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The All-Important Soil -- The Dirt on Dirt




Trowel, a large, clear jar with a lid, water, Sharpie pen


A stick of modeling clay, a little flour, a little light sand, and a little coarser sand or small gravel


Zip-lock bag and trowel or sturdy spoon for each student

Small empty yogurt cup or paper cup for each student

Sharpie pen

Pencil or sharp object to poke holes

Bean seeds

Water spray-mister

Bag of top-quality potting soil


Two small rocks and a piece of paper for each student


Pound bag of M&M's and a napkin for each student



The most important part of garden planning is to plan to have the best possible soil in which to grow your plants. Let's explore what makes good garden soil.


There are three types of soils: clay, silt and sand.


First, let the students feel the modeling clay. That kind of soil sticks together too much.


Next, let them feel a little flour between their fingers. That's what silt is like. If you've ever gone wading in a stream and picked up some soil at the bottom or on the side, that's silt. It's pretty fertile. But you don't want to have too much of it. You need a little clay - just not too much.


Then let the students feel the light sand and the coarser sand or gravel. You need a little of this in the soil to make it easier for the roots to push through and grow. But too much rock in the soil makes it drain too fast to do the roots much good, and makes the soil less fertile.


Now go out to the area where you're going to have your garden. Show the students how to hold and use a trowel. You can ask for a volunteer to dig about six inches down, and collect a soil sample that will fill the jar about half full. Fill the jar up with water.


The students should take turns shaking the jar for several minutes until it has mixed together well. Then let it sit still for one minute. At the one-minute mark, use a Sharpie to mark the layers that have settled.


The heavier materials will sink to the bottom, and the lighter materials will float to the top. You may have gravel or thick sand at the bottom, a thin line of brown silt, another thin line of darker soil, which is the clay, and then a lot of tan-colored water with some leaf or bark bits floating on top.


At the end of your time together, look at the jar again. Depending on which layer is the thickest, the students will know what the texture of their garden soil is. Sand on the bottom - silt in the middle - clay on top - then the water, and floating on the very top will be organic material. If there isn't any organic material, you need to add some!


Next, give each student a zip-lock bag, a digging tool, and instructions to go home or around the neighborhood to public places, like the school playground or a park, and collect a soil sample to bring to your next meeting.


They are NOT to seek good-quality garden soil, but dirt from places that have been neglected or where plants aren't growing a lot.


You could take the walk together and gather samples, making sure to get a variety of soil types.


About a cup of soil will do. Instruct them NOT to go onto private property, unless they ask first and are invited.


Compare the colors and textures. Is one soil sample kind of orange and sticky? That's clay soil - too sticky. Is another example gray and sandy? That will drain too fast for the roots to get much nutrition, and it's not very fertile. Are there hard clods? That'll be tough for plant roots to draw nutrients from. Do some bags contain gravel and a lot of sticks or even small bits of trash? That's not good for gardens.


Since plants have to get their nutrients from the soil in order to make their own food, the quality of the soil they're growing in is very important.


Let's compare how different kinds of soils work as an environment for plants.


Give each student a cup and let them write their names with the Sharpie. Make sure to poke a couple of drain holes in the bottom of each cup, and have each student fill their cup with the soil they brought. Put a bean seed on the top and gently push it down about two times as deep as the seed is wide. Gently push soil on top, and mist until moist.


Have the students bring their cups home, spray-mist or water sparingly every day, and keep in a sunny windowsill or counter. Set a date for them to bring them back in one month, and measure whose soil was the best based on whose plant has the most leaves. If their seed doesn't sprout, they should still bring their cup in and discuss what went wrong.


Now let each student feel a handful of the quality potting soil that you brought. When you gently squeeze a handful, and then release the pressure of your fingers, it should sort of stick together, and sort of fall apart - like the crumbles from a moist chocolate cake, or brownie mix with a little water in it.


The "tilth" of a soil refers to its physical condition. The kind of tilth that you want is called "friable" (pronounced "FRY-uh-bull" - but it doesn't have anything to do with French fries!). Friable soil is crumbly and moist. It is loose, will drain well, and is fairly easily crumbled even though it will loosely stay together.


Nice, friable soil is called "loam." That's what you want. There should be a little silt, a little sand, and a little clay, in good proportion. There should be a lot of tiny air pockets in the soil, to allow the roots of plants to get air, and to provide space for worms and microorganisms to work their magic. If clods form in good garden soil, they are easily broken up. The color should be dark brown, representing a lot of "organic," or live, soil parts, and there should be a distinctive smell of fresh earth.


Make one last bean planting with this quality soil, and you be the one to take care of it for a month. When the students bring theirs in, chances are, your plant will be the winner.


Where does soil come from, anyway? Here's how to find out:


Give each student two rocks, and a piece of plain, white paper. They are to knock the two rocks against each other for one minute. Notice what happens on the paper? Little crumbly pieces appear. That's how dirt gets started!


But there's a lot more to dirt than just rocks. Wash your hands with soap and water, then give each student a napkin, and open the pound bag of M&Ms. Distribute a handful of M&M's to each student's napkin.


As a group, decide which color of M&M will represent each of these soil components:


§         Rock

§         Clay

§         Sand

§         Decomposing animal manure (after a short time, it doesn't smell bad any more!)

§         Decomposing plant parts - leaves, grass, bark, etc. (humus, pronounced "HYOO mis")

§         Microorganisms (tiny creatures that break down the plant parts as their food)


So if your group decided that little bits of rock will be represented by brown M&M's, and a student has a lot of those, you could pretend that that student has rocky soil. What's to be done? Add fertile soil amendment (a word that means "change") in the form of compost - also known as humus -- decomposed plant materials rich in nutrients, that looks like soil, because it is!


If the red ones represent clay, and one student has an excess of red candies, that can be fixed by adding humus and a little sand to light up the texture, and perhaps tilling the garden, which means using a machine to turn it over and crumble it up.


Sandy soils can be fixed by adding humus.


If you decide that the yellow M&M's represent humus, and a student has a lot of yellow ones, that student's garden is going to be nice and fertile.


No matter whether you like or dislike the soil mix you got based on the colors of the M&M's, go ahead and eat them now! But remember - we don't eat dirt! We eat the stuff we grow in it!


By Susan Darst Williams • • Planning 09 © 2010


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