Soil biology is a branch of science that focuses on the organisms living in or interacting with the soil. These organisms can be microorganisms such as bacteria or fungi, or macroorganisms such as plants and animals.
Their activities instigate physical, chemical, and biological processes that take place above and below ground. In the long run, these organisms and their interactions shape the soil characteristics, which influence its ecosystem and the location where it is found.
As a science, soil biology involves the study of the ecological food web that takes place due to the interaction of these living organisms.
And since the ground serves as the interaction site for various organisms on earth, it also investigates the impact of their activities and their interactions on its properties and formation.
Soil originated from the disintegration of rocks in the pedosphere, the uppermost layer of Earth’s surface not occupied by water.
Natural rocks and those in man-made structures disintegrate when they are exposed to weathering agents such as water, wind, frost, temperature shifts, and other atmospheric gasses.
Despite having similar components, the ratio of each component in soils differs from one area to another. These differences arise from:
Hence, soil characteristics not only differ based on location but also when and how it is formed. And these components are broadly organized into four groups:[1]
Minerals are crystalline solids that aggregate to form a rock. Disintegrated rocks result in mineral particles of different sizes and species, which aggregate to form soil.
Minerals constitute about 45% of the overall components. They are often found as mineral salts, and most are acidic if dissolved. Common minerals found in soil include silicon dioxide, aluminum silicates, oxides of iron, and calcium carbonate.
Soil organic matter, or SOM, is decomposing dead plants and animals that can be transformed into humus, a dark, nutrient-rich, complex organic matter.[2]
Bacteria, fungi, and small animals such as mites, millipedes, earthworms, slugs, and shelled snails feed on dead plants and larger animals, breaking down complex molecules such as lignin, cellulose, lipids and proteins in the process.
Subsequently, these complex molecules are decomposed into simpler molecules, providing plants and other photosynthetic microbes with carbon and nutrients they can use to produce food for other animals.
Water fills the pore spaces between mineral and SOM particles or it’s absorbed onto the surface of these particles.
Air fills the open pore spaces that are not occupied by water. It is used by organisms that live underground for their respiration.
A soil’s uniqueness is reflected in its properties, which are recorded chronologically in layers, called horizons. These horizons are vertically combined to form a soil profile.
Each horizon differs in its thickness and can be characterized based on various physical and chemical properties of the ground.
Notable properties include:[2]
These properties are determined by the ratio and composition of the forming components, hence each characteristic is interwoven and influences one another. For example, soil color is dependent on mineral species, composition, amount of organic matter, and moisture.
Also, while minerals, ratio of minerals and organic matter, size of particles and pore space largely determine the soil structure and texture; these two latter properties affect the nutrient and water-holding ability, and the soil temperature, which ultimately affects its water content and biodiversity.[1,2]
Most large animals that come in contact with the soil are typically restricted to its surface. Plants, small animals, and microorganisms, however, interact with the soil surface and exert their influence beyond the uppermost horizons.
These organisms, which make up the soil biota or community, closely interact with the soil and play significant roles in developing soil profiles.[1]
For instance, plants penetrate their roots into the ground as they grow. Microorganisms and animals such as ants, earthworms, millipedes, slugs, and shelled snails inhabit and feed on plant products and litter on the L-layer.
Eventually, this L-layer becomes the O-horizon as the layer is being decomposed, while newer litter and organic matter are being formed on top of its surface.
The several living organisms found underground can be classified based on their sizes:[2]
Macro- and megafauna are organisms whose body widths are larger than 2 mm and they consume dead plants and organic litter. Examples include moles, earthworms, ants, millipedes, beetles, termites, and scorpions.
Mesofauna is a group of organisms whose body widths are between 0.1 mm to 2 mm. Microarthropods represent the most abundant and best-described mesofauna and act often as consumers. Examples are mites, springtails (Collembola), and pot worms.
Microfauna refers to organisms whose body widths are less than 0.1 mm. Microfauna include:[2]
Microorganisms in soil communities are microbes such as bacteria, yeast, fungi, and algae. They represent the most abundant and diverse group of organisms, a few notable microbes are:[1]
Common mycorrhizal fungi are:[2]
The biological activity of living organisms, especially microorganisms, provides the soil with nutrients and cycles them through the ecological food web of the site.
Food produced from plants grown aboveground is consumed by consumers that live in or below the ground. Wastes from food consumption, including plant, animal remains, and organic fertilizers, are decomposed, transforming them into nutrient forms that plants and other autotrophs can use to produce food.
The nutrients are cycled through the food web when consumers feed on the food and defecate or die, and decomposers digest the feces or consumers’ remains into the form of nutrients that plants and autotrophs can use.
Certain bacteria and fungi can enrich the soil with nutrients. Nitrogen fixers acquire nitrogen from the atmosphere, supplying it with ammonium while nitrifiers convert ammonium into nitrates.
Mycorrhiza can provide phosphorus in exchange for carbon sources derived from the plants they colonize. These minerals can be assimilated by plants and other autotrophs to produce food for consumers.
The ratio of soil-forming components determine its properties, including soil structure, texture, and moisture. To a certain degree, these properties are dictated by the biodiversity of the soil biota and their interactions with the soil.
The rate of organic matter decomposition and nutrient release relies on the species present on the location. In other words, the number of organisms and the species that decompose and transform organic matter into consumable nutrients affect the ratio of organic matter, water content, and air space between the components.
The composition of the soil dictates its structure and texture, which, in turn, prescribes the compatibility of the site for organisms to inhabit. Ultimately, the inhabiting organisms condition the soil fertility by acquiring or making nutrients available for autotrophs’ use, determining the suitability of its applications.[1]
Soil serves as a platform for the growth and development of microbes, plants, and animals that live above, in or below ground. Their interaction with the soil and with one another is depicted in a soil profile, consisting of several layers called horizons.
Overall, soil biology looks into the dynamics of the soil, in view of its occupying creatures and their impact on its properties and formation. Such understanding is useful in assessing its potential for use in agriculture and other human activities.
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