This is the day, above all other days, that the American table groans.

It is Thanksgiving, the national rite of plenitude symbolized by the roast turkey, the cranberry and the pumpkin pie. In some quarters, the day has come to represent more of an excursion into gourmandism than the festival of harvest and gratitude that it once meant to an agrarian nation.

Yet through the heartland, and at the tables where these things matter, a word of grace and benediction will be extended to the U.S. farmers who continue to make this nation's food bounty a thing of awe to much of the rest of the world. There's more of it, with more variety, generally at the lowest cost.

Recent statistics from the Agriculture Department tell part of the story. In 1984, only 11 percent of U.S. consumers' total personal consumption costs were for food -- the lowest amount ever spent for food in any country since the department began keeping records.

Canadians ranked second, with 13.4 percent of their spending going for food. Other figures included 21 percent in Western Europe and 30 percent for East Bloc countries, led by Poland's 37 percent and the Soviet Union's 25.6.

In contrast, Latin America averaged 35 percent (Honduras was high with 41.4 percent) and spending averaged 43 percent in the developing Asian nations. Japan and Hong Kong, which rely heavily on imports, averaged 19.9 and 18.8 percent respectively.

But the American cornucopia is achieved at high cost in terms of family stress felt in the most fecund corners of the land, in terms of the wrenching changes felt in a shrinking rural America, in terms of the enormous costs of federal farm support payments that often go to those who need help least.

If the official statistics are accurate, there are about 100,000 fewer farmers this Thanksgiving than there were a year ago. And a year ago, there were 100,000 fewer farmers than there were in 1985. And so on in 1984 and 1983, years of intense economic stress for those who send food to the city.

The reasons for this stress are many, and solutions are debated from Capitol Hill to the crossroads cafes in venerable outposts with names such as Rugby and Tupelo and Wytheville and Garden City.

Part of the debate, in fact, involves the symbolism of the Thanksgiving feast. In part because of government policies and in part because of massive surplus production, farm prices are on a downhill slide and the farmer's share of the cost of the feast diminishes each year, as it has done virtually without fail since 1980.

As incomes decline and farming expenses do not fall commensurately, profits narrow and the exodus from the land accelerates. Small farmers become tenants or leave; large farmers get larger and remain. Yet the land, still farmed by someone, yields its bounty.

Consider this example:

The American Farm Bureau Federation in Park Ridge, Ill., set out recently to calculate the cost of a traditional Thanksgiving dinner in 1987. Bureau home economists prepared a meal for 10, including turkey and all the trimmings, for $24.51. Last year the same meal, with food prices averaged from nine national markets, cost $29.75.

According to the Agriculture Department, the farmer's share of the retail food dollar in 1986 was 25 cents. Thus, from last year's Farm Bureau dinner the farmer would have received $7.43. The department has not produced a 1987 calculation, but assuming the farmer's share remains 25 cents, his take from this year's dinner would be $6.13.

Jim Hightower, the Texas commissioner of agriculture, paints a similarly dreary bottom line in his state. His department recently calculated that a turkey dinner based on average Texas retail prices will cost $2.52 per serving today. The Texas farmer's average net return would be 70 cents -- actually, just a shade better than Farm Bureau's national calculation.

In his holiday message, Hightower touched on another key issue: the portion of the food dollar that goes to processing and convenience.

The prototypical Texas dinner cited by Hightower included stuffing, at 19.8 cents per serving. "You're paying 20 cents for all the stuffing you can manage to load into your system," he said, "but the farmer who grew the wheat for the stuffing is losing half a penny per serving."

"If the stuffing is the store-bought variety, the box or plastic containing the bread crumbs costs 10 times the price of the crumbs themselves."

This gets into what Agriculture Department economists call the farm-retail price spread, basically the difference between what the farmer gets for his raw commodity and its price at retail. After four years of running almost neck and neck, the spread began to open in 1980 and it has widened every year since. In 1980, the farmer's share of the food dollar was 31 cents.

"The farmer's share could be going down, as it has for a long time, but this is not necessarily an equity measure," said Denis Dunham, an economist with the Economic Research Service. "The wheat farmer might get 5 cents out of the bread dollar and the egg farmer 60 cents of the egg dollar, but that does not mean the egg producer is doing better than the wheat producer."

Dunham's price-spread calculations for last year showed another widening between farmer and retailer, mainly because of labor and advertising costs. Yet food industry profits increased in 1986, largely from higher sales, the Economic Research Service said, while food costs were going up at a slower rate than consumer income.

There's another point worth noting when discussing the relatively good deal the American food consumer gets from the system. Dunham's data shows that food spending as a percentage of income varies considerably by income levels. The poorer one is, in other words, the more that food costs.

People getting between $30,000 and $40,000 spent 11.6 percent of their income on food in 1984. But Americans making between $5,000 and $10,000 spent 25.7 percent of their money on food -- almost identical to the Soviet Union.