50 USC 98b - National Defense Stockpile

(a) Determination of materials; quantities 
Subject to subsection (c) of this section, the President shall determine from time to time
(1)  which materials are strategic and critical materials for the purposes of this subchapter, and
(2)  the quality and quantity of each such material to be acquired for the purposes of this subchapter and the form in which each such material shall be acquired and stored. Such materials when acquired, together with the other materials described in section 98c of this title, shall constitute and be collectively known as the National Defense Stockpile (hereinafter in this subchapter referred to as the stockpile).
(b) Guidelines for exercise of Presidential authority 
The President shall make the determinations required to be made under subsection (a) of this section on the basis of the principles stated in section 98a (c) of this title.
(c) Quantity change; notification to Congress 

(1) The quantity of any material to be stockpiled under this subchapter, as in effect on September 30, 1987, may be changed only as provided in this subsection or as otherwise provided by law enacted after December 4, 1987.
(2) The President shall notify Congress in writing of any change proposed to be made in the quantity of any material to be stockpiled. The President may make the change after the end of the 45-day period beginning on the date of the notification. The President shall include a full explanation and justification for the proposed change with the notification.