Show Me Your Math

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Show Me Your Math

Soil Degradation

Soil Degradation: how do we know soil is better than other soil and what does that mean?

Curriculum links

  • Quantitative analysis
  • Measurement
  • Spatial reasoning
  • 3D shapes
  • Fractions
  • Critical thinking

Earth as an apple: How much soil is useful for growing food?

Grab an apple and hold it in your hands. Imagine it as the whole beautiful world sitting in your palm.

Take the apple, and with the help of an adult or older sibling cut it into 4 pieces.

In our world 3 of those 4 pieces would be covered with water. Set three pieces aside and you are left with ¼.

¼ of the world is land. 

Take the ¼ slice and slice it in half (what fraction are you left with?)

One ½ of the ¼ is the amount of land that is unsuitable for growing food because it’s covered in mountains, deserts and ice. You can set that half aside with the other pieces. 

⅛ of the world’s land is suitable for farming.

Carefully cut that remaining ⅛ into four equal pieces. Set aside three of those pieces. Those three pieces represent soil that is too poor to grow food and or is covered by buildings and roads. 

The ¼ of the ⅛ you are left with is the food growing soil available on earth (1/32)

Now very carefully peel the skin off the apple and set the rest of the apple aside. All plants grow in the topsoil which is a very thin layer relative to the earth’s crust. The skin of the apple that you just peeled off represents all the available land to grow food on earth. That is the land that feeds over 7 billion people fruits and vegetables. 

This is why it’s important to protect soil from getting degraded or lost. Once we lose it or damage it-it is incredibly hard to get it back.

What is soil degradation?

Soil degradation is damaging or losing soil so that it becomes unusable. We can degrade soils in a few ways. One way is to cause erosion by removing plants from the soil. 

You can see this in nature all the time. The roots of plants hold soil in place and keep it from falling away in a heavy rain. Next time it rains look at a patch of earth with grass and then look at earth without grass. In earth without plants the soil gets pushed around by the water and can get taken away as the water drains. 

How plants prevent soil loss

  1. Grab two buckets of water or cups
  2. In one cup or bucket drop a patch soil with some grass growing in it
  3. In the other drop a patch of soil with nothing growing in it
  4. Note the differences in the two cups or buckets, what do you notice?
  5. Think of a way we could measure the differences we see between the two cups. Could we weigh both patches of soil?
  6. How could we get soil back after it has washed away?

What plants do for soil

Soil Compaction

Another way that we destroy soil is by overworking it. In order for soil to grow plants it needs to have microscopic pores that allow air and water through. Plants are like you. They need water to grow but too much water is not good for them. Plants need water to drain so they can take what they need and then get a break.

When we overwork soil the microscopic pores become squished and get compacted and it takes a lot of work to get those pores back. Without pores the plants can’t grow.

Soil pores are also why good soil is so hard to find. Too many pores and the plants can’t hold on to water, and too few and they get soggy and over flooded. 

You can see compaction easily with a cup and some water.

What soil compaction looks like

  1. Take two cups and fill them with identical soil. To find soil, look somewhere plants are growing in a park or in your neighbourhood (don’t go into someone else’s garden bed though). 
  2. Fill cups with the same amount of soil and note how much of the cup the soil fills up.
  3. In one cup push the soil down as hard as you can. Note how much less space the squished up soil takes up. You have removed the trapped air pockets and reduced the volume of the soil. 
  4. Pour some water on both cups and watch what happens. 
  5. Does the water drain the same in both cups?
  6. How could you measure the difference between compacted and uncompacted soils?

What is a good soil made out of?

All plants have specific needs and characteristics that make them better suited for certain soils (think dessert = cactus). But there are a few key components of soil that make it healthy to the majority of plants that we eat and those are: drainage, nutrients, and water retention. 

Soil is a mix of inorganic and organic materials. 

You can think of inorganic as the rocks and minerals in the soil. When you rub soil between your fingers the gritty parts are the small fragments of rocks that have been weathered over thousands of years. 

Sand, silt, and clay are the inorganic components of soil. Good soil has a mix of all three and it’s usually called loam-you can impress your friends with that word ;). 

Characteristics of sand, silt, and clay

You are going to play the role of soil scientist and evaluate the sand, silt, and clay composition of a few soils. 

  1. Pick out some soils that interest you around your home or neighbourhood. 
  2. Back at your place take about 25g of one soil (about two tbsp) 
  3. Add water droplets to the soil until it stops absorbing water (the soils is saturated)
  4. Squeeze the soil in your fist as hard as you can-does it remain in a ball when squeezed? No? Then it’s sand! And you can switch to the next soil.
  5. If the ball did stay then you can begin to rub it between your thumb and forefinger smushing the ball and spreading it out. If the ball begins to look like a ribbon but then quickly falls apart-or you can’t even make a ribbon, then you have a loamy sand soil. And you can switch to the next soil. 
  6. If you are successfully making a ribbon then how long does your ribbon get before it breaks? Below there are some headings with lengths of ribbons. If your soil ribbon matches the length read the instructions below. 

Ribbon is less than 1 inch

  1. Add a lot of water to your soil and then rub it between your thumb and forefinger.
  2. If its gritty it’s a sandy loam soil
  3. If its smooth it’s a silty loam soil
  4. If its neither gritty and smooth its a loam soil

Ribbon is between 1 and 2 inches

  1. Add a lot of water to your soil and then rub it between your thumb and forefinger.
  2. If its gritty its a sandy clay loam
  3. If its smooth its a silty clay loam
  4. If its neither gritty or smooth its a clay loam

Ribbon is 2 inches and longer

  1. Add a lot of water to your soil and then rub it between your thumb and forefinger.
  2. If its gritty its a sandy clay 
  3. If its smooth its a silty clay 
  4. If its neither gritty or smooth its clay

So now you are a soil scientist! But what do these classifications mean for the soil?

Well soil needs drainage and nutrients. Sand offers great drainage, but doesn’t have many nutrients. Clay holds on to nutrients but doesn’t have great drainage. Silt has better nutrient retention than sand and better drainage than clay. Silt allows you to mix sand and clay to make a loam soil. 

Another test…

Throw soil against a wall! Make sure you are outside and the same distance from the wall for each soil ball you throw. 

This test won’t tell you the exact classification of soil but it will tell you what the general texture is. Make sure your soil is moist and then observe what happens when you throw it. 

Settling Test

One more test you can do-if you want to know the exact percentages of sand, silt, and clay in your soil is to do a settling test

Materials:

  • Straight edged, clear jar
  • Permanent marker
  • Ruler
  • Watch or stop watch
  • 1 tablespoon of powdered dishwashing detergent
  • Mesh sieve or old colander
  1. Using a mesh sieve or old colander, sift the soil to remove any debris (rocks, leaves, sticks, roots, etc.).
  2. Fill the jar ⅓ full of the soil 
  3. Fill the remainder of the jar with clean water but leave some space at the top
  4. Add 1 tbsp of powdered dishwashing detergent 
  1. Cap the jar and shake vigorously until the soil and water are one mixture (slurry)
  2. Set the jar down on a level surface and time for 1 minute
  3. After a minute, draw a line at the bottom of the jar indicating the coarse sand layer that has settled out
  4. Leave the jar in a level spot for two hours, mark the top of the new layer that has formed at the end of the two hours-this is the silt layer
  5. Leave the jar in a level spot for 48 hours-mark the top of the new layer with a permanent marker-this is the clay layer. 
  6. Using a ruler measure and record the height of each layer and use the formulas below to calculate the percentage of sand, silt, and clay

Calculate Soil Percentages

Height of sand layer ________inches / cm

Height of silt layer ________inches / cm

Height of clay layer ________inches / cm

TOTAL HEIGHT OF LAYERS ________inches / cm

% SAND=(sand height)/(total height) x 100 =___________ % SAND

% SILT=(silt height)/(total height) x 100 =____________ % SILT

% CLAY=(clay height)/(total height) x 100 =____________ % CLAY

Now you can use the soil triangle to classify your soil:

Great job soil scientists! For more information on the settling test and some photos check out: https://hgic.clemson.edu/factsheet/soil-texture-analysis-the-jar-test/

For a variety of fun soil tests check out:

http://www.fao.org/tempref/FI/CDrom/FAO_Training/FAO_Training/General/x6706e/x6706e06.htm

But what about organic matter?

In addition to sand, silt, and clay soil also needs organic matter called humus. 

This is fully decomposed plants and animals that stay in the soil for thousands of years. It’s rich in carbon and nutrients and allows for excellent drainage and water retention. You can tell a soil is rich in organic matter when it is a dark brown or black. Often when you buy soil it has been enhanced with lots of organic matter that helps plants grow.

When you make compost and add it to your soil-you are adding essential organic matter. 

An indication that soil has enough organic matter is if it has active microbes that feed on organic matter. How do we measure microbial activity? By burying underwear of course!

Materials

  1. Pair of white cotton underwear 
  2. Shovel  
  3. Flag 

Instructions 

  1. Dig a small trench and bury the underwear in the top 6 inches of the soil. 
  2. Leave the waistband showing a little and mark the place with a flag so you’ll be able to find it back again. 
  3. Leave the underwear buried for about two months. Dig it up carefully and wash it in a bucket of water to remove the soil if necessary.

If your underwear is broken down and full of patches then that means that microbes are active and have eaten it. This is a good sign that your soil has a healthy amount of organic matter.

If your underwear is completely intact then this indicates that there is likely not a large amount of organic matter in your soil. 

One more thing to measure

In addition to all of these microbes that live in soil I am sure you are aware of the many worms and insects that call soil their home. 

The healthiest soils usually have a good amount of earthworms. This is because earthworms add pores to the soil to help with aeration and also fertilize the soil with their poop!

To compare biological activity between soil patches, scientists will do earthworm counts. 

They section a piece of area of identical size in two patches of soil. Then they dig through the top 6 inches of the soil and count every earthworm they see.

Soil with more earthworms is usually better for growing. What soils do you think will have the most earthworms?

Soils in the forest, in the park, soils with plants, sandy soils?

Make a prediction and see what you find! You are a soil scientist now!

What do you think a “sinkhole” is? 

Sinkholes are holes in the Earth’s surface that are caused by chemical reactions. When karst is formed from dissolved sedimentary rocks, such as lime stone, gypsum, and salt, it weakens the structural integrity of the Earth’s surface, resulting in sinkholes. These geohazards can be created from the movement of ground water underneath the Earth’s surface, or from heavy localized rainfall.  

One sinkhole in particular that has made headlines in the past year is the Oxford Sinkhole. In Oxford, NS, a large sinkhole formed at the end of a parking lot due to excessive water runoff during heavy rainfall. The water dissolved the gypsum and salt under the parking lot, which resulted in the sinkhole.  

Oxford Before Sinkhole: The yellow dot shows where the sinkhole is

Oxford After Sinkhole: 

What types of rock do you think are at the surface of the Earth in Oxford, NS? Take a look at the aerial photo of Oxford, NS, notice the name of the lake?

Why do you think the sinkhole in Oxford formed? Notice the dark, circular spots in the lakes of Oxford? Do you think those are sinkholes?

Do you think sinkholes could form in your area? Visit the following website of a map that shows karst deposits in Nova Scotia. https://fletcher.novascotia.ca/DNRViewer/?viewer=karst

Try making your own “Sinkhole”! 

Materials

  • Foam cups
  • Sponge piece
  • Clear container
  • Water
  • Sand
  • Sugar cubes
  • Rocks
  • Ruler

Sinkhole in a cup!

  1. Poke a thumb sized hole in the bottom of a foam cup.
  2. Put a small square of sponge in the bottom of the cup
  3. Stack sugar cubes on top of the sponge 3 cubes high. This represents soluble rock.
  4. Fill the area around the cubes with one layer of rocks, then sand, and put a thin later of sand to just cover the cubes.
  5. Draw a line on the outside of the cup to indicate where the top of the sand it.
  6. Start a timer and submerge the cup about half way in water and observe what happens.

This may take a few minutes, but the sugar cubes will begin to dissolve which will result in a sinkhole. This is a module of a Ground Water sinkhole.

7. Record observation and the time they were noticed. Measure and record the diameter of the sinkhole model. How does the level compare to the line marked for the top of the sand and the beginning?

Sinkhole in a container!

  1. Put a layer of sugar cubes at the bottom of the container with dimensions 2x2x2
  2. Fill rocks and sand around the sugar cubes just like activity one, and just cover the cubes with sand
  3. Start a timer and slowly pour water around the area and see what happens. This is a model of a localized surface water sinkhole
  4. Record your observations and the time they were noticed.
  5. Measure the diameter of the sinkhole model.

Compare the two types of sinkholes: 

  1. What is the difference in how long it takes for the sinkholes to form?
  2. Why do you think one took longer than the other?
  3. How do the diameters compare?
  4. If the diameter of the Oxford sinkhole is 40m wide, how is your scale model comparable? Do the Math!

For more information on Sinkholes check out the follow links!

https://www.natgeokids.com/nz/discover/science/general-science/science-dr-karl-sinkhole/

https://atlantic.ctvnews.ca/video?clipId=1467430

Ask your family if they know of any sink holes near your home community. Many sinkholes today are just very deep lakes or bogs but at one time, they were sinkholes.

Resources for Older Students

Soil and Social Justice

Soil is so important that the location of farms and people relative to usable soil is an example of environmental racism. When African Nova Scotians first arrived in Nova Scotia as refugees from slavery they were given rocky, infertile land in many areas of the province including Lincolnville, Birchtown, and Preston, Nova Scotia. In addition, they weren’t even given the land! The land was leased to them and they were not allowed to sell it and move to a more fertile location (something white settlers were allowed to do easily). 

This resulted in African Nova Scotians being forced to work on the farms of white settlers or companies owned by white colonists in order to feed their families and themselves. 

Without land or property it is very hard to change your economic circumstances from generation to generation and so the choice to withhold land ownership from people of African descent and to only give them land that is infertile was a way of promoting the white supremacist agenda of early North America. This has been a huge contribution to the racial wealth gap we see today. 

First Nations communities faced and continue to face the same treatment. Using manipulation, white settlers forced First Nations to move into reserves that were purposefully chosen on unfertile land that could not support the population. First Nations were unable to gather food in their traditional territories and were unable to grow much of their own food on teh lands they were moved to. They suffered immensely and continue to face issues with food security today as local gardens and farms are hard to establish on northern reserves. 

White colonial powers understood the power of soil and so they used it to economically enslave racialized communities. Understanding why soil is fertile is important in reconciliation with First Nations and African Nova Scotians. We need to understand what was taken away before we can make it right.