* Soybeans edge lower after strong rally since mid-2020* Concerns over S. American supplies, Chinese demand to support prices (Adds quote in paragraphs 3-4, global wheat output forecast)By Naveen ThukralSINGAPORE, Feb 26 (Reuters) - Chicago soybean futures slid on Friday as U.S. weekly sales dropped after prices climbed to their highest in more than six years on the back of concerns over South American supplies.Corn and wheat eased but both market were poised to end February in positive territory.