What Do Plants Really Need?

Overview

Children are told that plants need sun, water, and soil. But how do they know that's true? This experiment moves them from belief to evidence. By growing identical seeds under different conditions — changing one variable at a time — they'll see for themselves what happens when you take away light, withhold water, or skip the soil. This is the scientific method in miniature: question, hypothesis, experiment, observe, conclude.

The Question

"Do plants really need ALL three things — light, water, and soil — to grow? What happens if we take one away?"

Background

Plants are autotrophs — they make their own food through photosynthesis. Light provides the energy. Water is both a raw material for photosynthesis and the transport system for nutrients. Soil provides minerals, anchorage, and a reservoir for water. Take away any one of these and the plant suffers — but the way it suffers tells you what each element does.

You don't need to explain photosynthesis to a 6-year-old. The experiment will show them the results. When they see a plant that's yellow and leggy from no light, or shriveled from no water, the conclusion draws itself.

Why bean seeds: Beans are ideal for this experiment because they germinate fast (3-5 days), grow visibly from day to day, and are large enough for small hands to handle. Radish seeds are a good alternative — even faster but smaller.

Hypothesis

Before planting, ask your child to predict:

"We're going to grow beans four different ways. I want you to guess what will happen in each group."

Write down their predictions in the notebook. Don't correct them — even if they predict the no-water group will grow fine. The experiment is the teacher.

Materials

See the frontmatter list. Total cost: under $10 if you need to buy seeds and soil. Everything else you likely have.

Procedure

Setup Day (30 minutes)

Step 1: Prepare the pots

Label 12 cups with masking tape: A1, A2, A3, B1, B2, B3, C1, C2, C3, D1, D2, D3. Three cups per group gives more reliable results than one.

Poke 2-3 drainage holes in the bottom of cups A through C using a pencil or nail (adult does this). Group D doesn't need pots.

Step 2: Plant the seeds

• Groups A, B, and C: Fill cups with potting soil to about 1 inch from the top. Push a bean seed about 1 inch deep into the soil. Cover gently. Water lightly until the soil is damp but not soaked.

• Group D: Fold paper towels into the bottom of the cups. Place bean seeds on top. Moisten the paper towels thoroughly.

Let your child do the planting. Show them how deep to push the seed. "The seed needs to be tucked in — not too deep, not too shallow. About the length of your fingertip."

Step 3: Place the groups

• Group A (control): Sunny windowsill. Will receive daily watering.
• Group B (no light): Inside a closed closet or covered with a box. Will receive daily watering. "These get water but no light. Let's see what happens."
• Group C (no water): Sunny windowsill. Will NOT be watered after today. "These get light but no water after today."
• Group D (no soil): Sunny windowsill. Will have paper towels kept moist daily. "These get light and water, but no soil."

Step 4: Start the observation journal

In the notebook, create a simple chart:

Daily Observations (5 minutes per day for 14 days)

Each day, your child:

1. Checks each group. Opens the closet for Group B (close it again after checking).
2. Waters Groups A, B, and D (not C).
3. Measures any growth with a ruler.
4. Records observations in the notebook: height in centimeters, color of leaves/stem, number of leaves, general condition (healthy, wilting, no change).
5. Draws a quick sketch every 3-4 days showing what each group looks like.

"What do you notice today? Is anything different from yesterday?"

The daily routine matters. Science is patient, consistent observation — not a single dramatic moment.

Record

By Day 14, the notebook should contain daily measurements and observations. The differences between groups will be dramatic:

• Group A (control): Healthy, green, tallest
• Group B (no light): Alive but pale yellow, thin, leggy — stretching toward any light source
• Group C (no water): Sprouted initially (using the seed's stored energy), then wilted and dried out
• Group D (no soil): Sprouted and grew for a while (bean seeds have stored nutrients), but growth will slow or stop as nutrients deplete

Analysis

Sit down with the completed journal. Look at the data together.

"Which group grew the best?" (Group A.)

"What was different about Group A compared to the others?" (It had everything — light, water, and soil.)

"What happened to the beans with no light?" Discuss the yellow color and thin stems. "Without light, the plant couldn't make its green color — that green is called chlorophyll, and the plant needs light to make it."

"What happened with no water?" Discuss the wilting. "Water is like blood for a plant. It carries nutrients and keeps the plant firm and upright."

"What about the beans with no soil?" This one is interesting — they often grow initially. "The seed had enough food stored inside it to START growing. But without soil to give it minerals, it ran out of food."

"Were your predictions right?" Go back to the hypothesis page. Compare predictions to results. No judgment — wrong predictions are valuable. "That's how science works. You guess, you test, and you learn from what actually happens."

The Explanation

"Plants need three things to grow: light for energy, water for transport, and soil for nutrients. Take away any one, and the plant suffers. Today you proved that with evidence — not because a book told you, but because you ran the experiment yourself. That's what scientists do."

Extensions

• Add a variable: What about temperature? Put a Group E in the fridge (with light from the fridge bulb and daily watering). Does cold slow growth?
• Music experiment (myth-busting): Some people say plants grow better with music. Set up a test group with music and a silent control. Is there a difference? (Spoiler: probably not — but the process of testing the claim is the point.)
• Longer observation: Keep Group A growing for a month. If it's a bean plant, it may flower and produce beans — completing the life cycle.
• Graph it: For ages 7-8, plot the growth data on a simple bar chart or line graph. Visual data representation is a real science skill.

Safety Notes

No significant risks. Remind children to wash hands after handling soil. If using mold-prone environments (dark closet), check Group B for mold growth — discard if it appears. Bean seeds are non-toxic but should not be eaten raw.