Guy Donald Smith, 73, former director of the U.S. Department of Agriculture's soil survey investigations who developed a system of soil classification that has become a world-wide standard, died of cancer Aug. 22 at his home in Ghent, Belgium.

Because of Dr. Smith's soil classification system, which he painstakingly developed over 20 years, more accurate information about the nature of soils can be exchanged between the nations of the world. Through better understanding and utilization of land resources, developing countries can increase food production and lessen their dependence on food imports.

Dr. Smith was a 1962 recipient of the Department of Agriculture's Distinguished Service Award. He was a past president of the American Soil Science Society and a fellow of the American Society of Agronomy, which awarded him its Soil Research Award in 1964.

The author of numerous publications on soil science, Dr. Smith joined the Agriculture Department in 1946 and worked in Iowa as a soil correlator. He moved here in 1950 and became director of soil survey investigations in 1952.

From 1964 until 1965, Dr. Smith was a visiting professor at the University of Ghent. After leaving the Agriculture Department in 1972, Dr. Smith returned for a year to the University of Ghent. He then spent two years in Trinidad as a soil consultant for the Organization of American States. He later did similar work for the governments of Venezuela and New Zealand. Dr. Smith resumed his post at the University of Ghent in 1978, and was with the university at the time of his death.

Dr. Smith was a native of Iowa and a graduate of the University of Illinois. After receiving a master's degree in 1934 from the University of Missouri, he worked in Illinois for two years for the Resettlement Administration. From 1935 until 1942, he was a professor of agricultural science at the University of Illinois. In 1940, he received a doctorate in agronomy from the university.

He served with the Army Air Forces in the China-Burma-India theater during World War II before joining the Agriculture Department in 1946.

His first wife, Jean Randall Smith, died in 1968.

Survivors include his wife, Yolanda Bultyn Smith of Ghent, whom he married in 1972; five children from his first marriage, Ann Spurgin of Charlottsville, and Lisa Brodyaga of Harlingen, Tex.; Guy Jr., of Albuquerque, N.M., Randall, of Takoma Park, and Arthur, of Baltimore; a brother, Curtis, of Phoenix, N.M., and six grandchildren.