As you read this, a little chunk of Iowa, one of the richest agricultural areas in the world, is just reaching the Gulf of Mexico. Along with it are little chunks of Illinois, Minnesota, Missouri and other states along the Mississippi River.

If you stood on the banks of the Mississippi at Memphis, you could measure about 40 tons of fertile topsoil flowing downstream every hour. Hour after hour, day after day, it keeps on rolling south.

Similar things are happening in other watersheds. The farm land that makes the United States the most productive agricultural nation in the world is washing away.

By estimates of the Department of Agriculture's Soil Conservation Service, American is losing about 5 billion tons of topsoil every year to water and wind erosion. Nature replaces some of it, conservation practices slow or stop some of it, but the net loss grows.

Additionally, a Council on Environmental Quality study reported this year that about 225 million acres of arid western land, an area about the size of the original 13 states, are undergoing "severe desertification" -- that is, loss of groundwater and high erosion that gradually makes the land unsuitable for cultivation. The potential consequences, CEQ said, are grim.

Such alarms have been heard of years, but they are ringing with increasing frequency. Congress is under pressure to create more forceful approaches to stanching the erosion of farm land under growing strain to produce more food for domestic and foreign markets.

With more than a third of U.S. farm production going abroad, there also is a growing realization that American farmers are subsidizing their clients by not including in commodity prices the cost of the toll on their land.

Iowa Gov. Robert D. Ray, concerned about the production pressure, has proposed a tax on farm exports to finance soil protection prgrams that neither farmers nor Congress is willing to take on. At Ray's request, the proposal is under study by the National Governors Association.

The Reagan administration, meanwhile, through Agriculture Secretary John R. Block, has told Congress that it is preparing proposals that will be sent to Capitol Hill by end of the year -- too late, obviously, to be a part of the comprehensive new farm legislation that will guide federal policy through 1985.

The 1981 farm bills prepared by the House and Senate Agriculture committees, soon to be debated in the two chambers, contain a variety of new approaches to the soil conservation problem -- special loans, matching grants for land repair, a volunteer worker program, emphasis on badly affected areas.

Committee sources maintain that this year's budget restraints, in combination with the pressures to renew the basic farm legislation, have forced postponement of more comprehensive or innovative approaches to soil conservation.

But in the view of the Natural Resources Defense Council, a major conservation organization, the proposed farm bills in the House and Senate offer little more than a piecemeal approach to the problem.

"It is a raging inferno and people are running at it with buckets of water," said NRDC staffer Thomas Barlow. "All of the approaches offered in the farm bills are needed, but none of them does anything major in bringing down soil losses." Part of the problem is excessive land disturbance during cultivation, the use of erosion-prone land for farming, insufficient rotation of crops. But another factor gaining increasing attention may be the land strain caused by federal programs and generally low farm prices, which encourage more production.

NRDE research and the CEQ study, prepared by Washington writer David Sheridan, conclude thqt the system of federal financial support for American farmers contributes to soil erosion by encouraging high-cost cultivation methods and overproduction on marginal land.

"The federal government subsidizes both the exploitation and conservation of arid land resources,= Sheridan wrote for CEQ. "But the subsidies for conservation are meager compared with those for exploitation. The net effect of the federal subsidies is to encourage production, not conservation.

"Federal subsidies are, in other words, a major force behind the desertification of the United States," he said.

Sheridan calculated that for every dollar spent by the government on conservationm $5 is spent to bolster or stimulate production.

The "subsidies" include price-support loans and target price payments for basic commodities, which stimulate conversion of natural grasslands to crops like wheat and cotton; low-interest loans for irrigation systems that mine ground water, and development assistance that encourages the urbanization and industrialization that vie for agricultural water supplies.

NRDC's Barlow said in a meeting with reporters last week that his organization is attempting to persuade Congress to take a new look at the situation as the farm bills reach the floor.

"There are some 30- to 40-odd programs of conservation outreach in the Department of Agriculture -- and in toto, they only do 20 percent of the job, by the department's own estimates," Barlow said. "The Soil Conservation Service reports that erosion rates exceed acceptable levels on over 140 million acres of crop land."

The NRDC approach calls for simpler, less expensive conservation practices than the complex systems traditionally promoted by SCS.Chief among these cheaper techniques are minimum tillage, strip cropping, crop rotation, planting of windbreaks and maintenance of vegetative cover on erosion-prone land.

To encourage that, NRDC urging Congress to authorize voluntary county-level prgrams that would result in the withholding of a farmer's federalfinancial assistance if he did not use the simple techniques prescribed by a board of county farmers.

The concept makes brows turn feverish on Capitol Hill, where virtually every proposal smacking of land-use planning in th last decade has been sent to early burial. Conservation Cross-compliance," as Barlow calls it, fits in that category.

But he and NRDE remain hopeful. "If authorized in law, it may turn out that this test of cross-compliance may not be tried by any county. So be it," Barlow said. "On the other hand, the arrangement may work in various parts of the country. If a few of these work, others will be less wary about joining in."