American farmers - and those in most other industrialized nations as well - have reaped abundant harvests of wheat, corn and other crops in 1990. And that's just the start of the bad news.

Consider what this is causing:- Agricultural surpluses have resulted in falling prices, so U.S. farmers are making less money.

- The lower earnings mean that American taxpayers must pay an extra $1.5 billion in minimum price supports to farmers. The target price for wheat, for example, is $4 a barrel. The current market price is about $1.50 a bushel less than that, so taxpayers must make up the difference.

- Because most countries have their own surpluses, the export market in grains has just about collapsed. In what little export market exists, heavily subsidized European farmers are undercutting American sales.

- The surpluses come at a time when the United States and other countries have just failed to negotiate an end to government subsidy programs. The continued dependency on subsidies this year will make that future goal much harder to achieve.

- Unsold surplus grains are going into storage. The backlog will tend to depress prices next year and even lower the target price so that farmers may earn less in 1991.

- The lower crop prices probably won't translate into lower prices for consumers at the grocery store. Middlemen, packaging, marketing and other costs account for 75 percent of the food dollar. What the farmer earns affects only 25 cents of each food dollar.

Back in the bad old days before government meddling in agriculture, farmers had good and bad years, good and bad prices. But when farm prices were low, at least consumers saw some benefit.

A free-market farm economy is long overdue. It couldn't work out any worse than the present government-subsidy tangle and it might evolve into something a lot better.