Misconceptions in Science – Plants, Soil and Photosynthesis

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One of the most common science misconceptions for upper elementary students is the belief that plants primarily obtain their mass from the soil.

This misconception arises from the observation that plants grow in soil and require nutrients from it. However, the truth is that plants obtain most of their mass from the carbon dioxide in the air through the process of photosynthesis. During photosynthesis, plants use sunlight to convert carbon dioxide and water into glucose and oxygen. The glucose is then used to create more complex molecules like cellulose and starch, which make up the majority of a plant's mass.

While soil does provide essential nutrients such as nitrogen, phosphorus, and potassium, these nutrients only contribute to a small portion of the plant's total mass. Understanding the process of photosynthesis and the true source of a plant's mass is crucial for students to grasp fundamental concepts in biology and ecology, such as the carbon cycle and the role of plants in producing oxygen and sequestering carbon dioxide.

A good way to demonstrate the concept of plants obtaining their mass primarily from carbon dioxide rather than soil is through a simple experiment using two plants and a scale. Select two plants of the same species and similar size. Label one as the "Control" plant and the other as the "Test" plant.

Weigh each plant (including the pot and soil) using a precise scale and record their initial weights. Place a transparent plastic bag over the "Test" plant and seal it to create an enclosed environment. This will limit the carbon dioxide available to the plant, reducing the amount of photosynthesis that occurs. Place both plants in a well-lit area with similar light exposure and temperature conditions. Ensure they receive adequate water during the experiment.

After 2-3 weeks, remove the plastic bag from the "Test" plant and weigh both plants again, recording their final weights. Compare the weight changes of both plants. The "Control" plant should have gained more weight than the "Test" plant, as it had access to more carbon dioxide and could undergo more photosynthesis.