NEW YORK—Gold closed below $900 for the first time in five weeks and copper sank to a six-month low Tuesday as falling crude prices and a stronger dollar fed selling of hard assets.

Other commodities traded mostly lower, with corn, soybeans and other agriculture futures falling. The declines are the latest sign that the commodities boom of the past year is at least temporarily losing momentum as a weak U.S. economy curbs demand for energy and raw materials.

Gold dropped more than $20 during the day as the dollar ticked higher versus the 15-nation euro, diminishing deman...