Backyard Biology

Nature stories from my backyard and beyond

Nature Activities

Living Soil

Friday, December 4, 2009

If I want to find biology in my back yard, I have to look no further than the ground right outside my back door. The soil at my doorstep is home to countless creatures, including insects, worms, spiders, protozoans, fungi, and bacteria. To see some of the larger creatures that live in the soil, try building a Berlese Funnel. Directions for building and using a Berlese Funnel can be found in an earlier version of Nature Activities. You might be surprised at what you can find living right at your doorstep.
But to get a really good idea of the life in the soil, you need to look for the really small creatures. You need to look for bacteria. It has been estimated by soil scientists that there are over a billion bacteria in a gram of soil. That is, in 0.035 ounces of soil. Therefore, if you grab a handful of soil from your back yard, you are holding more bacteria in your hand than there are people in the entire world! The reason you are able to hold so many creatures is that they are microscopic. In fact, if you were to magnify them a thousand times – as you can with a very good light microscope – they would appear as only tiny little dots. Meanwhile, if I magnified myself a thousand times, I would be over a mile tall!
But for all of their small size, they are indispensable. Some kinds of bacteria convert the chemical nitrogen into a form that is usable by higher organisms. Without this source of nitrogen, life couldn’t exist. Some bacteria decompose dead plants and animals, returning their nutrients back to the soil to be reused by other organisms. Some bacteria eat harmful chemical. Some bacteria are essential for the growth of certain plants. All in all, a soil would be pretty much devoid of life if it weren’t for the bacteria.

But for all of their importance, it is easy to see why these soil bacteria are so easily overlooked. They are just too small to see. So one way to see these bacteria is to grow them in large colonies (a large colony of bacteria consisting of millions of bacteria may be as large as the size of a dot on this page) and look at the colonies. Scientists have been doing this for ages. They grow their bacteria in Petri dishes containing a nutrient agar. Agar is the jello-like substance the bacteria are grown on and nutrient is the food provided for the bacteria.

While it is possible to buy professionally prepared agar plates, in this activity, you will learn to make a homemade version of these plates. Then you will collect some soil from your back yard and see what is growing there.
Scientists, with their special nutrient agar, are only able to “culture” a small percentage of the bacteria that live in the soil. With our homemade nutrient agar plates, we will be able to culture an even smaller percentage of the bacteria that live at our doorstep. But since the purpose of this exercise is to show that bacteria – any bacteria – are living in our oil, it doesn’t matter how many types we are able to culture, so long as we can culture some.
Keep in mind that we may also end up culturing some of the other organisms living in the soil – like fungi.

Caution: People who work with bacteria have a general rule. Since the harmful bacteria look the same as the good bacteria, they treat all bacteria as if they were harmful. While it is unlikely that you will encounter any harmful bacteria in the soil, never touch your bacteria.

When you have finished your experiment, dispose of your bacteria properly. Proper disposal method can be found at the end of the experiment.

Preparing the Agar Plates


  • Baby food jars or similar container
  • This is a substitute for the petri dish. You can use petri dishes if available.
  • Water
  • Agar
  • Also known as agar-agar, it can be bought in health food stores. This is the jello-like stuff that the bacteria will grow on.
  • Beef Bullion – Sodium Free
  • This is the nutrient source for the bacteria. Buy the packets instead of the cubes. It is easier to measure the amount you need.
    Since many bacteria can’t tolerate high salt – make sure to buy the sodium free bullion.ingredients image






  1. Put 50 ml of water into baby food jar.
  2. Add 0.2 grams of the beef bullion (about 1/16th tsp.) to the water
  3. Add 0.2 grams of the agar (about 1/16th tsp.) to the water
  4. Stir to mix the bullion and agar.
  5. Place the jar in a pan of water. Place the cover loosely on the jar. Do not screw the cap down on the jar.
  6. Cover the pan and boil the water for 30 minutes. This will sterilize the agar in the jar.
  7. After 30 minutes, remove the pan from the heat and allow to cool.

When the jar has cooled, the agar in the jar should be solid. Now you can screw the cover down on the jar. Your “petri dish” is now ready for bacteria.

Collecting the Soil Bacteria

  1. Put 100ml of water (about a half cup) into a jar.jar in pan image
  2. Place the jar, water and lid in a pan of water and boil for 30 minutes.
  3. After 30 minutes, remove the jar, water and lid from the pan – making sure not to touch the inside of the jar or lid. Set aside to cool.
  4. While the water is cooling, dig up some soil from your back yard.
  5. Add 10 grams of the soil to the water.
    To measure 10 grams of soil, make a Ruler Balance – see below for directions.
  6. Cap the jar and shake for a few minutes.
  7. Let the soil mixture settle for 10 minutes.

In ten minutes, the heavy dirt will sink to the bottom of the jar while the lighter bacteria will remain afloat in the water at the top of the jar.

Adding the Bacteria to the Agar Plate

  1. After the soil has settled for 10 minutes, stick a q-tip into the soil water.
  2. Gently drag the wet q-tip along the surface of the agar plate. Be careful not to press too hard. Don’t break the surface of the agar plate.
  3. Place the cap lightly on the agar plate and set aside in a warm place for a week. Do not screw the cap tightly in place. The bacteria need oxygen.

At the end of a week, you should see bacteria growing on the surface of the agar plate.agar plate image






Proper disposal of your bacteria
Whenever working with bacteria, it is always good to dispose of them properly even if you think they are completely harmless.

  1. Make a sterilizing solution of 10% bleach. Add one part bleach to 9 parts water. Make enough to fill a bucket enough that your petri dish will be completely covered.
  2. Open the cap to your Petri dish and add cap and dish to bleach solution. Let sit for 15 minutes.
  3. Now you can either throw out the baby food jar and agar or throw out the agar and clean up the baby food jar for your next experiment.

Ruler Balance
A simple serviceable balance can be made with a ruler, pen, coin and cardboard.

  1. Cut two pieces of cardboard about 1” x 2”. The exact size is not important as long as both pieces are the same size and weigh the same amount.
  2. Tape the pen onto a flat surface.
  3. Balance the ruler on the pen.
  4. Place a coin of known weight on one cardboard platform.
  5. The material you are weighing goes on the other platform.
  6. Move the coin to the desired spot so it will balance with the correct amount of the material you are weighing.

The formula for deciding where the coin goes is:
W1D1 = W2D2

W1 = weight of coin
D1 = distance of coin from center of ruler
W2 = weight of material to be measured
D2 = distance of material to be measured from center of ruler

Weight of coins Dime = 2.2 grams
Penny = 2.5 grams
Nickel = 5 gramsruler balance





In this picture, I need 10 grams of dirt, so I have two nickels to balance it at 6 inches.

To calculate for 4 grams:

W1 = 5 grams weight of coin
D1 = X inches distance of coin from center of ruler
W2 = 4 grams weight of material to be measured
D2 = 6 inches distance of material to be measured from center of ruler

W1D1 = W2D2
(5 grams)(X inches) = (4 grams) (6 inches)
X = 24/5
X = 4.8 inches
In this instance, you would place the nickel 4.8 inches from the center of the ruler and add dirt until the ruler is balanced.