From 1971 to 1973, American wheat farmers received $1.8 billion in federal payments for not planting crops. But $800 million of that covered land the farmers would not have planted anyway, according to the General Accounting Office.

The ineffective payments occurred, GAO said, because the Department of Agriculture allowed the farmers to include fallowed land, a common technique used for preserving fertility, in their so-called set-aside acreage. "Our conclusion (is) that some farmers were being paid for doing something - fallowing cropland - that they would have done anyway, GAO said.

The wheat payments were part of $7.8 billion paid to wheat, cotton and feed-grain farmers in the crop years of 1971 through 1973, the year the nation embarked on all-out production to meet domestic and foreign needs. The payments were designed to reduce surpluses and thus help raise farm prices.

The GAO report comes at a time of new surpluses following tight worldwide food supplies; the United States expects to have 1.1 billion bushels of wheat left over in storage when wheat harvesting for this year begins. Thus, the GAO recommended that new farm legislation being enacted this year not allow fallow land to qualify for any future set-aside payments, if Congress approves such programs.

But there is also increasing opinion that the nation can no longer politically or morally afford to pay farmers not to grow crops, for the whims of weather can abruptly plunge whole nations into famine.Agriculture Secretary Bob Bergland believes, for his part, that the government should financially help farmers store excess food stocks for the use in times of shortages.

Current farm legislation on Capitol Hill continues authority for set-aside programs. Bergland favors continuation of the authority but, a spokesman said, would use set-aside only as an extreme remedy to crop surpluses.

The GAO findings were the result of an investigation into why only 38 million acres of idled cropland came into production after the government ended payments that supposedly had kept 59 million acres not of production.

GAO wanted to know whether the government had paid million of dollars to keep nonexistent land, 21 million acres of it, out of crop production. The agency, which Congress uses to monitor several programs and executive agencies, concluded that the land existed but that it was not put into production, in most cases, because equal or similar amounts "were purposely left idle as a normal farming practice."

Some land, GAO said, was kept in pasture for grazing or had been converted to nonfarm use, such as highways, airports or industrial and urban development.

Thus, the GAO researchers concluded: "The 1971-73 cropland setaside expenditures were only partly effective in reducing production primarily because the [Agriculture] Department allowed summer fallowed cropland, normally idled regardless of set-aside programs to qualify for federal payment."

The GAO cited a department study indicating that summer fallow acreage "constituted virtually all of the set-aside cropland in North Dakota." The report specifically mentioned one farm where acreage increased only three acres although the farmer received set-aside payments for 39.

Fallowing is the practice of leaving land out of production to allow moisture levels to rebuild. It is more commonly practiced in drier parts of the nation, where wheat is a major crop. Thus the GAO study concluded that set-aside payments to wheat farmers for fallowed land were greater than for cotton and feed-grain farmers.

The study put no dollar figure on payments for fallowed cotton or feed-grain land, saying only that $800 million went to wheat farmers in a futile effort to reduce, production. The problem was "primarily" in wheat, GAO said.

The Agriculture Department responded that the set-aside programs were also intended to help farmers' income, and that if they hadn't been paid for fallowed land, they would have received money - "a major portion of the $800 million" - in other farm benefit programs.

The GAO report referred also to another disturbing trend - the loss of 2.7 million acres of farmland to non-agriculture purposes each year. This is partiallyt offset by 1.3 million acres of new cropland, for a net loss of 1.4 million acres of land a year.

In many cases, however, the lost land is fertile and more productive, with the new land more marginal and in need of vast amounts of fertilizer and energy-consuming irrigation.