If the Reagan administration has its way, the new four-year farm bill will be the final word on farm policy until 1985.
But just a month after the $11 billion farm bill squeezed through the House with a two-vote margin and was signed by President Reagan, farm belt congressmen already are lining up with fresh demands.
When Congress reconvenes Jan. 25, it will find an early crop of ''emergency'' proposals benefitting American farmers. Items on the farm-belt list include:
* Greatly expanded ''set aside'' programs to pay farmers for ploughing under as much as a quarter of their crops.
* Adjustments in loan rates, interest charges, and grain reserve rules to help out cash-short farmers.
* Further relaxation of federal regulations in areas such as pesticide use, worker safety, and food standards.
* Greater pressure on Japan and Europe in particular to guarantee freer access for US products.
Another proposal involves dairy farmers, who fought hard against the new farm bill's cuts in dairy program supports - and lost. The National Milk Producers Federation, recognizing that budget cuts will continue, is making the revolutionary suggestion that dairymen share the cost of a support program: The government would buy up to 3.5 billion pounds of milk products; producers would provide the money for any additional purchases.
Forecasts of improved harvests abroad and another bumper crop in the US are among the leading factors prompting the quick return to Congress. Severe winter conditions are affecting most farm states. But except for short-term damage to fruit and vegetables, the overall effect is to improve crop prospects by adding to soil moisture and reducing insect and weed problems. US Department of Agriculture (USDA) experts foresee an even larger domestic harvest this year than last. Moreover, favorable growing conditions from the Soviet Union, China, and Thailand to Brazil, Argentina, and Australia probably will result in reduced overseas demand for US agricultural exports. This early prospect of increased world supply continues to depress farm commodity prices.
Bill Mullins, chairman of the National Corn Growers Association, has charged that this situation is being made worse by ''inconsistent and unpredictable US government actions and inactions in international trade policy.'' In this view, Mr. Reagan has imposed a ''de facto'' embargo by responding to the Polish crisis with sanctions against the Soviet Union.
The 1981 Farm Bill contains a provision guaranteeing compensation if a ''selective'' embargo singles out farm exports, as happened two years ago. But many farmers say this is proving worthless because the threat of an embargo has cut sharply into exports already. They add that a total embargo still would hurt farmers most because nearly three-quarters of US exports to the Soviets have been agricultural.
USDA Deputy Secretary Richard Lyng's answer to such criticism is that Soviet, not US, policies are to blame. US exports to the Soviets have stalled, he says, ''not because of potential action of the United States government but because of potential actions of the Soviet government. There is nobody prohibiting their purchases. It is reluctance of the grain dealers or grain companies to make big commitments to that kind of a government at this particular point in time. This kind of uncertainty is something that our government simply cannot solve.''