When we're discussing soil, we're just scratching he surface

July 15, 2008

Soil is not dirt. Dirt is what you sweep up off the floor or what your mother told you to wash from behind your ears.

Soil on the other hand is quite different; Wikipedia describes soil as the naturally occurring, unconsolidated or loose covering of broken rock particles and decaying organic matter (humus) on the surface of the Earth, capable of supporting life. However, Webster captures the essence of soil by defining it as a medium in which something takes hold and develops.

In my line of work we talk about soil a lot. There are several things about soil or should I say people's perception of soil that bugs me. First, is top soil; top soil is regarded by many as some sort of "Holy Grail." As a retired Extension Specialist once said, top soil is merely the soil on top. There is nothing magic about top soil in and of itself.


Soil health or soil quality is what we are really talking about and not top soil. Top soil can get just as compacted as sub soil. But, first let us look at soil as a whole. This is called soil horizons which is a cross-section of the soil. The arrangement of these horizons is what makes up a soil profile. Soil Scientists use soil profiles and soil horizons to classify and interpret the soil for various uses.

Soils typically have four major horizons - the organic horizon (O) on the surface with the surface horizon (A) just below, then the subsoil (B), and the substratum (C). Hard bedrock, which is not soil, uses the letter R designation in a profile.

Now to soil quality which is defined as "the capacity of a soil to function and to sustain biological productivity, maintain environmental health and promote plant and animal health." Some of the soil properties that determine soil health include soil texture, depth of soil, infiltration, bulk density, water-holding capacity, soil organic matter, pH, electrical conductivity, microbial biomass, carbon and nitrogen and soil respiration.

Soil structure and soil tilth are very important, but still elusive concepts. Soil tilth refers to the state of aggregation of a soil. Aggregates are the combination of clay, silt and sand particles that are held together by physical and chemical forces.

Soil texture affects almost all other soil health indicators. Most soils in Washington County are "silt loams." This classification refers to the surface soil and does not take into account differences in clay content in the subsoil, impermeable layers near the surface, rock fragments and so forth.

Soil depth is the depth of soil to bedrock or to an impermeable layer. Soil depth determines how deep roots, water and air can penetrate into a soil. This, in turn, influences how much water can infiltrate the soil, how much water can be held by the soil and how much soil plant roots can occupy.

Soil organic matter consists of living, partially to fully decomposed organic materials. Soil organic matter is typically 1 to 5 percent of the total dry weight of topsoil, with lower amounts in the subsoil.

Bulk density is a measure of the mass of particles that are packed into a volume (e.g., a cubic foot) of soil. If bulk density goes up, porosity goes down. It is favorable to have a low bulk density so that water and air can move through the soil.

Earthworms are the "canary in the coal mine" so to speak when it comes to soil health. They generally increase microbial activity, increase the availability of nutrients and enhance soil physical properties. They also accelerate the decomposition of crop residue by incorporating litter into the soil and activating mineralization and humification processes. Earthworms improve aggregation and porosity, suppress certain pests or disease organisms and enhance beneficial microorganisms.

There are different types of earthworms: some live in the surface of the soil and make horizontal burrows, while others live in the vertical burrows that can be more than three feet deep. Some earthworms make permanent burrows, while other earthworms fill their burrows with excretions. Nightcrawlers are among the important earthworm species in agricultural soils.

So why all the information about soil, well you will soon be able to "Uncover the Secret World of Soil" as the Smithsonian's National Museum of Natural History puts it. Their new exhibition, "Dig it! The Secrets of Soil," will be opening Saturday. Again quoting from their news release, "There are more living creatures in a shovel full of rich soil than human beings on the planet. Yet more is known about the dark side of the moon than about soil."

Now that we have scratched the surface of soil so to speak, I trust you will remember soil is not dirt and top soil is not the same as healthy soil. Until next time, get your hands dirty in some soil or visit the Smithsonian exhibit.

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