Phosphate rock in the manufacture of fertilizers. Letter from the Secretary of Agriculture, transmitting, in response to Senate resolution of July 5, 1918, certain information relating to phosphate rock available for use in the manufacture of fertilizers, together with location of beds or deposits, quantity mined in the United States and how utilized, experiments that have been made by the departments as to the use of phosphate rock, including information as to the legislation necessary to make available these deposits of such rock on the public domain.