I wouldnt go with any of the Round Up from wal-mart it isn't straight chemical it is watered down. Id do what haystack said and go to a farm supply store and pick you up some glyphosate thatd be your best bet. Also if youve never planted in this area before i would reccomend a soil test and then fertilize accordingly. For soybeans you usually want about a 7 ph. Also round up is a contact killer so it will only affect the plants that are growing. After you spray though it'll take a couple weeks to get a complete kill off. Id say if the area has never been sprayed before you wont have to worry about many of the weeds being tolerant to the chemicals.