What Is Soil?
Chances are that you haven't thought a lot about the soil under your feet, but you may be surprised at the complexity of soil. Soil varies in its composition and the structure of its particles, and these factors are closely examined by farmers, who need appropriate soil for planting crops, as well as engineers who may need to understand how soil is going to hold up under different demands. Soil is also vitally important to the sustainability of an ecosystem because it serves as the natural medium for the growth of vegetation. In this lesson, you will discover just what soil is and which factors are looked at when determining the structure and the types of soil.
So, what exactly is soil? Soil can be defined as the organic and inorganic materials on the surface of the earth that provide the medium for plant growth. Soil develops slowly over time and is composed of many different materials. Inorganic materials, or those materials that are not living, include weathered rocks and minerals. Weathering is the mechanical or chemical process by which rocks are broken down into smaller pieces. As rocks are broken down, they mix with organic materials, which are those materials that originate from living organisms. For example, plants and animals die and decompose, releasing nutrients back into the soil.
Soil Profile and Soil Horizons
Now, if you look down at the soil under your feet, you cannot tell very much about that soil. So, when you study soil, it's helpful to grab a shovel and dig a hole that is big enough to reveal a vertical section of soil that ranges from the surface to the underlying rock, referred to as a soil profile. The soil profile is somewhat like the soil's fingerprint, and it will differ from other soil samples based on factors like its color, texture, structure and thickness, as well as its chemical composition.
Each layer of a soil profile is referred to as a soil horizon. These horizons are identified by letters. Horizon A is the upper layer, closest to the surface. You can think of this horizon as the topsoil. In fact, you can use this as a memory jogger to help remember the order of the horizons. The letter A is at the top of the alphabet and refers to the topsoil layer. As you move deeper into the layers of the soil profile, you have horizons B and C, giving us the three main horizons.
Types of Soil
There are three basic types of soil: sand, silt and clay. But, most soils are composed of a combination of the different types. How they mix will determine the texture of the soil, or, in other words, how the soil looks and feels.
One type of soil is sand. Sand within soil is actually small particles of weathered rock. Sand is fairly coarse and loose so water is able to drain through it easily. While this is good for drainage, it is not good for growing plants because sandy soil will not hold water or nutrients.
This definition is from Soil Taxonomy, second edition.
soil - Soil is a natural body comprised of solids (minerals and organic matter), liquid, and gases that occurs on the land surface, occupies space, and is characterized by one or both of the following: horizons, or layers, that are distinguishable from the initial material as a result of additions, losses, transfers, and transformations of energy and matter or the ability to support rooted plants in a natural environment.
The upper limit of soil is the boundary between soil and air, shallow water, live plants, or plant materials that have not begun to decompose. Areas are not considered to have soil if the surface is permanently covered by water too deep (typically more than 2.5 meters) for the growth of rooted plants.
The lower boundary that separates soil from the nonsoil underneath is most difficult to define. Soil consists of horizons near the Earth's surface that, in contrast to the underlying parent material, have been altered by the interactions of climate, relief, and living organisms over time. Commonly, soil grades at its lower boundary to hard rock or to earthy materials virtually devoid of animals, roots, or other marks of biological activity. For purposes of classification, the lower boundary of soil is arbitrarily set at 200 cm.
Soil is a vital part of the natural environment. It is just as important as plants, animals, rocks, landforms, lochs and rivers. It influences the distribution of plant species and provides a habitat for a wide range of organisms. It controls the flow of water and chemical substances between the atmosphere and the earth, and acts as both a source and store for gases (like oxygen and carbon dioxide) in the atmosphere. Soils not only reflect natural processes but also record human activities both at present and in the past. They are therefore part of our cultural heritage. The modification of soils for agriculture and the burial of archaeological remains are good examples of this.
Soil, together with the plant and animal life it supports, the rock on which it develops, its position in the landscape and the climate it experiences, form an amazingly intricate natural system – more powerful and complex than any machine that man has created. Soil may look still and lifeless, but this impression couldn’t be further from the truth. It is constantly changing and developing through time. Soil is always responding to changes in environmental factors, along with the influences of man and land use. Some changes in the soil will be of short duration and reversible, others will be a permanent feature of soil development.
"It is difficult to rate the importance of the different soil functions, since all are vital to our well-being to some extent," it says. "However, the function of supporting food and agricultureworldwide is fundamental for the preservation and advancement of human life on this planet."
Healthy soils for a healthy people and planet:
Most of us know that: no soil, no sustenance. Famines are driven by soil degradation, as poor farming practices lead to soil loss through erosion and leaching of nutrients from the soil. Anyone who has done even a little gardening recognizes how the quality of the soil can change the outcome of the harvest. But soil serves us in so many other ways, FAO points out.
1. Since soil is the basis for plant growth, it contributes to the maintenance of both the natural and planted landscape. It supports the forests, wetlands, jungles, prairies and grasslands that spawn the planet's amazing vegetative biodiversity. Those plants—some of which we are still discovering—provide food, fuel, animal feed, medicine and raw materials for clothing, household goods and other essentials. Plants in turn help prevent soil erosion.
2. Soil also supports animal biodiversity, above and below ground. It's essential to the lives of both wildlife and domesticated livestock. And the soil itself is teeming with a fathomless number of micro-organisms and insects as well as familiar organisms such as earthworms that maintain soil quality, provide nutrients, break down toxic elements and interact with water and air to help maintain a healthy natural environment.
3. Soil is important in providing an adequate water supply and maintaining its quality. Soil and the vegetation it supports catch and distribute rainwater and play a key role in the water cycle and supply. Soil distribution can impact rivers, lakes and streams, changing their shape, size, capacity and direction.
4. The water absorption properties of soil play a role in reducing pollution from chemicals in pesticides and other compounds.
5. Soil provides both the foundation and base materials for buildings, roads and other built infrastructure.
6. Soil holds the key to Earth's history, containing and preserving artifacts of the planet's past, both its natural and its human/cultural antecedents. You can thank soil for those dinosaur fossils every kid loves to see at a natural history museum as well as the relics that tell us how our own human story evolved.
7. And critical to Earth's future, soils and how we use them play an important role in helping us to address climate change. Soil organic matter is one of our major pools of carbon, capable of acting as either a source or sink. Soil contains the fossil fuels that drive climate change when extracted but when left underground give us the chance to reduce the greenhouse gas emissions that drive climate change and reach our eventual goal of a zero-emissions world.
The FAO Soils Portal provides a wealth of information about what is being done and what can be done to maintain the beneficial qualities of soils around the globe.
