President Carter signed into law yesterday a bill establish a nationwide system of monitoring foreign purchases of American farmland, but he warned that overall trade policy opposes unnecessary restrictions on international investments.

The law directs the secretary of agriculture to evaluate the effects of foreign investment on family farms and rural communities and requires reporting by all foreign citizens who hold or acquire a significant interest in farmland.

Carter said many farmers have expressed concern that increased foreign investment has driven up land prices and that foreign investors may be more willing to subdivide or divert the land to other uses.

"While recent surveys by the Commerce Department and the General Accounting Office suggest that foreign ownership of domestic farmland is still very low," Carter said in a statement, "I recognized that we need more information on farmland ownership patterns before we reach definite conclusions."

He cautioned that there would not be immediate results from the studies and that the information would have to be analyzed carefully.

"The policy options that arise from such analysis will have to be considered in the light of overall U.S. trade policies, including our opposition to unnecessary restrictions on international investment flows," he said.

"It will be particularly important to evaluate whether various economic trends, including higher real estate prices and absentee ownership of land, are actually due to foreign investment, and whether they reflect other domestic social and economic factors."