Question about controlling variables in experiment

[imsorrywhat]

Hi,
I am considering doing a project on runoff of harmful materials (such as cow dung) into rivers and streams. Specifically, the effectiveness of different plants in absorbing/preventing it reaching the water in the first place. My vague plan for this so far would be a large box on a slight lean, separated into 3 compartments that allow liquid to travel through them. Compartment 1 to pour in contaminated water, 2 for plant of choice, 3 to collect runoff. I would test the amount and concentration of contamination in before placing in compartment 1, and then again when collected in compartment 3.

I am wondering if a. this is a decent plan/idea to begin with, and b. any suggestions on how to control size of a plant -- I am considering using flax, as it is very common and native around here, but when fully grown is also taller than me. Is it fair to use a not yet fully grown or smaller version, with a mention that any runoff absorbing qualities would likely be increased as it grew larger (or is this a bit too unscientific).

Thanks

[LeirD]

Hi!

This all sounds pretty interesting and viable! I think it is hard to outright guess the difference between heights of the flax plants that you're using, so, instead, you could try to test it. Set up 3 of your 'large boxes' (with the 3 compartments each) and place a short, medium, and tall flax plant in each of them respectively. This way you can directly measure and compare the amount of filtration/absorbency from each height of plant to the other heights. Also, I would note that if all three plants show roughly the same amount of absorbance, then either 1) the height of the plant does not matter or 2) the dirt is doing much of the filtering. To test this second point, you could set up a box with only dirt and no plant at all.

Once you know how much the plant height influences your results you can continue with testing other types of plants!

Hope this helps!

[MadelineB]

Hello,
One suggestion to add to the excellent suggestions by the previous expert ... I suggest that you record the heights of the plants at the beginning and the end of your experiment. You might also measure the roots of each plant, since that could be where the filtration is taking place. To get those measurements, maybe weight the (dried and cleaned) roots at the end of the experiment.

Best of luck with your project!

Madeline