Rain Forests: Loss of Nutrients in the Soil
Soil is the top layer of Earth's surface that supports plant life. It is composed of particles from rock mixed with humus (material in soil formed by decayed plants and animals). Fertile soil contains an abundance of humus, thus it contains an abundance of nutrients needed for plant growth.
Decomposition is the breakdown of substances into simpler parts, such as the decay of plants and animals. The rate of decomposition of dead plants and animals in a tropical rain forest is very fast. For example, if it took 12 months for a plant to decay in an area of Texas where much of the year is hot and dry, it might take only 6 weeks for the same plant to decay in an area of a tropical rain forest where it is hot and wet year-round. Even though there is rapid decomposition and an abundance of materials being decomposed, very few of the nutrients accumulate in the soil of tropical rain forests. Instead, they are quickly absorbed by plants or washed away by rain.
Because the soil of tropical rain forests is so nutrient-poor, areas cleared for growing crops (plants grown for food) are soon not very productive. As the nutrients in the soil are used up, planters move on to cut down more rain forest trees. Because of this, as more of the tropical rain forest is destroyed, more barren infertile land is produced.
To demonstrate the loss of nutrients in tropical rain forest soil.
- 9-ounce (270-mL) paper cup
- six 5-ounce (150-mL) paper cups
- round coffee filter
- 1 cup (250-mL)soil from outdoors (potting soil will work)
- red food coloring 3-ounce(90-mL)
- paper cup
- tap water
- Using the marking pen, label the large cup "soil" and the six 5-ounce (150-mL) cups with numbers 1 through 6.
- Use the pencil point to make 15 or more small holes in the bottom of the soil cup. Line the inside bottom of the lower half of this cup with the coffee filter.
- Place the soil inside the filter-lined soil cup.
- Add 5 drops of food coloring to the soil in the cup.
- Stand the soil cup in cup number 1.
- Fill the 3-ounce (90-mL) cup with water, then pour the water into the soil cup.
- Allow the water to drain out of the soil-filled cup and into cup number 1.
- Repeat steps 5 through 7 five times, using the remaining five cups.
- Compare the color of the water in each cup. How does the color of the drained water in cup 1 compare to that in cup 6?
The water in cup 1 is very red, but the redness of the water in each of the later cups decreases, so that by cup 6 it is very pale or not present at all.
The red food coloring represents nutrients in the soil of a tropical rain forest. The water draining out of the cup containing the soil represents surface runoff (rainwater that is unable to soak into the ground and moves over its surface). Nutrients in the soil that are soluble (able to dissolve—break up and thoroughly mix with another substance) mix with this water and are transported to nearby water-ways, such as streams or rivers. The decrease in the color of the draining water indicates that as more and more rain flows across the soil, the soil's nutrients can be washed away. This process of washing away nutrients is called leaching.