1. How Many Microorganisms Live in a Pinch of Soil?
The number of microorganisms in 1 g (about 0.035 oz) of soil is said to be "over 100 million" [4]. However, this is a wide estimate that changes greatly depending on the counting method and soil type. Different sources show ranges from millions to over hundreds of millions.
Besides microorganisms, soil animals also live there. They are divided by body size: large (larger than 2 mm (0.08 in), like earthworms, ants, and centipedes), medium (0.2–2 mm (0.008–0.08 in), like mites, springtails, and nematodes), and small (smaller than 0.2 mm (0.008 in), like protozoa) [4]. By diet, they are divided into three groups: decomposers that eat dead plants, predators that eat other animals, and herbivores that eat living plants. Decomposers are thought to be the most numerous group [4].
These creatures vary in size and diet. Who eats whom? Let us look at fallen leaves as an example.
2. How Do Many Soil Animals Break Down Fallen Leaves Step by Step?
A nature observation resource from Shimane Prefecture explains the journey of fallen leaves [4]. First, earthworms, woodlice, springtails, and mites eat the leaves and break them into small pieces. Predatory creatures like pseudoscorpions, centipedes, and spiders then eat these bugs. Moles and small birds eat those predators.
Leftovers and droppings are decomposed by other insects or microorganisms. The nutrients created soak into the soil with rain and are absorbed by plant roots.
Here, animals and microorganisms share roles. When animals break down leaves, it becomes easier for microorganisms to decompose them. The work of mixing soil to improve aeration and drainage is mainly done by animals like earthworms [4]. So far, we have looked at "being eaten and decomposed." But how do usable plant nutrients appear during this feeding?
3. Why Do Nematodes That Eat Bacteria Release Nutrients for Plants?
In the soil, there are nematodes (roundworms) that eat bacteria. When they eat bacteria, the nitrogen from the bacterial body enters the nematode. However, when more nitrogen enters than the nematode needs, the excess is released outside the body as "ammonium." Plants can use this ammonium directly [4].
Think of it like passing uneaten food to a friend at the next seat. But the nematodes are not eating for the plants' sake. It is just that what remains happens to reach the plants.
In soil with nematodes, the reaction where nitrogen changes into a form plants can use (mineralization) is said to be fast [4]. The "leftovers" from the eaters create the flow of nutrients in the soil. If these creatures are present, do plants always benefit?
4. Do Plants Always Benefit When Soil Animals Are Eating Microbes?
A review article summarizes that many studies show nematodes that eat bacteria and fungi help plant growth by advancing nitrogen mineralization. However, the effect changes depending on the species of nematode, microbe, and plant, as well as food conditions (C/N ratio) [4].
In pot experiments with protozoa that eat bacteria, nitrogen mineralization and plant nitrogen absorption increased [4]. But in this experiment, when water disappeared from the tiny spaces in the soil, the protozoa's activity stopped. For small creatures, water is a prerequisite for movement.
This means we cannot say for sure that "soil with eating creatures is good for plants." Positive effects can occur, but they depend on conditions. So far, the soil food web was described as a single line: "eat leaves → eat bugs." This map is now being reviewed.
5. Do Springtails Really Eat Fallen Leaves?
Springtails were long thought to feed on fallen leaves. However, a 2021 study (Biology Letters) showed that many springtails in forests have carbon in their bodies that is "younger" than fallen leaves. They are thought to rely on carbon from living plants, which was recently made by photosynthesis [4].
Some predators that eat springtails also showed carbon ages younger than fallen leaves. It is believed that what springtails eat affects the entire food web [4].
A presentation from the Forestry and Forest Products Research Institute stated that many springtails in the studied forest relied on carbon younger than leaf litter. The soil food web is not just a "brown food web" driven by dead organic matter, according to the explanation [4]. This does not mean the previous chapter's flow was wrong. It means the main players can change in one study. How vast is the underground world?
6. What Share of Earth's Species Live in Soil?
In 2023, a paper published in PNAS estimated that "soil is considered the habitat for about 59% (range: 44–74%) of Earth's species." This is about twice previous estimates [4]. This is an estimate based on many documents, not a count of each species. "About 60%" is a rough guide and should be read with its range in mind.
Another review (Nature, 2014) argued that underground organisms influence plant diversity and ecosystem functions, and determine responses to environmental changes. The University of Manchester stated there are "millions of species and billions of individuals" in one grassland or forest [4]. However, these numbers are rough estimates by researchers, not data from a complete count.
Diverse organisms live in the soil beneath our feet and are connected by eating. Let us look for this at the end.
7. How Can You Observe Feeding Under a Fallen Leaf?
Try this: In a park or forest, turn over one fallen leaf and look with a magnifying glass. Write down what creatures you see and what they seem to be doing. The creatures you see are only a tiny part of the underground world.
For safety, do not touch biting insects like centipedes with bare hands. After observing, wash your hands and return the leaves to their original spot.