Biology STAAR (State of Texas Assessments of Academic Readiness) Practice Test

How does phosphorus typically return to the soil in the phosphorus cycle?

1. Through evaporation

2. By plant decay

3. From animal excretion

4. Through condensation

The correct answer is: By plant decay

In the phosphorus cycle, phosphorus primarily returns to the soil through the process of plant decay. When plants die or shed their leaves, they decompose, and the organic material is broken down by microbial activity. This decomposition releases phosphorus back into the soil in forms that are available for uptake by other plants. This process is crucial because phosphorus is a vital nutrient for plant growth, playing a key role in energy transfer and genetic material formation. Although animal excretion contributes to the phosphorus cycle as well, the primary mechanism for returning phosphorus to the soil comes from the decay of plant material. The other processes, like evaporation and condensation, relate to the water cycle rather than the phosphorus cycle, further emphasizing the significance of decomposition in recycling this essential nutrient.