While soil is a term that different people use differently, topsoil is the recognizable crumbly ‘mud’ found in gardens and outdoor areas.
Soil is the top layer of the Earth’s crust, containing minerals, chemicals, and organisms that help support plant and animal life.
Soil scientists usually study the earth’s surface to a depth of about 1.2 meters.
Categories of Soil
Soil is generally classified into four categories:
- Clay — is the finest type of soil, with a diameter of fewer than 2 micrometers. A micrometer is a thousandth of a millimeter.
- Silt — this has a diameter of 2-60 micrometers.
- Sand — Soil of this size and above can be seen with the naked eye.
- Stones — particles larger than 2 mm are classed as stones, which can include grit and gravel.
Clay, Silt, and Sand are known as ‘fine earth.’
How is Soil Formed?
Soil is not formed overnight; it can take up to 1000 years for just an inch of ground to form! Therefore, time is a critical component in the formation of soil. Some other factors that aid the shape of the earth are:
- Living organisms like plants, animals, fungi, and bacteria.
- Topography is the slope of the land where the soil is forming.
- The climate where the soil is forming
- Parent material is the minerals and rocks that disintegrate to form the soil.
Why is Soil Important?
It can be easy to disregard soil and classify it as unimportant because it is often considered dirt. However, soil plays a crucial role in supporting life on Earth. Here are some of the ways that soil is so important:
- Plants: The majority of plants need soil to grow. Soil is vital for plants as it provides them with food and nutrients. Moreover, plants use the earth to anchor themselves to the ground using their roots.
- Atmosphere: Soil releases gases, including carbon dioxide, into the air, impacting the atmosphere.
- Living Organisms: Lots of living things, such as animals, plants, fungi, and bacteria, rely on the soil as a place to live.
- Nutrient Cycles: Soil is vital in cycling nutrients, including the carbon and nitrogen cycles.
- Water: The soil aids in the filtration and cleansing of our water.
Residual Soil
Residual soil is essentially soil material that has come about as the result of weathering and decomposition of rocks that have not been transported from their original place. Residual soil takes many years to form, as mechanical and chemical weathering transforms solid rock into soil.
The process of the formation of residual soil can be laid out in these steps:
- The bedrock, the layer of solid rock beneath the soil, breaks due to weathering from various physical processes, such as ice wedging.
- Water, oxygen, and carbon dioxide seep into the cracks of the bedrock and cause chemical weathering.
- Plants become established and produce biological weathering.
- Weathered materials collect and eventually form soil.
- The soil slowly develops soil horizons as each of the layers of soil becomes altered. This is because the top layer of soil is exposed to the most weathering, and each of the soil layers is changed slightly less after this. This is because the top layer of soil is the first to come into contact with water and air.
Layers of Soil
There are many layers of soil, also known as horizons, which are all labeled in descending order from the top layer to the bottom:
O (humus or organic): This layer is mainly composed of organic matter, like decomposing leaves. This layer can be thin in some soils, thick in others, and not even present in others.
A (topsoil): This horizon is composed mainly of minerals from the parent material, with some organic matter incorporated into the mix. Topsoil is an excellent source of material for plants and other living organisms to live off.
E (eluviated): This layer of soil is leached of clay, minerals, and organic matter. After this, a heavy concentration of sand and silt particles of various resistant materials remains. This layer is not present in some soils but is often found in older and forest soils.
B (subsoil): The subsoil is rich in minerals that have been down or been leached from both the topsoil and the eluviated soil layers.
C (parent material): This layer of soil has little to no evidence of weathering or other alterations of the soil particles.
R (bedrock): This layer of soil is made up of a mass of rock such as granite, basalt, quartzite, limestone, or sandstone.
The focus is mainly on the three main horizons, which even the simplest of soils possess: the topsoil, the subsoil, and the horizon.
Let’s dive deeper into each of these layers of soil:
- Topsoil
The topsoil, known as the A horizon, tends to be the darkest layer of soil as it contains a lot of organic material. This layer of soil is home to the most intense biological activity, as many insects and small animals burrow through it. Plants also reach their roots down into the topsoil. These plant roots play a crucial role in keeping this layer of soil in place.
In the topsoil, minerals often dissolve in the fresh water that moves through this layer to be carried to the lower soil layers. Other tiny particles can also get transported to these lower soil layers as water seeps further into the ground.
Plant roots help to hold this layer of soil in place. In the topsoil, minerals may dissolve in the fresh water that moves through it to be carried to lower soil layers. Small particles, such as clay, may also get taken to reduce layers as water seeps into the ground.
- Subsoil
This layer of soil is located directly below the topsoil and is made up mainly of minerals and leached materials. In addition, the subsoil is home to humus, a dark, organic material that forms in soil from decaying plants, animal matter, and clay. However, the subsoil does not contain as many macroscopic and microscopic organisms that reside in the topsoil, making it rich in organic matter.
- Bedrock
This layer of soil, also called the C horizon, is a layer of partly altered bedrock. Some weathering has taken place in this layer of soil, but not so much so that the original rock cannot be seen.
It would be impossible to go into detail about all the different types of soil, as there are thousands out there, each of which has unique characteristics and features. Let’s instead look at a few common types of soil:
- Sandy soil
Sandy soil is characteristically dry and warm. This type of sand is also typically very acidic and low in nutrients. Sandy soils are also categorized as light soils because they contain a large amount of sand and very little clay.
One of the defining features of sandy soil is its excellent water drainage. However, this water drainage has downsides, so this soil struggles to absorb nutrients. It is much quicker to warm up in spring than more clay-heavy soils, but it is also more likely to dry out in summer.
Adding organic matter can improve this sandy soil’s nutrient density and water-holding capacity.
- Clay Soil
Unlike sandy soil, clay soil is very heavy and high in nutrients. In winter, clay soil stays pretty cold and wet, while it tends to dry out in summer. This is because clay soils comprise over 25% clay, with spaces between the different clay particles. These spaces allow clay soils to retain such a high level of water.
Clay soils are regarded as a gardener’s nightmare, as they drain slowly and take a long time to warm up in summer. This, in combination with the fact that they dry out in summer, makes them very difficult to work with.
- Silt Soil
Silt soil is very light, fertile soil with a high moisture retention level. Silt soil has such a high level of moisture retention as it is composed of medium-sized particles. However, these particles tend to get washed away with the rain as they are excellent. However, this can be stopped by adding organic matter, which helps the silt soil particles form into clumps.
- Chalk Soil
Chalk soil is pretty versatile because it can be either light or heavy. However, it always has a high alkaline concentration due to the calcium carbonate within its structure. As a result of its alkaline content, chalk soil does not support the growth of ericaceous plants, such as azaleas, blueberries, camellias, and rhododendrons. Such plants require much more acidic soils.
