Roy Douglas Hockensmith, 75, a retired official of the Agriculture Department's Soil Conservation Service who was an authority on the soils of Latin America, died Monday at Sibley Memorial Hospital. He had a heart ailment.

Mr. Hockensmith retired from Agriculture in 1972 as assistant deputy administrator for soil survey, a post he had held for 20 years. He joined the Soil Conservation Service in 1937 as a regional soil scientist in Amarillo, Tex., and came to Washington two years later.

During his years with the survey he participated in a number of international conferences and served with several United Nations projects dealing with land and water use. He also had been a consultant to the government of Brazil during the 1960s and helped direct soil studies for the location of new farming areas in that country.

Mr.Hockensmith was the recipient of the Agriculture Department's Superior Service Award in 1959, its Certificate of Merit in 1967, and the Soil Conservation Service's Hugh H. Bennett Award in 1975.

He was president of the Soil Conservation Society of America in 1962. He was a fellow of the American Society of Agronomy and a member of the American Society of Range Management. He also was a member of the Cosmos Club.

Mr. Hockensmith was a native of Gallatin, Mo., and earned bachelor's and master's degrees at the University of Missouri. He taught agronomy at Colorado State University before beginning his federal career in 1934 with the Federal Land Bank in Wichita, Kan.

Survivors include his wife, Edith M., of Washington, and three brothers, Edward, Donald and Jack, all of Gallatin, Mo.