A COMMON THREAD runs through these fragmentary reports: In Arizona a copper producer, Phelps Dodge, is suspending production at three smelters and laying off several thousand people. In Brazil, the government's price support system is acquiring enormous--and enormously expensive--stocks of surplus coffee. In the American Midwest, farmers' income last year was the lowest since the 1930s, and the price of corn is now sharply lower than it was a year ago. In Bangladesh, a similar fall in the price of a different crop, jute, deepens the distress of one of the world's poorest countries.

The world's economy is now going through a slow process of deflation--and, as usual, deflation bears heavily on the prices of commodities. Among the agricultural products, this downward pressure has been aggravated by the very large crops that are the response to shortages of the 1970s.

You have read more than enough about the falling prices of oil and the worldwide glut. Much the same forces are now dominating the markets for other commodities as well: wheat and aluminum, cocoa and cotton, rubber and sugar. In many of them, as in oil, the oversupply is being aggravated by the dumping of inventories as interest costs rise.

The United States is the world's largest producer and exporter of raw commodities, and the American economy is feeling this general decline in the markets. But the effects here are minor compared with those in the one-product countries. Jute accounts for nearly two-thirds of Bangladesh's export earnings.

The International Monetary Fund publishes an index of the prices of 35 important commodities. That index is now lower, in terms of the manufactured goods that commodity earnings can buy, than at any time since the IMF first bega computing it in 1951.

Manufacturing industries and the people who work for them are usually pretty well equipped to protect their prices and wages from erosion in times, like the present, of falling demand. But commodities are sold on auction markets to the highest bidders, and the producers' influence over those prices ranges from little to zero. Public policy in the United States, and most of the other industrial countries, is now successfully slowing down inflation. But policy that restrains the prices of manufactured goods has a far more drastic effect on the volatile markets in which the commodities are traded. For the countries that depend on these markets, the outlook for stable growth is deteriorating rapidly--at a time, incidentally, when the drop in OPEC oil revenues will leave the commercial banks with less money to lend them. The consequences of American economic policy are not confined to American markets.