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Nature and farming · Soil and fields

Soil is alive

A handful of fertile soil is home to billions of bacteria, metres upon metres of fungal threads and countless tiny creatures. Explore soil layers, humus, Darwin's earthworms and mycorrhiza, and find out why 1 cm of topsoil takes hundreds of years to form.

  • 17 min
  • 2 simulations
  • Mini quiz

Updated:

Preview

The question

Pick up a handful of soil from a garden. To your eyes it looks like an ordinary brown powder, with no life in it at all. Yet that handful may hold more bacteria than there are people on Earth, fungal threads that could add up to hundreds of metres in total, and hundreds of tiny animals. Is soil simply a place where living things live, or is it a kind of living system in its own right? And what does this invisible crowd have to do with whether a field is fertile?

Try it yourself

No account needed: change the values and check your answer. Some simulations ask for a prediction before they start; it doesn't matter whether it's right or wrong.

Soil layers

Order the soil profile

Select a layer and move it with the up and down buttons; the buttons work with the keyboard too. The drawing of the profile updates with every change. When you have finished ordering, press Check.

Prediction

Predict first

Where do you think the darkest layers in a soil profile are?

It doesn’t matter whether you’re right; the simulation opens once you choose.

Soil layers

Division of labour underground

Choose a job from the list for each organism; a job can be given to more than one. When you have matched them all, press Check.

Prediction

Predict first

What do you think is the main workforce that turns dead leaves on the forest floor into humus?

It doesn’t matter whether you’re right; the simulation opens once you choose.

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Embed the simulation on your site

Paste this code into the HTML of your blog or school site. Visitors try the simulation without an account; no progress is saved.

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Frequently asked

5 questions

Is soil just crumbled rock?

No. Mineral particles are the skeleton of soil, but soil also holds humus, water, air and countless living things. A common textbook example for a fertile loam is roughly 45% minerals, 5% organic matter, 25% water and 25% air by volume.

Isn't it bad to have fungi and bacteria in soil?

Some cause disease, but the great majority break down dead matter and return nutrients to the cycle, or work together with plants. Mycorrhizal fungi, for example, carry water and phosphorus to roots. Without them the nutrient cycle would stop.

Why does humus matter so much?

Humus holds water like a sponge, keeps nutrient ions on its surface so the rain does not wash them away, and sticks particles together into loose crumbs. Soil that is dark and crumbles in your hand is usually rich in humus.

Where does the smell of earth after rain come from?

The best-known part of that smell is a molecule called geosmin, made by some soil bacteria, above all Streptomyces. The human nose can detect geosmin even at very low concentrations.

Doesn't lost soil grow back by itself?

It does, but very slowly: according to the FAO, forming 1 cm of topsoil can take hundreds to a thousand years depending on conditions. On a bare, sloping field a single heavy downpour can carry away several millimetres. That is why terraces and plant cover matter.