GRAINS-Soybeans firm as China continues to buy U.S. supplies

SYDNEY, May 15 (Reuters) - U.S. soybean futures rose on Tuesday, extending two-day gains to more than 2 percent, as fears of dampened Chinese demand for North American supplies eased, though gains were checked as planting progress exceeded market forecasts. FUNDAMENTALS * The most active soybean futures on the Chicago Board of Trade were up 0.6 percent at $10.24-1/4 a bushel by 0220 GMT, having firmed 1.5 percent on Monday. * The most active corn futures were up 0.8 p