Science in the garden: Why soil matters

4 Jan

Winter is a great time to rest, whether you’re a plant or a person, so we’re all taking advantage of the lull in the garden at George Watts Montessori. Some of our vegetables, like the cabbages and carrots, are close to being harvested, and everything else has already been plucked or slowed down until spring.

Mulching with straw

Given the natural hiatus, it’s an ideal time to do some soil-and-erosion lessons with students in the garden.

Soil happens to be a garden lynchpin, in more ways than one. Obviously, you need healthy soil to grow things.

But soil is also a great way to connect the garden with the science lessons kids have to learn anyway — like, lessons about humus and soil absorbency.

My granddaddy, 1970s

What I think the garden can do that a book can’t is help kids understand why they should care about soil in the first place.

These are two of the activities we’re going to do with our elementary school students to directly connect science curriculum (N.C. Standard Course of Study) with the school garden.

You can find these sorts of hands-on activities from a range of sources, but this is from one of my favorites — Evergreen, an inspiring Canadian group that aims to deepen the connection between people and outdoor spaces. (I was lucky enough to hear a presentation by the director of the group’s Learning Grounds program at this year’s NC Outdoor Classroom Symposium.)

Work with your teacher liaisons on this. I found these specific activities after sitting down with a Lower Elementary teacher (1st, 2nd, 3rd grades) who is also on our school’s science committee. She told me about what will appear on science benchmark tests, what science “kits” classrooms receive during the year, and what she needs to cover as part of the N.C. Standard Course of Study for science.

Ask teachers what lessons they want to spiral review in the garden. What activities make sense? What materials should the garden have to make it a true outdoor classroom?


LESSON: The value of topsoil

THE BIG QUESTION: Why is topsoil important in our world? How does healthy topsoil affect our lives?

Worldwide, 25 billion tons of agricultural topsoil are swept away every year. That’s 7% of the globe’s good growing land every decade. In these activities students will come to appreciate how little topsoil there is in the world and how it can easily be lost through erosion. In the garden, students will discover soil stewardship techniques to build soil health and protect soil from erosion.

Activity #1: If the Earth Were an Apple…

Use an apple to demonstrate the need for soil stewardship.


• Apple

• Sharp knife


1. Ask the students what they know about soil. What is it? Why do you think plants need it? How does it help humans?

2. Show the students the apple and give the following demonstration:

Let’s say this apple is the Earth.

Cut the apple into quarters and set three of the quarters aside.

Three quarters of this apple represent all the oceans on earth. The remaining quarter represents all the land on earth.

Cut the remaining quarter in half and set one piece aside.

One half of the land is unfit for humans; it is either too hot, like a desert, or too cold, like the north and south poles.

Cut the remaining piece into quarters and set three of them aside.

Of the land that humans can live on, only this small piece is land that we can grow food on. The rest is too rocky, or there isn’t enough sun for plants to grow.

Peel the remaining piece.

This thin peel represents the thickness of the soil in which we grow our food. It is only about three feet deep. This tiny portion is the only area out of the whole earth where all the right conditions exist to grow food. Enough food has to be produced on this small bit of land to feed all of the people on earth.

3. With so little soil in the world, what should people be doing to take care of it?


Activity #2: Erosion

It takes over 100 years to produce just an inch of soil, and in many parts of the world existing soil is lost as much as 18 times faster than new soil is formed. This activity demonstrates to students how this important resource can be lost through erosion by rain and wind when the garden is left bare for the winter.


• Four waterproof trays — you can use old baking sheets with rims

• Sod to fit snugly in one of the trays

• Soil

• Watering can


1. In the winter, there is usually plenty of rain and wind. What do you think the rain and wind will do to the soil? When soil or rocks are moved from one place to another by rain or wind, we call this erosion.

2. Ask the students to make predictions about what will happen when the wind blows across the tray filled with sod and the tray filled with soil.

3. One at a time, hold the trays filled with sod and soil over one of the empty trays. Invite a few students to blow across each tray to simulate wind. Notice how much soil collects in the empty tray.

4. Lean the two sod- and soil-filled trays on an angle against a wall, with the base of each resting inside one of the empty trays to catch residual water and soil

5. Ask the students to make predictions about what will happen to the soil in each tray when water is poured across the top of them.

6. Pour an equal amount of water across the tops of the two trays for 5 seconds and observe the runoff that has collected. Which tray lost the most soil?

7. What happens to soil when the wind blows on it or it gets rained on? What might prevent soil from eroding? What could we do in our garden to prevent erosion?

We can point to three erosion-prevention tactics we’re using in our garden:

1. COVER CROPS (We’ve planted crimson clover.)

One of the ways students can protect their garden soil during the winter is to plant a cover crop. Cover crops are usually non-edible crops and often include a mix of different plants with roots that can keep soil from washing away during heavy rains and draw nutrients from deep down in the soil. Planting a cover crop also provides competition for weeds; well-established cover crop will shade out weeds entirely. In the spring, the cover crop is pulled up and added to the compost or cut down, left to dry and dug into the soil.

2. OVERWINTERING CROPS (In our garden, garlic and onions are examples of overwintering crops; we’ll harvest them in the spring.)

In addition to planting cover crops, a portion of the garden can be set aside for planting overwintering crops. These crops start their growth slowly in the fall and winter and do the rest of their growing when the weather warms up the following year.


Another way for students to protect the soil in the garden is to cover it with mulch. Mulch is a layer of organic matter put on the surface of the soil and may include materials such as straw, hay, leaves or compost.

A useful analogy is the annual shedding of leaves in the forest, creating a layer of leaf litter on the forest floor. Both fallen leaves and mulches provide a layer of insulation, suppress weeds, decompose to add nutrients and organic matter to the soil and provide a place for helpful insects to hide. Just like the forest floor doesn’t turn itself over, there is no need to dig mulch into the soil. Natural processes help to incorporate the mulch into the soil, and any mulch that hasn’t decomposed by the spring can be raked up and added to the compost.

** When you’re finished with this activity, return soil to one of the vegetable beds, the compost pile or to the bin where you found it.

** Lesson Extension: Have students walk around the school grounds and identify evidence of soil erosion. If they have access to a video camera, use it to document the examples. How could the examples of erosion be prevented?

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  1. Making the jump from garden to Living Classroom « Growing Gardeners - February 8, 2010

    […] soil can be lost when rain and wind cause erosion — and why kids should care. (I posted the text for this lesson […]

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