The Secretary may make appropriate adjustments in the support price for any commodity (excluding cotton) for differences in grade, type, quality, location and other factors. The adjustments shall, so far as practicable, be made in such manner that the average support price for the commodity will, on the basis of the anticipated incidence of such factors be equal to the level of support determined as provided in this Act. Beginning with the 1991 crops of wheat, feed grains, and soybeans for which price support is provided under this Act, the Secretary shall establish premiums and discounts related to cleanliness factors in addition to any other premiums or discounts related to quality.