Experiment Details

• Grade Level: 6th–8th Grade (adaptable for elementary through high school)
• Key Topics: Soil Science, Plant Growth, Scientific Method, Variables
• Estimated Time: 1 hour setup, 3 weeks observation
• Mess Factor: Low to medium (soil and water spills possible)

Purpose: To discover how different types of soil—potting soil, clay, and sand—affect the way a common plant grows.

Hypothesis: If plants are grown in different soil types, then the plants in soil with better nutrients and drainage will grow taller and healthier than those in poorer soil.

Step-by-Step Instructions

1. Label each pot clearly with the soil type: “Potting Soil,” “Clay Soil,” and “Sand.”
2. Fill each pot about 4 inches deep with its respective soil. Try to keep soil level the same in all pots.
3. Plant one seed about 1 inch deep in each pot. Cover gently with soil.
4. Water each pot with 1/4 cup of water, making sure soil is moist but not soggy.
5. Place all pots in a sunny location (inside on a windowsill or outside) where they’ll get the same amount of sunlight.
6. Water the plants every two days with the same amount of water (about 1/4 cup).
7. Measure the height of each plant every 3 days using a ruler. Record the measurements and observations in your notebook (plant height, leaf color, number of leaves, etc.).
8. Continue for 3 weeks or until you see clear differences in growth.
9. At the end, compare the average height, health, and appearance of the plants in each soil type. Which grew tallest? Which had the healthiest leaves?

Soil Science: Why Does Soil Matter for Plants?

Soil isn’t just “dirt”—it’s a complex mixture of minerals, organic matter, water, air, and living organisms. The quality and type of soil can make a huge difference for how plants grow. Healthy soil provides:

• Nutrients: Plants absorb essential minerals from soil, such as nitrogen, phosphorus, and potassium.
• Water retention: Good soil holds enough water for plants to use, but also drains so roots don’t drown.
• Air spaces: Roots need oxygen! Dense or waterlogged soil can suffocate roots.
• Support: Soil anchors plant roots and provides a stable place to grow.

Comparing Soil Types

• Potting soil is specially designed for plant growth, with a blend of peat moss, compost, perlite, and nutrients. It’s light, holds moisture well, and provides plenty of air pockets for roots.
• Clay soil tends to be heavy and dense. It holds water tightly (sometimes too much), can be slow to drain, and may limit air flow to roots. Some nutrients are present, but the density can make it hard for roots to grow and “breathe.”
• Sandy soil is loose and drains quickly. It has big particles and lots of air spaces—but it doesn’t hold water or nutrients well, so plants may dry out or miss out on minerals.

Why Test Soils?

Farmers, gardeners, and scientists test soil to see what plants will grow best in different places. The same plant can thrive in one soil but struggle in another. Understanding soil science is crucial for growing food, restoring habitats, and protecting the environment.

Plant Science: Roots, Leaves, and Growth

When a seed sprouts, roots grow downward to search for water and nutrients, while shoots grow upward to find light. Roots need soil that lets them spread out and absorb what they need. If soil is too dense (like clay), roots struggle. If soil is too loose (like sand), roots may not get enough water or nutrients. Potting soil strikes a balance for most garden plants.

What’s Happening in This Experiment?

• Plants in potting soil will usually grow taller, with greener leaves and stronger stems.
• Plants in clay soil may show slower or stunted growth, with pale or curled leaves due to lack of air or root space.
• Plants in sandy soil may struggle to sprout or show wilting/yellow leaves due to dryness and low nutrients.

The experiment teaches kids (and parents!) how important it is to keep water, sunlight, and all variables the same—so that soil is the only thing changing. That’s good science!