surface that crumble when you touch them with your fingers.
The particles stick together due to physical, chemical, and biological activity and create large continuous passageways through the soil.
This network of pores within and between the aggregates constitutes a key aspect of soil structure since it influences many of the processes critical to soil functioning.
More stable aggregates improve soil density, porosity, infiltration, drainage, aeration, water-holding capacity, and resistance to erosion [2].
The ecological role of soil organisms
All the set of living organisms present in a soil is formally known as “edaphon”.
This group not only includes microorganisms like bacteria, fungi, and protozoa, but also plants (through their roots), and a wide variety of animal species such as insects (like ants), mites, nematodes, earthworms, and burrowing vertebrates [3].
Many of these soil organisms are often called “ecosystem engineers” since they influence and improve soil functions in many different ways [4].
Soil organisms directly add organic matter and nutrients in the form of plant litter (especially root litter), carcasses, fecal material, and microbial residues.
Plant roots and fungal hyphae physically bind soil particles.
Fungi are particularly important since they release substances such as glomalin, which functions as a glue that also promotes aggregate stability [2, 6].
Animals like earthworms create bio-pores that increase oxygen and water infiltration (bioturbation) and promote the formation of organo-mineral compounds and water-stable aggregates which increase soil water-holding capacity [7].
Their excreta also improve soil fertility since it is rich in nitrate, as well as available forms of phosphorus, potassium, calcium, and magnesium [8].
Since living soil organisms directly affect soil structure and aggregate stability, they are crucial to maintaining the land’s ability to support ecosystems and society.
You can find more information about the soil structure at the following links: [1, 2, 7]
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