What soil does, and what it does not do
If plants don’t eat soil, why do they grow in it?
Soil was not on the plant’s shopping list. Yet most plants you see grow in it.
So what is all that dirt for? It must do something.
A kitchen pantry stores food until you need it.
You eat the food, not the shelves. Soil is a bit like a pantry.
Soil is also like a pair of shoes. Shoes grip the ground, so you don’t slip.
Roots grip the soil, so the plant stands up tall.
Up close, soil is tiny bits of rock and bits of old, rotted plants.
Between the bits are gaps. Water fills some gaps, and air fills the rest.
- A · soil bits
- B · water
- C · air gap
- D · roots
Stir sugar into water, and it seems to vanish. It breaks into bits too tiny to see.
We say the sugar dissolveTo break into pieces too tiny to see and spread all through water, like sugar stirred into tea.
See it in the glossary.
Plant food in soil works the same way. It dissolves into the water in the gaps.
Roots take in that water and the food in it.
Roots need food dissolved in water, not the soil itself.
So mix plant food into water, hold the plant up, and add air. The plant can grow with no soil at all.
- A · net pot & seedling
- B · roots
- C · air stone
Your turn. Whose job is it?
Sort each job. Does soil do it, or not?
Which jobs does soil do?
Tap a card, then a bin. Or press 1 or 2 on a card.
- Nothing here yet
- Nothing here yet
Which jobs does soil do?
The answer:
Soil's job
- Holds the plant up Roots grip the soil, so the plant stands.
- Stores plant food Soil holds plant food until roots take it in.
- Holds water for roots Water clings in the small gaps.
- Keeps air near the roots Air fills the gaps that water doesn't.
Not soil's job
- Makes the plant's sugar Leaves make sugar from light, air, and water.
- Gives the plant light Light comes from the sun or a lamp.
- Is the plant's food Plants don't eat soil. They take in food dissolved in water.
Check 1 of 4
Which job does soil do for a plant?
Show the answer
It holds the plant up. Right. Roots grip the soil, the way shoes grip the ground.
Check 2 of 4
You stir sugar into water. It seems to vanish. What happened?
Show the answer
It dissolved. Right. It broke into pieces too tiny to see. It's still there.
Check 3 of 4
Soil gets packed down hard. There are no gaps left. What do the roots miss most?
Show the answer
Air. Right. Air lives in the gaps between soil bits.
Check 4 of 4
Why can a plant grow without any soil?
Show the answer
Its food can be dissolved in water. Right. Roots take in food mixed into water, with or without soil.
You just learned what soil does for a plant, and what it does not do.
Next up: Roots: drinking, eating, breathing.
Go deeper: the willow tree testOpenClose
In the 1600s, Jan Baptist van Helmont planted a 5-pound willow in 200 pounds of dried soil.
For five years he gave it only water. The willow gained about 164 pounds. The soil lost only about 2 ounces.
He decided the tree was made from water. We now know much of its weight came from carbon dioxide in the air.
Soil is also full of life. Fungi, bacteria, and earthworms break down dead plants and release nutrients for roots.
Clay and rotted plant matter hold some nutrients on their surfaces. That slows how fast water washes them away.
Small gaps between soil grains hold water against gravity, like a sponge. Bigger gaps drain quickly and let air in.
Sources
- Cogger, C. A Home Gardener's Guide to Soils and Fertilizers. EM063E. Washington State University Extension, 2014 (revised 2020). Link
- Gatiboni, L. "1. Soils & Plant Nutrients." In Moore, K. A., and L. K. Bradley, eds. North Carolina Extension Gardener Handbook, 2nd ed. NC State Extension, AG-831, 2022. Link
- Jones, G. B., S. Goode, and R. Simes. Soil: The Dirty Secrets of a Living Landscape. EM 9304. Oregon State University Extension Service, 2020. Link
- Jones, C., and K. Olson-Rutz. Plant Nutrition and Soil Fertility. Nutrient Management Module No. 2, 4449-2. Montana State University Extension, 2016. Link
- U.S. Geological Survey. Water, the Universal Solvent. USGS Water Science School. Link
- Kaiser, C., and M. Ernst. Hydroponic Lettuce. CCD-CP-63. Center for Crop Diversification, University of Kentucky College of Agriculture, Food and Environment, 2016. Link
- Nelson, M., G. Langellotto, and L. Nackley. Hydro Hints: Deep Water Culture. EM 9455. Oregon State University Extension Service, 2025. Link
- Saupe, S. G. Willows and Photosynthesis: Analyzing van Helmont's Classic Experiment. Concepts of Biology (BIOL115), College of St. Benedict/St. John's University, 2004. Link
- Shell, H. R. "van Helmont, Jan." Plant Sciences. Gale, via Encyclopedia.com. Link
- Schwarcz, J. The Powder of Sympathy. McGill University Office for Science and Society, 2022. Link