The first would be easier to set up if you can get a dozen or more volunteers to help. You must set up your theory and show how you monitored the experiments.

The second will take at least one week to start seeds and establish sufficient plant growth to measure. Also to do a good study you will need a lot of plants. And you will be measuring very minute quantities of detergent or you will kill all the plants.

If done well I think you will learn more and do better with the first.

Plants is pretty basic. It’s perfect. You can control all your variables and only manipulate the plant food. Keep your project simple…so you can explain the results properly.

Experiment #1 has way too many variables to consider.

I wouldn’t start messing with having to get informed consent to do research on human beings.