What is soil?

Soil, being the foundation of life, is of great importance to human and nature

Written by Ethlyn Koh
Photos by Goh Shang Ming

Every day as you lay your feet on the ground and walk the earth, have you ever wondered what lies beneath? Soil. This material exists on the outermost part of Earth’s crust, forming the surface of land and sometimes regarded as “skin of the earth”. 

The uses of soil are endless. 

Soil is an important natural body as it supports agriculture. Most of the food we consume can be traced back to soil because it is the original source of nutrients needed to grow and produce food. Soil also plays a crucial role in the water cycle. Not only does soil store and filter water, providing a clean supply of water, it too improves our resilience to floods and droughts, especially in the face of climate change. On top of that, soil is a habitat for a wide variety of organisms. It houses microscopic organisms to soil fauna of larger sizes – for example, earthworms, springtails, burrowing rodents, etc. Soil is undeniably an extremely valuable and vital ecosystem that delivers ecosystem services, enabling life on Earth, fundamental to our survival.

So what exactly is soil? 

The Soil Science Society of America defines soil as the unconsolidated mineral or organic material present on the immediate surface of earth, serving as a natural medium for the growth of land plants. Others describe soil as layers of generally loose mineral and/or organic material that are affected by physical, chemical and biological processes at or near the planetary surface and usually hold liquids, gases, and biota (living things) and support plants. 

Composing of a mixture of minerals, water, air, organic matter, and decaying remains of living things that once lived, the components of soil fall into two distinct categories: biotic and abiotic factors. The biotic factors encompass both the living and dead – for instance plants, insects and even soil microorganisms such as archaea, fungi, algae and more. The abiotic factors on the other hand represent non-living things which include minerals, water and air. Commonly found soil minerals such as nitrogen, phosphorus and potassium are amongst the essential nutrients needed for healthy plant growth followed by calcium, magnesium and sulphur. The combination of these factors ultimately determine the properties of soil – its texture, structure, porosity, chemistry and colour. But that’s a topic for another day. 

Healthy soil is crumbly and darker in colour due to the abundance of organic matter in it.

Undoubtedly, soil builds and supports the foundation of a community, a nation, and basically any form of life. The giving nature of soil provides us and other forms of life with an abundance of resources. 

Soil can also be described as a repository of memory, holding records of the past of our planet, our evolutionary history of how far we have come. It may also store secrets and possibilities that have yet to be discovered to a sustainable future. 

Essentially, all life depends upon the soil… There can be no life without soil and no soil without life; they have evolved together.

Dr Charles E. Kellogg

So while you keep your feet on the ground, stay grounded and stop treating soil like dirt.


Certini, G., & Ugolini, F. C. (2013). An updated, expanded, universal definition of soil. Geoderma, 192, 378–379.

Es, H. (2017). A New Definition of Soil. CSANews (Madison, Wis.), 62(10), 20-21.


Soil Biodiversity

Bacteria, insects and earthworms break down organic materials e.g. fruit peels and dry leaves in the composting bin. Same goes to the real soil
Bacteria, insects and earthworms break down organic materials e.g. fruit peels and dry leaves in the composting bin. Same goes to the real soil

Soil organisms constitute more than 25% of discovered biodiversity on earth. However, much of them remain unexplored and receive little attention compared to aboveground organisms.

Though less visible, these organisms are responsible for various ecosystem functions such as:

  • nutrient cycling
  • pollution remediation
  • disease control
  • water infiltration
  • supporting agro-ecosystems etc. 

The ecological processes in soil are mainly driven by interactions between soil microorganisms and plants, especially their underground roots. The soil microbes (microscopic organism), mainly bacteria and fungi, break down dead organic matter e.g. fallen leaves and release minerals and carbon compounds into the soil. These nutrients will be reused by plants for development. Some microbes establish mutualistic relationships with plants. For example, the mycorrhizal fungi transport water and minerals to the plant, while they receive carbon in return. 

The soil microbes also suppress plant diseases by competing with disease-causing organisms, colonising or consuming them.

Soil microorganisms are important in maintaining soil structure and retaining water.

The sugar-rich secretion of bacteria or threadlike filaments of fungi bind soil particles into small aggregates which are physically and chemically stable.

The microbes are eaten by larger soil organisms i.e. the protozoa and nematodes. These small animals are then eaten by their predators such as insects, centipedes, spiders and scorpions. This underground food web is connected to aboveground food web as soil-dwelling animals become the food source of animals that live on the ground such as birds, snakes and frogs. 

Aside from organisms in the grazing food chain, there are animals that feed on dead plant materials. Unlike decomposer, these animals need to orally ingest the organic matter and digest it inside their bodies. Some examples of these detritus-feeders are woodlices, beetles and termites. 

A pleasing fungus beetle feeds on fungus and decomposing matter.

Cave cricket lives in leaf litter.

Apart from that, the earthworms which feed on leaf litter and soil are known as ecosystem engineers as they produce nutrient-rich castings and create pores in soil. The castings are important for soil aggregate formation and plant growth, while the pores in soil facilitate water movement, increase water infiltration and alleviate flooding.  


  1. Ingham, E. R. (n.d.). Soil Bacteria. Retrieved from USDA Natural Resources Conservation Service Web site: 
  2. Editors. (2017, November 05). Difference between Detritivores and Decomposers. Retrieved from


Conserving Soil Biodiversity

Black, fluffy soil with dead plant roots

Like aquatic and terrestrial organisms, soil organisms are threatened by a series of environmental issues. The major threat that they face is habitat loss, which results from land conversion, pollution, climate change and invasive species. Agricultural activities such as “tillage” alter composition of bacterial communities and reduce diversity of soil fungi and larger animals. Construction of roads, buildings and street pavement damage the soil structure and destroy  soil organism’s habitat.

Habitat degradation occurs when the soil is polluted. Pollutants such as heavy metal and excess nutrients change the soil environment chemically, usually in an abrupt and profuse manner. This makes the soil condition unfavourable for many existing soil microorganisms. As a result, only a few pollution-tolerant species survive and dominate the community. The overall microbial diversity and activity decrease. 

The alteration of environmental parameters as a result of climate change also affects soil organisms. Increased concentration of atmospheric CO2 triggers responses of soil fungal communities. Such responses are reflected in the amount of living plants in the area. Quantity and frequency of rainfall and changes of temperature also impact underground animals such as insects. However, the impacts vary by taxon (unit used by scientists to classify organisms) and ecosystem as some are more resistant to environmental changes while some are more vulnerable.  

The intrusion of invasive species such as exotic plants brings changes to the soil environment as well as underground microbial communities.

Their roots release a new combination of chemicals e.g. sugars and enzymes into the soil. The type and amount of chemicals are different from the ones released by original plant communities. This affects the activity and population size of microbial community at the rhizosphere i.e. portion of soil surrounding roots of living plant as its biological and chemical properties are influenced by the roots. The invasive plants also impact the soil organisms by interfering with nutrient cycling e.g. the legume plants, or changing the amount of litter and root inputs. 

Conservation measures to support soil biodiversity include managing natural areas, restoring degraded ecosystems, adopting sustainable farming practices and adapting urban areas for both nature and people. Identifying undisturbed land and protecting it are important to sustain soil biodiversity as the habitat quality of soil organisms is maintained. Other than that, both artificial and natural revegetation of disturbed land help soil microbes and fauna to re-establish. 

Dry leaves are used for mulching and composting. They help to retain soil moisture, regulate soil temperature and suppress weed growth.

Sustainable farming practices are also important in conserving soil biodiversity. Reduced tillage, crop rotation, planting of cover crop and retention of litter are some useful measures to improve soil quality as well as support soil biodiversity. Allocating spaces for greenery and wildlife in urban planning, establishing green roofs and rain gardens, reduced soil compaction and using mulch as groundcover are some of the ways that encourage soil biodiversity in urban areas.


Alizabeth M. Bach, K. S. (2020). Soil Biodiversity Integrates Solutions for a Sustainable Future. Sustainability, 2662. 

Nihorimbere, V., Ongena, M., Smargiassi, M., & Thonart, P. (2011). Beneficial Effect of the Rhizosphere Microbial Community for Plant Growth and Health. BASE, 327-337. Retrieved from