Mention "sustainable agriculture" to U.S. farmers, and you create an instant argument.

Supporters claim that sustainable agriculture - which basically involves soil conservation and minimal use of pesticides and chemical fertilizers - is necessary to maintain farm land's long-term productivity.Opponents generally agree on the importance of soil conservation, but argue that chemicals are often essential production tools.

Robert O. Blake, an expert on global conservation issues, belongs in the first group. He thinks sustainable agriculture is particularly critical for the Third World - and he makes some points that even opponents of sustainable agriculture will likely agree with.

Mr. Blake belongs to the Washington-based Citizens Network for Foreign Affairs and is chairman of its Committee on Agricultural Sustainability for Developing Countries.

The Citizens Network promotes "understanding of the importance of U.S.-Third World relationships to U.S. economic, social and environmental well-being."

Within 20 years Mr. Blake thinks the Third World will have 40 percent more people and up to 40 percent less arable land. He worries that "the chance for the next generation to feed itself" is threatened by "poor planning and poor conservation methods."

He sees these problems in Third World agriculture:

* Marginal land, like that often found on hillsides and in rain forests, is being farmed too intensively. Such land quickly plays out unless farmed with great caution. For example, recently deforested farm land in Brazil's Amazon Basin is already wearing out.

* A lot of good land is also being pushed too hard. That doesn't hurt the land right away, but long-term damage eventually will be done unless farmers ease up.

The United States is familiar with that. In the 19th century, farmers in some Southern states pushed their fertile land much too hard, and some of that land still hasn't completely recovered.

* Pesticides are often overused or misused. That prematurely increases insect resistance to the pesticides and leads to unnecessary crop damage.

* Irrigation techniques need improvement. India and Pakistan, for example, are losing farm land because of poor irrigation practices.

So why isn't the Third World doing a better job of managing its farm land?

There seem to be a couple of problems. For one, Third World farmers are often desperate to feed their hungry families and neighbors. Maximizing short- term production - regardless of long-term consequences - probably seems necessary to them. It probably would to you and me, too.

Another problem: Third World farmers usually don't have much access to technical expertise. In contrast, U.S. farmers can easily get plenty of information about the most efficient way to irrigate or use a certain pesticide.

So Mr. Blake thinks Third World farmers should get more training and guidance in the proper use of production tools like pesticides and irrigation.

That makes sense. Even U.S. farmers who oppose some aspects of sustainable agriculture will agree that land should be farmed judiciously and that chemicals must be used with skill. Many Third World farmers are doing

neither - to their own eventual detriment.

Does that mean that sustainable agriculture is the best way for the Third World to go? Hard to tell. A good case can be made that many Third Nation nations will need to rely heavily on pesticides and chemical fertilizers to feed their skyrocketing populations.

Even so, it's obvious that Third World farmers must pay more attention to the environment in order to ward off eventual disaster.

U.S. farmers, meanwhile, have a stake in what their Third World counterparts do. Unless Third World nations stabilize agricultural production, their economies will fall apart. If that happens, they won't have any money to buy food from us.

And from a humanitarian viewpoint, all Americans should be disturbed by the danger of worsening food shortages in the Third World - a dismal scenario that might be unavoidable unless Third World farmers do a better job of taking care of their land.