As expected, Washington's trial balloon about planting liquid mines along West Germany's border with East Germany and Czechoslovakia has been shot down in Bonn.
The idea was initially floated - very cautiously - by Brig. Gen. Anthony A. Smith, Defense Secretary Caspar W. Weinberger's executive assistant in charge of NATO affairs. He described to a group of NATO parliamentarians a week ago the potential advantages of a new liquid explosive for mining West Germany's border as a passive conventional defense against any surprise tank attack.
Such explosives have already been tested by the United States and West German armed forces in South Korea and West Germany. A peacetime pipeline could be laid out that could be easily loaded by simply pumping the explosive through the line.
General Smith added, however, that no proposal has been made, and that there are strong political constraints against any such project. These constraints, he suggested, include the reluctance of citizens to fortify their borders when they are not at war. Elaborating on this, the general noted that no Texan or North Dakotan would want to put this kind of highly visible fortification along his own border in peacetime.
This observation accurately foreshadowed the West German reaction once the story broke in Bonn a few days later. The Defense Ministry spokesman said immediately that neither NATO nor the German Army has any such plans, that such ideas were rejected in the past and would again be rejected in the future, and that any such notion would be ''absurd.''
Peter-Kurt Wurzbach, Defense Ministry state secretary added that this idea smacked of a Maginot mentality and contradicted NATO reliance on a mobile rather than static defense. And only one major newspaper, the Suddeutsche Zeitung, carried favorable editorial comment. More typical was the Bonn General Anzeiger's allusion to President Reagan's microphone test about bombing the Russians: ''Is this too a macabre joke?''
There are several reasons why the subject is so touchy here. The West Germans have an aversion to anything that imitates the ugly East German wall - even if barriers on the West German side would be designed to keep aggressors out and not to keep citizens in. Anything that makes the division of Germany look or feel more permanent goes against the grain.
Furthermore, North German areas close to the frontier are heavily built up. While such a project would presumably not pose a danger to villagers or townsfolk in peacetime, the construction would be disruptive to civilian life.
West German sensitivities on this score are also heightened by history. Back in the early 1960s the US wanted to plant a belt of atomic mines along the East-West German boundary. It required strenuous efforts for Bonn to convince Washington that such a guarantee that any war would immediately go nuclear would not be acceptable to West German voters.