American farm income will fall to its lowest level in a half century, the U.S. Department of Agriculture forecast yesterday, confirming what most farmers have predicted for months.

The department projected that net farm income this year will be $19 billion, the lowest mark, when adjusted for inflation, since 1933.

The forecast indicated that net farm income, the traditional measure of agricultural stability, will drop for the third consecutive year, although not by as much as many trade sources had predicted. The figure represents farmers' income after payment of production expenses.

The report cautioned, however, that the picture could change substantially in final accountings, with commodity prices remaining depressed and with the prospect of record corn and soybean crops and a near-record wheat crop.

Although the net farm income figures are adjusted to 1967 dollars, making comparisons with income figures from Depression years possible, department economists stress that today's farming situation is far different from that a half-century ago. Farmers today have more off-farm income and, while income totals are comparable, there were about 6.5 million farmers in 1933 as opposed to 2.4 million today, meaning that more money now is spread among fewer farms.

The farm picture would be gloomier if not for direct government payments to farmers of about $4 billion, three-fourths of it in the last quarter of the year. These include 1982 deficiency payments, advances on the 1983 crop and disaster payments.

"Major uncertainties surrounding these forecasts are fourth-quarter crop conditions, crop and livestock prices later this year, advance direct payments to participants in 1983 commodity programs and production expenses," the report said.

As an example of the uncertainties in the report, the USDA projected that corn prices will range between $2.35 and $2.55 per bushel by the end of the year. That would represent a major and improbable price climb from the August average of $2.19, the lowest mark for this basic grain since 1977.

USDA's projection also said the value of U.S. agricultural exports is expected to drop by about 8 percent, or about $3.3 billion in fiscal 1982, to $40.5 billion. Although a record volume of 165 million tons is projected, this would be the first year-to-year decline in the value of farm exports since 1969.

There was a slightly upbeat side to the USDA report. Departmental economists said that livestock receipts will increase 2 percent from last year's level, reaching a record high of $70 billion, and that the recent steep inflation in production expenses has slowed substantially.

The report said production expenses are expected to increase only 2 percent this year, compared with 9 percent last year and 10 percent in 1980. This year's 2 percent, if it materializes, would be the smallest increase since 1964.

At the same time, the report indicated that total production expenses for 1982 will exceed cash receipts, which are projected to be 1 percent lower than last year's level, another indication of the tight cost-and-income squeeze in which farmers find themselves.

The report released yesterday had been awaited for months by agricultural organizations and members of Congress, who heavily criticized Secretary John R. Block last winter when he ordered an end to the department's tradition of periodically publishing the projections.

Legislators complained that Block and the Reagan administration were attempting to suppress bad news in an election year as Congress was moving to rewrite and strengthen support programs in an attempt to increase farmers' incomes.

Block, however, insisted that the projections frequently had been off the mark and that he did not intend to continue publication of income forecasts that he considered more speculative than factual.

The subject remains controversial, and the Senate Agriculture Committee has scheduled hearings later this month on legislation proposed by Sen. Alan J. Dixon (D-Ill.) and others that would require monthly publication of the projections.