Standards (Microsoft Word Document)

Materials:

• soil samples
• water
• a clear, plastic container
• a ruler

Background

Soil composition is determined by the amount of sand, silt, and clay particles present in a given soil. Depending on the proportions of each, soils can feel very different and positively or adversely affect the way plants grow. Check out Spotlight on Soil for additional details.

Exploration

1. Use a “ribbon test” to describe the general composition of each soil sample. Explain that the ribbon test isn’t exact, but scientists often use it in the field to get a sense of a soil’s composition because it’s so easy to implement (all you need is a little water). To do this, add water to a small clump of soil until it makes a moist ball. Then gently roll the ball of soil between your palms. If this results in a nice, long “ribbon” of soil, the sample contains a high proportion of clay (thus sticks together well) and can be described as a clay soil. If the ball crumbles, the sample has a high sand content and can be called a sandy soil. If the result is somewhere in between these two extremes, you have loam: a soil with a good balance of sand, silt, and clay. Working in small groups, ask students to determine the composition of each sample.

2. For a more precise test, create "mudshakes" from your samples and observe the components as they settle. For each soil sample, have students fill a clear container about two-thirds full of water, then add enough soil to nearly fill the jar. You can also add a pinch of laundry detergent to help the soil components separate well. Label each jar to keep track of the samples. Shake the jars vigorously then place them on a flat surface where they won’t be disturbed for a couple of days. During this time, observe the jars as the particles settle into layers. The larger particles, sand, are heaviest and will settle at the bottom, followed by silt and then clay. Some clay may stay suspended, making the water at the top cloudy. Organic matter will float on or just below the water’s surface. Once the samples have settled into layers, you can calculate the percentage of each component in each sample — and therefore the general type of soil — by measuring the height of each layer and translating it into a percentage of the whole (height of each component divided by height of entire sample).

3. Compare the mudshake results to those of your ribbon test. How accurate were your ribbon test findings? 

Making Connections

Branching Out

- Learn about the macro- and micronutrients needed by plants for good development and growth. Where do these nutrients come from? How do plants use them? Obtain a do-it-yourself soil test kit and determine the nutrient content of your soil samples (you can get these from your local Cooperative Extension office or some garden centers).