Friday, September 23, 2011

Components of Productive Soil

At my Agriculture workshop this summer we had an excellent lesson presented to us on the components of productive soil. I loved that the presenter did it just as he would with the children, so we got to participate and understand how to do it with kids. As adults, we understand the idea of percentages, so I had to find some way to make that concept accessible to my students. First I thought of a pie chart but I couldn't find anything that was already divided into 20ths. So we used nickels and I told them that you should think of soil as a dollar. There are four things that make up good healthy soil that plants grow well in. What could they be and how much of each is needed?

I had four paper lunch bags labelled A, B, C, and D. They are already on the tabletop; you don't pass them around. Each volunteer is called to come up and feel what is in their bag -- without looking -- and to describe what they feel to the class, not to say what they think is in there.

Bag A is several handfuls of rocks. Crushed rocks (parent material) and minerals make up 45% of our soil. After the person who had bag A got the class to guess that it had rocks, I asked the class to guess how many nickels worth of Crushed Rocks/Minerals were in productive soil, soil that is good for plants to grow well in. Half? More than half? Less than half? I told them that it was a little less than half, nine nickels. I laid out nine nickels in a row on the floor and put a label by it that said
45 cents
crushed rocks and minerals

Bag B is a glass with Water in it. After the class figured out that water was in bag B, I asked them, how much water do you think needs to be in the soil for it to be good for plants to grow in? It is five nickels (25%). I laid out five nickels in another row, under the first row, and put a label by it
25 cents

Bag C had a handful of hay to represent Organic Matter. I asked the class, once they knew what was in the bag -- having them try to hear the description and guess what's in the bag is so much fun for them! -- how much compost and decaying plant and animal matter do you think needs to be in the soil for it to be rich and healthy? We have a vermicomposting bin in our class as well as individual student composting bins, so we have made a big deal of this topic. They thought it would be 25 cents out of the remaining 30. Which is what adults usually guess, also. They were surprised to find that it is only 5%, one nickel!
One nickel in the next row with a label by it
5 cents
decaying organic matter

So what's in bag D?

Students couldn't wait to find out. They knew that it was a large percentage since 25 cents was remaining. I called the final student to feel inside the bag and guess. This is a tricky one to describe since Bag D is empty. The final component is Air. Can you believe that arable soil is 25% air? Our workshop presenter explained to us that the reason our grass dies when you walk on it in the same path over and over is that the soil gets so compacted that the plants can't breathe.

The last five nickels in the final row with a label
25 cents

Our presenter had a nice suggestion for doing this lesson with older students. His idea for a modification was to give the children all four components and have them mix them up into different proportions (you'd have to buy already crushed minerals of course). Then have them plant seeds in the different mixtures and observe and record how they grow. From this you can lead the students to come up with their own conclusions about the best percentages.

By the way, those interested in fertilizing with minerals in addition to compost might like to visit

The other lesson he presented to us is the famous "Apple Lesson." This can be found in many places online but I think the best description of it is here. If you want a version which also discusses how much food is in the ocean, I recommend this lesson.

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