How do you try to stop Pakistan or some other foreign country from building a nuclear weapon? One thing the U.S. government is trying is to make it difficult for anyone to buy from American-based companies the raw materials, equipment and technical know-how necessary to get into the nuclear bomb business.

The bureaucratic tool being used is the Commerce Department's authority to impose export controls on specific items found to be significant in the long and complicated bomb-building process. Powerful support for this approach is coming from the intelligence agencies, which are gathering every bit of information they can on what the Pakistanis and others are trying to purchase that could be of use and from whom.

The latest illustration is a new rule that imposes controls on the export of extremely pure calcium and magnesium, published in the Sept. 16 Federal Register (page 45939).

There are very few uses for the pure forms of these two commodities, termed "nuclear grade," other than in preparation of a metallicized form of uranium used either to make pellets for a nuclear reactor, or for a bomb. Pure calcium, for example, is added to a chemical compound containing enriched radioactive uranium. The uranium metal that results is stable enough to be worked on a metal lathe and put into the precise shape needed for a bomb.

Asked what led to the new rule, an Energy Department official said there was information that "a country we do not want to assist" was in the market for these commodities. He added that since 1979 similar controls have been placed on nine other items that could be used in bomb-building and were being sought by countries that U.S. intelligence agenices said wanted them for that.

One cautionary note: a scientist involved in the U.S. nuclear program said last week that it's not that difficult to make pure calcium from the impure form.

His most depressing observation was, however, that if a country like Pakistan is now looking for calcium, it must already have solved the problem of gathering the enriched uranium or plutonium it would need--and that means their scientists may be within a year of being able to test a device.

The Senate has just passed a federal farm bill that could cost taxpayers in the billions of dollars to support such crops as sugar, wheat, corn, cotton and tobacco, particularly if they are overproduced and in surplus.

But what would you think of a federal farm commodity program that even in the face of a 40 percent production increase from one year to the next and a massive surplus hanging over the market, still won't cost the taxpayers more than a modest administration fee?

The product is almonds, and the way the industry and government are cooperating in handling what is termed an "emergency situation" in the California almond world is described in the Sept. 1 Federal Register (page 43824).

For reasons that the Agriculture Department and the growers still cannot fully understand, the 1981 almond crop has been "the largest in history," according to the notice, leaving "worldwide supplies approximately twice world demand.."

So in July the Almond Board of California recommended that only 75 percent of the 450 million pounds of almonds expected to be harvested this year be marketed.

Under the federal almond program, farmers sell their produce to 24 regulated wholesalers or middlemen called handlers. The handlers, in turn, abide by marketing orders proposed by the almond board and approved by the Agriculture secretary.

In this year's extraordinary situation, the board determined that no less than 25 percent of the crop, or roughly 105 million pounds, will be withheld from the market and kept stored in reserve by the handlers, with no compensation in the form of loans or other payments coming to them from the federal government.

That still would leave a marketable crop of some 40 million pounds above the amount sold last year--and that's a lot of almonds.

The reserves controlled by the handlers could be sold only to new markets or other noncompetitive outlets approved by the board. But for the most part they would be held until next year when some almond analysts expect the trees, as one Agriculture official put it, "will be tired and thus not as fruitful."