The Plant-Soil Relationship

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Students investigate the relationship between plants and soil.


  • Student observation journals
  • Chalkboard or dry-erase board


Without plants, the earth would be barren, its surface unprotected from the effects of sun, wind, and rain, and its soil composition too poor to sustain life. Plant roots help to prevent erosion, and when plants die, they become the raw material for worms, insects, and microbes to build the nutrient-rich humus that supports robust food webs. (Recently, researchers have discovered that living plants secrete excess carbohydrates through their roots to encourage growth of microbes!) You can learn more about soil composition in Spotlight on Soils.

Laying the Groundwork

1. As a class, discuss whether and how soil is important for plants. Ask, What do you think soil does for plants? (Provides a place to anchor roots, nutrients, water, air.) Ask, Have you ever seen plants growing without soil? Where? Explain that some plants, including certain aquatic and parasitic plants, have particular adaptations that allow them to meet their basic needs without soil. Ask, Do plants need soil?

2. Delve deeper into the plant-soil relationship. Ask, Other than mineral particles, what is an important part of soil? Share the background information about microorganisms -- fungi, bacteria, and other decomposers -- and discuss the role they play. 


1. Find out what your students know or assume about the plant-soil relationship. Ask, Plants need soil, but does soil need plants? Why? Record all answers and supporting reasoning, then visit the schoolyard or a nearby park to make observations. Have teams investigate the soil in different areas (e.g., garden beds, lawns, weedy patches, woods, a compacted area along the edge of a driveway or sidewalk) and record their observations in their journals. Make a second chart, summarizing these findings and take a vote. Ask, Who thinks soil needs plants?

2. Collect some soil from your schoolyard. Put it in a large zippered plastic bag along with some organic materials (vegetable scraps, plant clippings, old leaves). For comparison, add these same organic materials to a second bag, but do not add soil. Moisten the contents of the "compost" bags, seal them, punch a few air holes in them, and leave them in a warm part of the classroom for a week or two, while the class observes what happens. Then ask, Are the contents changing? How? What do you think might be causing this change? Where have you seen examples of once-living things changing and decomposing outdoors? (Rotting logs, moldy garbage, compost piles.) Did some materials seem to decompose more quickly than others?

Making Connections

Discuss the explorations. Ask:

- In what ways do you think plants depend on soil? Based on your observations, can you imagine how soil might depend on plants or animals? In what ways do animals -- including humans! -- depend on soil?

- Why do you think materials might break down quickly in soil? What do you think might happen to once-living things that decompose in soil? How might these once-living things help to support life?

Branching Out

- Find out how plants grow in different types of soils. For instance, fill a pot with heavy clay soil, another with sand, and a third with loam, and plant bean seeds. Provide equal amounts of water, fertilizer, light, etc., to each pot and chart the resulting growth. Students might also find out how worm castings or compost affects plant growth.

- Investigate plants that are adapted to survive in specific soil conditions (e.g., many bog plants are carnivorous because acidic bog soils lack adequate nitrogen).

- Conduct a simple simulation to get kids thinking about erosion. Fill a shallow pan with soil, prop it up about two inches at one end to create a slope, and set up a collecting basin below the pan for runoff. Holding a watering can a foot above the soil, sprinkle 'rain' for a minute or two. If necessary, help students make connections between the simulation and what can happen outdoors, then discuss techniques farmers and gardeners use to reduce or prevent erosion. (Mulching, terracing, cover crops, adding organic matter to improve a soil's water absorption.)

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