the answer to your question is the amount of time and the effect it has on each material as well as the fact that the nitrogen cannot change it is an unchangeable variable whereas everything else the time, the substance of the subject, the possible formation of nodules can change without changing the identity of the experiment itself otherwise, you have a dependent variable. I hope this helps :-)
Answer: Nitrogen
Explanation:
An independent variable can be changed, altered or manipulated by the experimenter so as to observe the result of such manipulation on the test variable or experimental variable or dependent variable.
Here, nitrogen is the independent variable. The effect of which can be observed on growth of the roots of hair, height of the plants and nodule formation all these factors are the examples of the dependent variables.
