I am not sure about the soybeans. There may be some useful information regarding their nutritional content on the beautiful dragons site. (beautifuldragons.com I *think*).
As for feeding, once a day should be fine. I believe most people say that you should remove their food after a certain timeframe (like an hour), but I have found that my Uros like to snack on the greens even after they get crunchy. I usually feed them, then leave the food in their tanks until I feed them the next day. If you are giving them a treat like banana, or other fruit, you have to be a little more careful leaving them out that long though.
I hope that helps!
Elizabeth
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1.0 Mali Uro Archimedes
0.0.1 Egyptian Uro Zuberi Mosca Khu (Mosca)
0.0 Fish
0.1 Sulcata Minnie
1.1 Iguanas Flik and Loki
0.1 Newfoundland Jasmine
0.1 Feline Winter
Indiana & Wisconsin