Welcome to Science Buddies! You have an excellent and unique idea for a science project. Here is a project idea from this website that you could adapt for your project that includes useful details for setting up the experiment. http://www.sciencebuddies.org/science-f ... #procedure
Most of the research on these compounds has been done on mammalian cells. http://en.wikipedia.org/wiki/Daidzeinhttp://en.wikipedia.org/wiki/Genistein
Here is an interesting reference article that suggests a biological function of isoflavones in plant growth:http://www.bashanfoundation.org/horst/h ... trogen.pdf
The Wikipedia article has a unreferenced statement on the function of isoflacvones in plants, to help protect against pathogenic fungi and other microbes and to stimulate symbiotic bacteria for nitrogen-fixing. You should look further and try to find actual scientific references for this information. http://en.wikipedia.org/wiki/Isoflavones
The analysis of isoflavones would be difficult, so it would probably be best if you could contract the supplier of the food supplements and ask if lot-specific analysis is available. Then you could purchase that product and request the certificate of analysis from the supplier. If this is not available, you will have to rely on the product label for concentration information.
Thanks for including the details on the inactive ingredients. It would be best to use the pure isoflavones, if these are available. However, n0ne of the ingredients in the pills contain potassium, nitrogen, or phosphorus, which would be expected to affect plant growth. However, the element magnesium is the central atom in chlorophyll molecules. So for your negative control, you will have to find out how much magnesium is in the pills and add it in the same concentration.
You can find magnesium at your local pharmacy; it is sold as Epsom salts.http://www.lclabs.com/MSDS/D-2946MSDS.php4http://www.chm.bris.ac.uk/motm/chloroph ... hyll_h.htm
Daidzein and genistein are not soluble in water. Here is information for daidzein, which suggests first dissolving it in DMSO and then diluting 1:10 in an aqueous buffer:
How are you going to measure plant growth in your experiment?