This year's value of U.S. agriculture exports to the Soviet Union is expected to climb 50 percent to a three-year high of $1.4 billion, according to Agriculture Department economists.

"Behind the gain will be much larger wheat shipments at higher prices and much bigger soybean and soybean meal sales," the department's Economic Research Service said Monday.Farm product sales to the Soviets exceeded $1.9 billion in 1985 but plummeted to a 13-year-low of $658 million in 1986 before rebounding to $938 million in calendar 1987.

A major factor has been large sales of U.S. wheat at subsidized prices under the department's Export Enhancement Program.

"The U.S. value share of Soviet grain imports may be above 30 percent for the first time since 1984 and (may comprise) over 8 percent of total agricultural imports," the report said.

Although total Soviet agricultural imports may increase 5 percent in 1988 from last year's estimated $16 billion, the value will still be below the $19 billion average during the first half of the 1980s.

"Grain imports now cost about $3 billion a year instead of $7 billion as in 1981-85," the report said. Despite a possible 20 percent rise in the per-ton cost of imported grain in 1988, this "unit value" will still be around one-third lower in the early 1980s.

Kathryn Zeimetz, chief author of the report, said the forecast of $1.4 billion this year is still far below the $2.9 billion in the value of U.S. agricultural sales to the Soviets in 1984.