New Orleans — Day and night, ships glide down the Mississippi River past New Orleans, loaded with Midwestern grain headed overseas, part of a farm-export business now earning $46 billion a year.
Along with the grain, however, an even more constant export slips past this trading city -- an estimated 40,000 tons an hour of precious topsoil washed from the Midwest's farmlands.
America's grain export boosts the US economy and helps feed the world. But increasingly, agricultural experts warn that American grain exports will suffer unless the United States acts quickly to reduce soil losses caused by current farming practices.
''Conservation of land resources should become more prominent in the minds of the world's decisionmakers,'' insists R. Dudal, director of the United Nations Food and Agriculture Organization's land and water development division.
This Belgian soil conservationist says soil erosion problems require priority treatment, ''because unlike many other natural resources, land is irreplaceable. When you speak about air pollution, for instance, or water pollution, these are things which can be restored, problems which can be reversed. But the destruction of land is something which is final.''
Dr. Dudal explains that protecting the world's productive soils is vital because population increases mean world food demand ''could increase by 50 percent in the next 20 years and could more than double again in the first half of the next century.''
Dudal, in New Orleans along with some 800 other soil experts to attend a Soil Conservation Society of America conference discussing ''the politics of conservation,'' calls for additional research on a global scale to assess the world's agriculture lands and provide information needed to protect the world's soil resources.
On the basis of past research, he says, UN studies show that 17 developing countries already cultivate 90 percent of their potential arable land. This situation, he says, means that these countries can't afford to lose any land to erosion. In an increasingly interdependent world, he adds, it also means that countries such as the US and Argentina must protect their agricultural productivity in order to provide an exportable surplus for food-deficit countries.
One sign of progress Dudal points to is the international support for the Food and Agricultural Organization's 1981 World Soil Charter. This charter, he says, calls on governments to:
* Recognize that ''among the major resources available to man is land, comprising soil, water, and associated plants and animals: the use of these resources should not cause their degradation or destruction because man's existence depends on their continued productivity.''
* Manage farmland ''for long-term advantage rather than for short-term expediency.''
* Give high priority ''to optimising land use, to maintaining and improving soil productivity, and to conserving soil resources.''
* Provide ''proper incentives at farm level and a sound technical, institutional, and legal framework'' to encourage soil conservation.
Peter Myers, head of the US Department of Agriculture's Soil Conservation Service, agrees about the seriousness of the soil-loss situation. He told the Soil Conservation Society conference here that currently, ''A third of US cropland loses so much soil it reduces the ability of the soil to produce food and fiber.'' Mr. Myers promised that despite the Reagan administration's budget cuts, ''soil and water conservation will remain a federal priority.''
Neil Sampson, executive vice-president of the National Association of Conservation Districts, warns that despite such promises, federal spending on soil and water conservation has declined seriously.
Louisiana state Sen. Samuel Nunez tells of one example of the problem: 47 square miles of Louisiana land lost per year as the Gulf of Mexico eats into coastal wetlands damaged by man. Senator Nunez says, ''Soil conservation programs are vital programs which affect the whole security and welfare of this nation.'' He says his state is taking the initiative on its wetlands problem by spending $35 million in state money - hoping this will shame the federal government into increasing rather than cutting spending for such measures.
But Washington insider James Giltmier warns that soil conservationists will need to become skilled political activists if they want to hold onto federal dollars. This former assistant to Sen. John Melcher (D) of Montana says that for congressmen, ''it often appears that our conservation programs are pouring money into a bottomless pit.''