* Wheat ticks up but not far from 3-1/2 year low * Soy falls 0.4 pct on f'cast for rains in Argentina (Adds details, quotes) By Naveen Thukral SINGAPORE, Jan 21 (Reuters) - U.S. soybeans eased on Tuesday, falling further from last week's three-and-a-half week top as forecasts of rains in Argentina eased concerns over