AS CONGRESS gets down to writing a new farm bill, one of the few points of general agreement -- apart from the obvious fact that something is the matter down on the farm -- is that it doesn't make sense to subsidize farming on highly fragile land. Deciding what to do about it is, of course, another matter. The Senate Agriculture Committee will soon be considering amendments that, taken together, could help save both farmers and topsoil.

Wind-and water-caused erosion met or even surpassed historical records last year in areas extending from the Pacific Northwest through the Great Plains and Midwest into farming areas of the Southeast. Almost half of the country's cropland is losing soil faster than it can be replaced. Soil run- off is not only a farm problem; each year it costs the country billions of dollars in pollution of water supplies and damage to reservoirs, lakes, dams and hydroelectric facilities.

Many specialists worry that erosion will worsen as financially pressed farmers abandon conservation measures and as more farmland falls into the hands of absentee owners. Nothing can be done about bad weather (except complain), but much can be done about the farming practices and congressional policies that increase the weather's toll in topsoil. A few years ago, Sen. William Armstrong drew congressional attention to the fact that farm subsidies tied to crop production had encouraged the plowing up of millions of acres of fragile grasslands while adding to unneeded and expensive crop surpluses. A more self-defeating set of policies is hard to imagine.

Although Sen. Armstrong's original measure failed to clear Congress last year, proposals now being offered by Sens. Jesse Helms, Robert Kasten and Richard Lugar would not only accomplish the purpose of the original bill but strengthen it significantly. Most important are provisions that would deny all federal subsidies to farmers who plow highly erodible land -- with exceptions given only to land farmed in the last five years, and then only if conservation practices were used -- and establish a reserve that would permanently convert 30 million of the most erodible acres to less damaging use. A final provision sponsored by Sens. Kasten and Edward Zorinsky would extend the same much-needed protection to wetlands.

Taking these lands out of production would not only be a boon to the environment but would also help boost farmers' incomes and cut taxpayers' costs by avoiding production of surplus crops. Congress doesn't get many chances to do so much good so quickly. This year it should act -- before another few billion tons of precious topsoil start blowing in the winds.