Commencing July 1, 1936, the proceeds for each fiscal year derived by the Board from the sale of power or any other products manufactured by the Corporation, and from any other activities of the Corporation including the disposition of any real or personal property, shall be paid into the Treasury of the United States on March 31 of each year, save and except such part of such proceeds as in the opinion of the Board shall be necessary for the Corporation in the operation of dams and reservoirs, in conducting its business in generating, transmitting, and distributing electric energy and in manufacturing, selling, and distributing fertilizer and fertilizer ingredients. A continuing fund of $1,000,000 is also excepted from the requirements of this section and may be withheld by the Board to defray emergency expenses and to insure continuous operation: Provided, That nothing in this section shall be construed to prevent the use by the Board, after June 30, 1936, of proceeds accruing prior to July 1, 1936, for the payment of obligations lawfully incurred prior to such latter date.