Soil

What is Soil?

Soils are complex mixtures of minerals, water, air, organic matter, and countless organisms that are the decaying remains of once-living things. It forms at the surface of land – it is the “skin of the earth.” Soil is capable of supporting plant life and is vital to life on earth.

Soil, as formally defined in the Soil Science Society of America Glossary of Soil Science Terms, is:

1. The unconsolidated mineral or organic material on the immediate surface of the earth that serves as a natural medium for the growth of land plants.
2. The unconsolidated mineral or organic matter on the surface of the earth that has been subjected to and shows effects of genetic and environmental factors of: climate (including water and temperature effects), and macro- and microorganisms, conditioned by relief, acting on parent material over a period of time.

So then, what is dirt? Dirt is what gets on our clothes or under our fingernails. It is soil that is out of place in our world – whether tracked inside by shoes or on our clothes. Dirt is also soil that has lost the characteristics that give it the ability to support life – it is “dead.”

Soil performs many critical functions in almost any ecosystem (whether a farm, forest, prairie, marsh, or suburban watershed). There are seven general roles that soils play:

1. Soils serve as media for growth of all kinds of plants.
2. Soils modify the atmosphere by emitting and absorbing gases (carbon dioxide, methane, water vapor, and the like) and dust.
3. Soils provide habitat for animals that live in the soil (such as groundhogs and mice) to organisms (such as bacteria and fungi), that account for most of the living things on Earth.
4. Soils absorb, hold, release, alter, and purify most of the water in terrestrial systems.
5. Soils process recycled nutrients, including carbon, so that living things can use them over and over again.
6. Soils serve as engineering media for construction of foundations, roadbeds, dams and buildings, and preserve or destroy artifacts of human endeavors.
7.  Soils act as a living filter to clean water before it moves into an aquifer.

Soil Profile

There are different types of soil, each with its own set of characteristics. Dig down deep into any soil, and you’ll see that it is made of layers, or horizons (O, A, E, B, C, R). Put the horizons together, and they form a soil profile. Like a biography, each profile tells a story about the life of a soil. Most soils have three major horizons (A, B, C) and some have an organic horizon (O).

The horizons are:

O – (humus or organic) Mostly organic matter such as decomposing leaves. The O horizon is thin in some soils, thick in others, and not present at all in others.
A - (topsoil) Mostly minerals from parent material with organic matter incorporated. A good material for plants and other organisms to live.
E – (eluviated) Leached of clay, minerals, and organic matter, leaving a concentration of sand and silt particles of quartz or other resistant materials – missing in some soils but often found in older soils and forest soils.
B – (subsoil) Rich in minerals that leached (moved down) from the A or E horizons and accumulated here.
C – (parent material) The deposit at Earth’s surface from which the soil developed.
R – (bedrock) A mass of rock such as granite, basalt, quartzite, limestone or sandstone that forms the parent material for some soils – if the bedrock is close enough to the surface to weather. This is not soil and is located under the C horizon.

Value of Soil

Social issues and soil quality

Nutrient cycling, water regulation, and other soil functions are normal process occurring in all ecosystems. From these functions come many benefits to humans, such as food production, water quality, and flood control, which have value economically or in improved quality of life. People can increase or decrease the value of soil benefits because land management choices affect soil functions. Thus, it is important to understand what benefits we derive from soil and their value so we can appreciate the importance of managing land in a way that maintains soil functions.

What are the social benefits of soil?

People tend to emphasize benefits with the most direct, private economic value. In rural areas, this is usually plant growth especially as  crops and rangeland, but also a recreation areas. In urban/suburban areas, the most direct economic benefits of soil relate to structural support for buildings, roads, and parking. Landscaping, gardening and parklands may also be valued economically.

Those are all on-site, short-term benefits. That is, the landowner who decides how to manage the soil also reaps the benefits (and costs) of those management decisions. In contrast, many important benefits are long-term or go beyond the land being managed. The landholders who make the management choices and pay the costs of managing land may not be the same people who are affected by the landholders decisions. Society should discuss the value of these off-site benefits and to what extent the land owner or society should pay to maintain these soil functions.

Public, off-site benefits of soil relate to the following resource issues:

Water quality of streams, lakes, oceans, and groundwater

Air quality, especially particulates

Greenhouses gases, including carbon dioxide, methane, and nitrous oxide

Biodiversity

Water flow and flood control

Sustainability and land productivity

Aesthetics

Summary of soil benefits

Soil Function	Benefit of Value to Humans
	On-site	Off-site
Nutrient cycling	Delivery of nutrients to plants Carbon storage improves a variety of soil functions	Enhances water and air quality Storage of N and C can reduce greenhouse gas emissions
Maintaining biodiversity and habitat	Supports the growth of crops, range land plants, and tress May increase resistance and resilience to stress  Reduces pesticide resistance	Helps maintain genetic diversity Supports wild species and reduces extinction rates Improves aesthetics of landscape
Water relations	Provides erosion control Allows on-site water recharge of streams and ponds Makes water available for plants and animals	Provides flood and sedimentation control Groundwater recharge
Filtering and buffering	Can maintain salts, metal and micronutrient levels within range tolerable to plants and animals	Improves water and air quality
Physical stability and support	Acts as a medium for plant growth  Supports buildings and roads	Nutrient Stores archaeological items Stores garbage
Multiple functions	Sustains productivity	Maintains or improves air and/or water quality