Does using home ingredients help radishes grow faster than store bought fertilizer?
Think of the independent variable as the one that you have direct control over--it's up to YOU, not determined by the unknowns, the experimental organism, etc.
So your QUESTION is whether home stuff or fertilizer is better for radish growth.
Your null hypothesis (i.e. the "nothing special happens" view) is that NO, there's no difference in the two treatments.
Your experimental organism is the radish.
So--what treatment of radishes will allow you to compare the two additives (fertilizer, home ingredients).
That is your independent variable, the one that you determine and control.
What OUTCOME will you measure in order to judge/determine how each treatment succeeds? That is the dependent variable--the one that depends on the validity of the hypothesis and on the unknown behavior of the organism.