The Carter administration is planning a $300 million project that would use satellites to forecast worldwide harvests, ranging from rice in Vietmam to wheat in the Soviet Union.

The project has the tentative approval of at least a dozen countries whose crops would be involved and the definite approval of the White House, which is understood to have allocated $30 million to get the project under way in fiscal 1980.

The project still has no formal name, but will make use of the existing Landsat satellite and a follow-on Landsat to be put into earth orbit late in 1980 to forecast harvests of wheat, barley, rice, soybeans, corn, cotton and sunflowers in all the major crop-growing regions of the world.

The project would cost an estimated $300 million over its first six years. Most of the cost would be picked up by the Agriculture Department and the National Aeronautics and Space Administration. The Interior Department, the State Department and the National Oceanic and Atmospheric Administration would also pay a share.

Most of the money would be spent to buy computers, hire people and contract with companies and universities to begin the complex task of forecasting crop yields from satellite photographs of crops as they progress from planting to harvest.

At least one reason for undertaking such an ambitious project is the failure by the Agriculture Department and the Central Intelligence Agency in 1977 to forecast correctly wheat harvests in the Soviet Union and soybean harvests in Brazil.

Both harvests were badly overestimated, with the result that the United States was ill-prepared for the heavy foreign buying of both crops here, buying that led to higher prices for American consumers.

The State Department's Agency for International Development (AID) is also backing the project, in part because it would provide an early warning of impending crop disasters around the world. AID gives disaster assistance to many countries, much of it in food, when harvests are wrecked by blight, drought or too much rain.

The satellite forecasting project, informally called the "Agricultural Initiative" at the agencies that would be involved in it, is the outgrowth of an experiment run a year ago by NASA and the Agriculture Department called Large Area Crop Inventory Experiment (LACIE). That involved the use of the Landsat satellite now in orbit to forecast wheat harvests.

Sources inside the Agriculture Department described the LACIE project as a "mixed success" underestimating by 15 to 20 percent spring wheat harvests in Canada and the United States but coming within 1 percent of the spring wheat crop for the Soviet Union.

Photographs taken by Landsat with the permission of the Soviet Union were analyzed together with weather information provided by the Soviets to produce a forecast of 91.4 million tons at least a month before spring harvest began. The Soviets said their spring wheat harvest was 92 million tons.

In explaining the dramatic difference between a 1 percent error and a 20 percent mistake, Agriculture officials said the Soviet crop was easier to forecast from satellite pictures because their fields are larger and they don't grow other crops like barley and alfalfa alongside wheat that "confuse" the satellite.

"The major problem in the U.S. and Canadian pictures was in differentiating between spring wheat and barley," one Agriculture source said. "A real farmer can't even tell the difference until their heads come up, and we were looking at pictures that showed more than an acre of the stuff growing side by side."

Sources said that part of that trouble will disappear with the launch of the next Landsat satellite in 1980 because its cameras are sharper than those aboard the current satellite. The next satellite will focus on fields no larger than 90 feet across, while the existing one can only pick out fields 120 feet across.

The cameras on Landsat take infrared pictures, too, returning photos of crops that appear different if they are immature, diseased or stricken by drought. The next Landsat will have improved infrared cameras aboard.

No fewer than a dozen countries have been talking with the United States about a satellite service to forecast harvests. They include Mexico, Australia, Argentina, China, Brazil, Canada, Thailand and Burma.