Looking for a field study you can do from your classroom? This activity will give your students a closer look at dirt, I mean soil! Soils serve many functions, including being a medium for the growth of plants, the recycling of nutrients and wastes, and purifying and storing of water.
The four main building blocks of soils are minerals (45%), water (25%), air (25%), and organic matter (5%). The particles in soil are composed of broken pieces of rocks that have been altered by the physical and chemical processes of weathering and erosion. The arrangement of soil particles form the structure of soil and provide open pore space that is filled by water and air. The more pore space in your soil, the more water the soil can hold.
This activity shows students how to classify the texture of a soil. Soil texture is determined by classifying soil by particle size into the categories of gravel, sand, silt, and clay. Gravel particles are the largest, at approximately 2 mm in diameter. Think of the sand you see at the beach; if you look closely you can see each grain of sand! These grains of sand are still fairly large, averaging about 1 mm in diameter. On the other hand, silt and clay particles are much smaller, almost invisible to the naked eye, averaging 0.02 mm for silt and less than 0.002 mm for clay. Most soils contain a mixture of sand, silt and clay in different proportions, so that is what we will focus on in this lesson.
To have your students test their soil for texture you will need the following materials:
- A clear plastic or glass jar with a lid for each student
- Soil from each student’s home (about 1 cup)
- Dish soap
- Masking tape
To get started, have each student bring in a clear plastic container with a lid that seals and a ziplock bag with soil from their lawn or garden. You will only need about a cup of soil per student. Use masking tape to label each student’s jar and soil baggie with their name.
Place about a cup of soil into the jar and fill the jar about 3/4 of the way full with water. Add a drop of dish soap; you don’t need much! This will help break up some of the soil clumps. Fasten the lid tight and have each student shake their jar for about five minutes. Then place all the jars somewhere out of the way where they won’t be disturbed.
The next day have students measure the depth of each particle layer. You should be able to use color to tell the difference between layers. Since the sand particles are the largest (and therefore heaviest) they settle out first, filling the bottom of the jar. Above the sand layer is silt, and above silt is clay. Anything floating on the surface of the water is organic matter.
Have the students classify their layers by particle size. Then have the students use their rulers to measure the total depth of soil in the jar and the depth of each layer. Dividing the depth of each layer by the total depth will give the students an approximate percent of that parcel size in the soil.
If the soil is > 50% clay it is classified as a clay soil; the kind of soil that sticks to your shoes. If the soil is > 50% sand it is a sandy loam. If the soil is >50% silt it is a silty loam. And finally if the soil is a mix of all texture types it is considered a loam.
Students can then compare their soil texture to other students and discuss where they collected their soil from.