ShareRocks and Soils - Sizing It Up!
The solid part of soil is made from mineral
and organic matter. You have already learned a little bit about organic
material. Now it is time to learn more about the inorganic parts of soil.
Do you remember what other solid materials are in soil? (Hint: You need
them to be healthy.) The answer, of course, is
minerals. Minerals come from rocks. Minerals become part of the soil in
several different ways. Sometimes minerals are added to the soil from volcanic
eruptions. Most minerals however, come from rocks that have been broken
apart over a very long period of time.
Size wise
Just as daisies, dinosaurs and doughnuts come in different
sizes, so does soil. Soil
particles come in different sizes. Scientists group these into three categories.
Sand has the largest particle size. Clay is the smallest size. Guess where
silt fits in? If you said in between the two, you are absolutely correct.
Knowing this can help you to understand the type of soil that you have
in your yard or your garden.
Here is an example of how to visualize the
relationship between sand, silt, and clay. If a particle of sand were a
basketball, then silt would be the size of a softball, and clay would be
the size of a golf ball. The size of soil particles is important. The amount
of open
space between the particles has a lot to do with the ease that water moves
through soil and the amount of water the soil will hold. The best kind of
soil for plants allows water to move easily enough to be absorbed by the
root system and slow enough to allow water to stay in the soil for continued
use by the plant.
A soil with a high percentage of sand, allows water to move too quickly. The roots of plants have little time to absorb the moisture. Because clay particles are so small, they pack tightly together holding water tightly. Plant root systems cannot get to the water. Soils that have the ideal mixture of sand, silt, and clay are considered to be the best type of soil for plants. These soils are called loams.
Do it!
You can use your sense of feel to help determine the
type of soil you have in your garden. Sand feels gritty. Silt feels smooth,
soft and somewhat slick. Clay feels sticky and often leaves a color stain
on your fingers. Soils that have a lot of clay can be pressed together
through the thumb and forefinger to create a ribbon of soil.
You be the scientist!
Problem:
How can you determine the
composition of the soil in your yard?
Materials:
- 250 - 500 ml. (one or two cups) of soil from your yard or garden.
- One liter (one quart) jar with a tight fitting lid
- 15 ml. (one tablespoon) of a powdered water conditioner such as Calgon®
- Water
- Ruler (metric)
Procedure:
- Place the soil in the jar.
- Add the water conditioner.
- Fill the jar approximately two thirds full of water.
- Place the lid on the jar and secure tightly.
- Shake the jar until the ingredients are thoroughly mixed.
- Allow the jar to sit undisturbed for about 72 hours.
- Carefully bring the jar to your work area.
- Observe your soil. If your soil is typical of most soils, you will notice a layering of the soil. These layers are the sand, silt, and clay.
- Using a ruler, measure the length (in cm.) of the three layers from the bottom of the jar to the top of the soil. Do not include the water in your measurement. If you have some other organic material floating at the top, you do not need to measure it.
- Record the total volume of the soil in centimeters.
- Next, measure and record the total volume of each layer of soil. (You may not have all three layers in your soil sample. If this is the case, compare your soil to a classmate's soil to find out which layer you are missing in your soil sample.)
- Ask your teacher or parent to help you determine the percentage of each layer.
- Which type of particle will be found at the bottom, middle and top?
Why? To find the answer, highlight the box below.
Answer: Because sand is the biggest and heaviest of the three particles, it will sink to the bottom first, so the bottom layer is sand. Silt will be the middle layer. You will find the clay at the top. - The ideal soil is called a loam. It is a mixture of 40% sand, 40% silt, and 20% clay. What would have to be done to your soil to make it a loam? HINT: You would have a difficult time taking these tiny particles out of your soil.
- Write a summary or draw a picture of your findings. Discuss these findings with your parents. Maybe you can actually do something to make your garden more productive. And that is what science is all about!
Analysis:
- What kind of soil particle was the most common in your soil sample?
- From what kind of rocks do you think your soil was formed?
- How did your soil sample compare from the soil samples of your friends?
