Soil Microbiota: The World of the Earthworm

A quick look at the base of your nearest tree may seem to offer nothing more than broken-down soil particles and pebbles, but this deceiving surface hides underneath it a whole world of living creatures operating to keep functioning a system that lets life thrive on Earth. The world of the soil’s microbiota, although mostly unseen by the human eye especially due to its large existing underground, is a fascinating one that has been around since an estimated 4 billion years ago.

A microorganism is a living organism that is invisible to the human eye and can thereby only be seen under a microscope. These can be categorised into bacteria, fungi, protozoa, algae, nematodes, and actinomycetes. Just one teaspoon of soil contains billions of microbes, more than the number of people on the planet. Scientists estimate that there are around one trillion species of microorganisms on Earth, of which 99% are yet to be discovered. It’s evident that there’s still much about these organisms that we don’t know, and research into this world has had recent beginnings and advances as scientists try to understand how their functioning can be advantageous against synthetic chemicals in soil. What we do know, however, is that these microorganisms make plant and animal life possible by contributing to certain processes that keep the life cycles going.

One such process is the nitrogen cycle, in which soil microbes convert ammonium to nitrates that are then absorbed by plants. Bacteria may be the most important microbes to aid in soil processes, acting along with actinomycetes to break down nutrients to be absorbed by plant roots, with certain actinomycetes also acting as antibiotics for the plant. Microbes like fungi also help keep the environment clean in addition to providing plants nutrients by decomposing dead organic matter through the process of decay.

The microorganisms aren’t alone in this job, however. They are aided by animals such as earthworm that rely on these microbes for nutrition. Earthworms may be the most important animals involved in the processes of keeping soil healthy and fertile. It’s fairly common knowledge that worms are found in the soil and that they take care of it, but the exact way they do this is a very interesting albeit-less widely known process.

These invertebrates have a segmented body structure that allows them to freely move through soil and create burrows or channels in the process. They feed on the soil’s microorganisms as well as decaying roots, leaves, and any other organic matter they may come across in the soil. As a result, they affect the soil microbial structure, although this relationship is still under study. Even the microbes found in their digestive tract help in the process of breaking down organic matter. This stimulation of existing microbes in the soil as well as the conversion of organic matter into microbes is how earthworms aid in the process of recycling nutrients in the soil. They produce an energy-rich mucus in the soil which activates the soil’s microorganisms. 

In addition to this, the channels that they create as they move through soil help increase soil porosity, allowing more air and water to seep into the soil to be easily absorbed by plant roots, and these roots are also able to grow more easily due to this. They move through the soil by rhythmically contracting and extending their body muscles in a process known as peristalsis, similar to how our food pipe functions. They also reduce soil-borne diseases by ingesting bacteria and help bind the soil.

Considered as the ecosystem’s engineers, they are crucial to agricultural growth by helping in the creation of vermicompost. This eco-friendly compost is made of organic matter broken down by earthworms, and when mixed with soil drastically increases the nitrogen, potassium, and phosphorus content that plants require to grow. The earthworms involved in vermiculture are different from soil-dwelling worms, however, and instead include the Brandling worm, the tiger worm, or the red wiggler. This is a much ecologically advantageous alternative to chemical fertilizers as they do not drastically alter the soil’s chemical composition for long periods of time or many crop cycles and are less hazardous to human health in the long run.

These hermaphrodites often occur in temperate and tropical soils. They can broadly be divided into three types, depending on their activity level and depth in the soil. Epigeic species are surface-dwellers, smaller in size and feeding on organic plant remains. As a result, they are better adjusted to variations in atmospheric conditions. Endogeic species live one level deeper, also feeding on organic plant and soil matter as they move through the soil, creating burrows and worm casts in their trail. Anecic species live deep in the soil and form very long burrows that reach the deepest layers of the Earth. They feed on organic matter that they come across in these depths. 

A high number of earthworms is usually an indicator of healthy soil, but this may not always be the case. Earthworms are not always advantageous to the soil, depending on the types of plants that the soil is hosting. For instance, an invasive species of earthworms has detrimental effects on temperate forests by excessively breaking down nutrients in the soil whereas these forests require thick layers of organic matter to be slowly broken down. As a result of the earthworms’ faster processes, the soil nutrients quickly get recycled and leached, and younger plants are not able to adapt as quickly as required to grow. Earthworm burrows in forests may also have negative effects by allowing the easier passage of water through the forest floor which can lead to still-standing water or flooding.

Earthworms can certainly be helpful in infertile soil by increasing the nutrient level along with organic matter, but as demonstrated by their activity in forests, their processes are advantageous only in moderation. Research is still underway with regards to exactly how they interact with soil microbiota and just how beneficial they are to the environment at large, but their biological, chemical and physical contributions to soil health cannot be ignored. Out of the 5000 and above species of earthworms discovered, only around 16 species do more harm than good. Despite it all, they are still the farmer’s and gardener’s best friends and the environment's custodian.

This world of activity underneath the soil below our feet is always going on, even when we’re unaware of it. But the next time you see leaves falling from tree branches and accumulating by the tree’s base, you can be well assured that billions of microbes and plenty of earthworms are making their way to them to keep our soil and environment healthy and clean.

Authored by Pooja V, Intern @ PfA Bangalore