Sooner or later, someone no doubt would wonder about the world celery seed situation, so here's some of the good news and bad news crossing the desk of Rex E.T. Dull, the man at the Agriculture Department who keeps tabs on the spice trade.

The good news is that prices early this year were down from last year's exceptional highs, which were caused by civil strife around Amritsar, India. But the bad news, according to late intelligence, is that unrest is boiling up again and nobody knows what will happen to celery seed.

India, it turns out, supplies most of the world's celery seed. And any savant who knows his potato salad knows that potato salad is nowhere without celery seed from India.

The nutmeg picture, on the other hand, is a bit clearer from Rex Dull's vantage point. Indonesian nutmeg is doing well; nutmeg from Grenada, however, is doing poorly in general and here in particular because it contains more fat than American buyers want.

After the U.S. invasion of Grenada in 1983, the Soviet Union canceled a contract to buy 500 tons of nutmeg a year and the island's warehouses now hold a huge surplus, Dull said recently. An American firm's promotion of a Grenadian nutmeg "kit," with graters and recipes, hasn't made much impact either, he added. To top off the problem, the Soviets recently rejected a new sales feeler put out by Grenada.

Arcana like this are the sugar and spice of Dull's life. He has been the Foreign Agricultural Service's keeper of data on tea, spices, cocoa and essential oils for 24 years and never, by his account, has it been a bore.

"I've always enjoyed working with the spices, but you never get to where you know it all," he said. "Because of our budget cuts, we've had more work and I was assigned to work on vegetables and wine and olives as a result. Last year we were running ragged around here. I was a nervous wreck."

Dull's reports and market analyses, published periodically by the Agriculture Department, apparently are snapped up eagerly by people who have a need to know about his subjects. He said the spice trade association just bought 500 copies of his "Tea, Spices and Essential Oils" report for 1985. Dull's "Cocoa," which came out in March, was another big seller.

As a result of his total immersion in the fine print of spices and the like, Dull has become a walking encyclopedia of things you always wanted to know but probably never dared ask about.

For example, most of our mustard seed comes from Canada and perhaps the biggest purchaser in this country is the R.T. French Co., the mustard manufacturer. "You will see turmeric as one of the spices in the statistical tables imports hit 1,800 metric tons in 1984 . They color the mustard with turmeric because people expect it to be yellow," Dull said.

Dull's data also disclose that oregano imports continue moving right on up on the charts. "Oregano has become very popular, due to the popularity of pizza, spaghetti, the Italian foods. Same thing for basil, which is increasing," he said.

Or consider the sesame seed. Imports were 36,759 metric tons last year by Dull's reckoning. The country's largest user probably is the McDonald's fast food chain, which promotes the sesame seed bun.

The hot item in the spice business these days happens to be pepper. Dull's last circular reports that tight supplies and rising prices have hit the world pepper market, with production having fallen below consumption.

But pepper, like so many other farm products, is an up-and-down commodity. Brazil boosted plantings and exports to record levels in 1981 and 1982, but when prices dropped, farmers reduced plantings and "neglected cultural practices," Dull reported. Some Malaysian farmers switched to cocoa; bad weather zapped India's 1984 crop.

So, with tight supplies, world prices for black and white pepper nearly doubled, and have continued their sharp climb on the New York spot market this year, according to Dull's research.

The bottom line of all this comes to Dull's conclusion that the American diet probably is more varied than ever. U.S. imports of condiments, seasonings and flavorings last year reached a record 195,855 tons, valued at $271 million.

An awful lot of buns, pizzas and what-have-you, in other words.