It's clearly time for Washington to start prodding its allies to make good on their financial promises to support Operation Desert Shield in the Persian Gulf.

With less than three weeks left in 1990, America's allies have given less than half the money they pledged to support U.S. military operations in Saudi Arabia this year.The worst foot-draggers are Japan and Germany, which have delivered barely one-fifth of the money and materials they promised. Only the exile government of Kuwait has given all that it promised.

Saudi Arabia, where the vast majority of allied forces are located, is the only ally to make an open-ended offer of support. Fine. But a strong case can be made that the Saudis should be quicker to pay and maybe even ought to foot the great bulk of the bill.

So far, the Saudis have forked over $987 million worth of support, including food and water plus fuel for warplanes. But the Saudis clearly could afford to do much more in view of the fact that their country has amassed $30 billion in additional oil revenues since Iraq's invasion of Kuwait last Aug. 2 sent oil prices to record highs.

What about more troops as well as more money from the allies participating in Operation Desert Shield?

Actually, the performance on this score seems satisfactory. Neither Japan nor Germany has sent armed forces to the gulf. That's understandable in view of the restrictions imposed on those nations after World War II. But Japan and Germany could provide greater logistical support.

Though the great bulk of military personnel in Saudi Arabia are American, forces of varying sizes have also been dispatched by Britain, France and 24 other allied nations. More troops from these allies would increase the difficulty of coordination among forces speaking different languages.

At this point, no one can be sure exactly how much the bill for Operation Desert Shield will come to. But they can be sure that the tab must be paid even if shooting never breaks out. They also can be sure that public support in America for efforts against Iran will wither if Congress finds it necessary to increase taxes sharply, as some lawmakers have been suggesting.