Soils

Think back to a time when you travelled through a verdant forest. You may have marvelled at the density of the trees or the bountiful symphony of birds. Similarly, think back to a time when you have visited a beach. These rocky or sandy coastal areas may have been sparsely populated with hardy grasses. Both of these ecosystems support widely different organisms, in large part due to the types of soils that support them. There are over 100,000 types of soils on the planet, and each interact with organisms in their own unique way. Soils store water, carbon, and biota. One small tablespoon of soil can contain more bacteria than China's human population of 1.4 billion. In this section, we will dig into the world of soils, and their importance to our living environment.

Soils Meaning

Soils are life sustaining. On a planetary scale, they are a wafer-thin layer on the outer surface of the Earth. Yet, this thin soil layer substantially supports the biosphere. They are both a biotic and abiotic component of ecosystems that supply food and energy to most autotrophs. Soils contain abiotic components such as nutrients, water, and oxygen. Soils also contain a multitude of bacteria, as well as nematodes, earthworms, and insects. The most life sustaining soils are themselves living. Soils are teeming with bacteria, insects, fungi, nematodes, and protozoans. Human agriculture provides us with a bounty of food, thanks to our understanding of soils. Soils are so important that we have instituted soil management practices to protect them. So, what exactly are soils?

Generally speaking, soils are mostly composed of minerals at about 45% of the volume. Second most, water and air fill in the spaces in between the minerals, at about 25% of a soil's composition each. Lastly, organic matter is the least abundant of the soil components, at about 5%. Soils vary differently, and the above numbers are loose observations. Some peaty soils may be more organic rich at 30% composition organic matter, while some soils may be arid, i.e., too dry to support vegetation, at less than 10% water content.

Soils Horizons

Soils naturally tend to form into distinct layers, which soil scientists have differentiated and named as soil horizons. The combination of all the soil horizons is called the soil profile (Fig. 2).

Each soil horizon is unique and has its specific characteristics.

• O Horizon: The topmost of the soil horizons, exposed to the atmosphere. It is usually rich in humus, a black soil rich in organic matter, such as leaf litter, small and microscopic organisms and animal waste.
• A Horizon: Also called topsoil, these soil horizons are typically rich in organic matter. Infiltration water flushes this soil layer from its clays, iron oxides and aluminium oxides, a process called eluviation.

• B Horizon: Known also as subsoil, these horizons commonly demonstrate clear structure. Within these horizons is a zone of illuviation which is rich in clays, iron oxides and aluminium oxides that have percolated from the soils of the A horizons above it.

• C Horizon: Also called the substratum, these soil horizons generally has a low organic matter content and is mostly insulated from factors that alter soil. It may be rich in water-soluble salts such as calcium carbonate or sodium.
• Bedrock: Not technically a soil horizon, this bottom layer is made of rock and marks the end of the soil profile.

Soils Types & Characteristics

Classification is generally based on:

• The four components of soils
• The size of the minerals, also called mineral fraction
• The presence or absence of water and air
• Organic matter content

The organic matter content category is too diverse to be able to classify constructively. In soils, the mineral fraction are categorized as such (also see Fig. 3):

• Clay: The finest of the soil grain sizes. Clay particles are less than 0.002 mm in diameter.
• Silt: Silt particles are a moderately fine soil mineral, whose diameter ranges between 0.002 mm and 0.06 mm.
• Sand: A coarse, yet still small, soil particle size, which identifies mineral particles between 0.06 mm and 2 mm.
• Pebbles, cobbles, boulders: The largest of the soil particle sizes, used to classify large stones.

Of the four classes listed above, only the first three are used to classify soils into subgroups. The percentage, or fraction, of a soil containing a particle size will determine its type, or soil texture. For example, a soil composed of 20% clay, 40% sand and 40% silt is called loam. A different soil composed of 45% clay, 5% silt and 50% sand is called sandy clay.

Based on texture, soils are grouped into 12 categories, which can be seen in a soil textural triangle diagram (Fig. 4). To use a soil textural triangle, draw a line on the triangle parallel to the orientation of the numbers for each mineral fraction. The intersection of all three lines will identify the type of soil based on your composition values.

Soils Structure

Beyond texture, soil scientists use another qualifier to describe soil, called structure.

To describe soil structure, there are five major classifications (Fig. 5). They are:

1. Structureless: Loose soils exhibits little to no peds, such as a dune or a beach. Massive soils are cemented together into one large block. Hardpan, or caliche, is an example of a massive soil, cemented together by calcium carbonate. Structureless soils are most often observed in C horizons.
2. Granular: Soils with this structure have small rounded peds, smaller than the size of a pea. These soils usually have a high organic matter content.
3. Platy: These soils form distinct thin plate like structures, piled on top of each other, akin to the pages of a book. These soils can be indicative of a compacted soil, or a young, undisturbed soil.
4. Blocky: Blocky soils have larger peds, bigger than the size of marbles. Angular blocky soils tend to have more clay than subangular blocky soils. This structure is most commonly observed in B horizons.
5. Prismatic, or columnar: These soils create long vertical cylindrical peds, sometimes up to a meter long. Prismatic soils have peds with more angular edges and generally a higher clay content. This structure is most commonly observed in B horizons.

Soils Colour

Soils appear in a wide variety of colours, and we can use this characteristic to deduce some traits about soils. Red, orange, yellow and brown soils indicate oxidation and a well oxygenated soil. On the other hand, a grey soil, a blue-grey soil or a greenish grey soil indicates reducing conditions, and a soil that is often water saturated. Black and dark soils are indicative of a high composition of organic matter. Calcite and other salt deposits can make soils appear light or off-white, especially in arid soils. Soil colour can be used to differentiate between horizons. For example, a black soil typically indicates O horizons, whereas C horizons tend to be the lightest soils in the soil profile, due to high concentration of white salts, natural deposits of minerals left behind after evaporation.

Soil Formation

The mineral fraction of soil consists of small minerals. These small minerals were once attached to a larger rock fragments, but erosion and deposition subsequently occurred to form these soils. Soils can form in situ, from the local rock parent material around it. Alternatively, soils can form after transport by gravity (colluvial), by water (alluvial, lacustrine), by ice (glacial) or by wind (aeolian).

You may be wondering what causes such a wide variety of soils. There are multiple soil forming factors that affect the physical nature of soils and how they originate. These factors are climate, organisms, relief, parent material, and time, often remembered by using the mnemonic term “CLORPT”.

• Climate: The amount of water a soil receives is strongly correlated to its climate. An arid climate will yield dry soil with oxygen rich oxidative conditions, whereas a wet climate may contain nutrient poor soils because nutrients get flushed out of the soil profile.
• Organisms: Organisms supply soils with nutrients through their waste or their death and subsequent decay. Animals alter soils by burrowing into them, and plants alter soils by exuding root acids that can release nutrients sequestered in peds. Humans, without the use of good soil management practices, have
• Relief: Relief describes the shape of the land's surface. Soils on severe slopes lose precipitation water to surface runoff, whereas soils on flat land experience more water infiltration from precipitation. The direction of the slope, or aspect, can also affect how much sunlight a soil receives in a day, which can affect the soils' moisture level and vegetation.
• Parent Material: The mineral fraction of soils is simply broken down pieces of larger rock material. Weathering is the erosion of rock material by its environment, such as rain, frost heaving or root activity.
• Time: The longer a soil sits undisturbed by major events, the more long-term effects alter it. Compaction, leaching, decomposition, illuviation, and erosion.

Uses of Soils

Soils are critical to an ecosystem, and offer multi-functional uses to humans. In an ecosystem, soils are important because they serve as an anchor for many plant's roots, while also nourishing the plant with nutrients and water. Furthermore, many living organisms reside in soils. Fungi, microbes, nematodes, protozoans, insects, and some mammal, reptiles and amphibians, all call soil their home (Fig. 6).

Fig. 6 This soil food web exemplifies the rich biodiversity found in soils. USDA-NRCS.

Soils also naturally act as a filter. When precipitation or floods pushes water vertically down into a soil profile, it carries with it various contaminants. These contaminants can bind to soil particles, while water percolates through. Soil also acts as a natural recycling centre by decomposing dead organic matter. Decomposers, such as worms, bacteria and fungi, break down organic waste into nutrients that can be reused by plants and other organisms.

Humankind has long thrived because of agriculture on rich soils. Furthermore, soils have long been used as a construction material. Early civilizations used clay soils to build bricks and pottery, a practice still in use today. More contemporary human uses of soils are to fabricate glass, with sand, or makeup, with clay. Sustainable soil management practices are vital to protect these beneficial soils uses.

Soils Degradation

Soils degradation occurs naturally, either rapidly due to a sudden occurrence, or over long periods of times, such as decades or centuries. Rapid soil degradation is usually due to extreme weather, geologic or biologic event. For example, flooding, landslides or overgrazing from a booming animal population all have the potential to dramatically and suddenly degrade a soil. Slower soil degradation is often linked to erosion. Long-term soil degradation is typically driven by either wind erosion, or water erosion, in the form of rain splash or water runoff. Some soil management practices can prevent soil degradation, such as the installation of wind barriers or the use of cover crops for wind erosion management practices and stormwater redirection for water erosion management practices.

Soils degradation can also be accelerated by human activities, such as over-cultivation, pollution, or construction.

• Excessive tilling of agricultural soils breaks down soil structure, which flushes out the nutrients that were sequestered in the peds.
• Unsustainable logging practices, such as clear-cutting, increases soil erosion because the lost trees can no longer protection around erosion.
• Pollution, such as excessive ash or dust, can suffocate the O horizon, exposing the soils to more erosion.
• Severely toxic pollution can outright destroy soil biota.
• New construction regularly kills vegetation and breaks down soil structure, leading to increase erosion and increased nutrient leaching.

Since agriculture and shelter are important for our survival, we cannot simply stop these activities. Humans have instead put into place good soil management practices to lessen human accelerated soil degradation.

To quantify soil loss and to enhance agricultural soil management practices, soil scientist have devised an equation, called the Universal Soil Loss Equation (USLE). The equation measures the weight of soil lost per year unit area (A), using six factors:

1. Erosion due to rainfall (R)
2. Erodibility of soil (K)
3. Slope of soil (LS)
4. Crop type (C)
5. Type of agriculture practice (P)

The Universal Soil Loss Equation is the product of these six factors:

A = R · K · LS · C · P

In conclusion, soils are the thinnest part of the Earth's crust, almost like a skin. It is somewhat simple in that it is made up of minerals, organic matter, water and air Yet, it is teeming with millions of organisms and its physical properties are detailed and categorized at length. This is because understanding, and preserving, soils is crucial to sustaining life on Earth.