An experiment tests if plant species affects soil nitrogen levels. Different species are planted, and nitrogen levels measured. What is the independent variable?
- Plant size
- Soil nitrogen levels
- Measurement method
- Plant species (correct answer)
Explanation: The hypothesis is that plant species affects soil nitrogen levels through their different root systems, growth patterns, or nitrogen-fixing abilities. The experimental design plants different species and measures resulting nitrogen levels, making plant species the independent variable - the factor being deliberately varied by researchers. Soil nitrogen levels would be the dependent variable (what's measured), while factors like measurement method would be controlled. Plant species serves as the manipulated factor to test its impact on soil chemistry.