The United States must build new chemical weapons to redress the Soviet Union's "decisive military advantage" to wage chemical war, administration officials told the Senate Appropriations Committee yesterday.

Flatly contradicting civilian experts who testified Wednesday, witnesses from the Defense and State departments said the U.S. chemical weapons stockpile is not enough to force the Soviets into "a deterrent posture" in which they would be required to put on protective gear and use decontamination equipment.

The Reagan administration is seeking $705 million in fiscal 1983, mostly for defensive items such as protective equipment and troop training. About $54 million would be for acquisition of new "binary" chemical weapons, which if approved would be the first produced in 13 years.

They are called binaries because they contain two separate chemicals which do not become lethal nerve gas until they mix after the projectile is fired.

"It is not a question of the total amount of chemical agents," said Theodore Gold, deputy assistant secretary of defense. Witnesses for the administration acknowledged that we have no evidence to show that the Soviet has a bigger stockpile, or that its weapons are better deployed.

Rather, Gold said, the United States lacks "militarily useful weapons."

He said the most critical deficiency of American chemical weapons is that none will reach beyond battlefield artillery range to hit such back-of-the-lines areas as airfields and supply stations.

The weapons proposed for 1983 are artillery shells that wouldn't reach beyond the battlefield either, administration officials said.

The United States has an aerial tank for spraying behind the lines, but planes carrying it are vulnerable to modern anti-aircraft weapons, Gold said.

Also, current chemical bombs contain only "non-persistent" nerve gas that soon evaporates rather than sticking to surfaces for several days as does "persistent" nerve gas.

The Soviets, said Richard Wagner, assistant secretary of defense, already have additional ways of delivering chemical weapons, such as air-launched rockets and mobile missiles with a range of more than 200 miles.

While critics at Wednesday's session said the United States already has the capability to fight at least a 90-day war in Europe regularly using chemical weapons, administration witnesses questioned those figures yesterday.

The critics said that if 5 percent of all artillery rounds fired were chemical it would be more than adequate. But Gold said that "the judgment of our military is that a few times 5 percent is a much more appropriate mix" of ordinary and chemical artillery shells, so the United States could not carry on chemical war as long as the critics contend.

A vote on an amendment to strike out the funds for binary weapons is expected today or early next week.