DURING the 1970s, while productivity in the economy as a whole grew hardly at all, agricultural productivity grew at a healthy 5.5 percent a year. But there was a cost. The increase in production was achieved in large part through farming practices that amount to "mining" the land.

The phrase is an apt one because, for all practical purposes, topsoil is a nonrenewable resource like coal or oil. Though it is continually produced, it takes nature 100 to 1,000 years to replace one inch of topsoil. The current average rate of erosion on American farmland is one inch every 30 years. In the most productive areas of the Corn Belt, the soil is blowing and washing away twice that fast. And in more fragile areas, losses from erosion are stripping the land of one inch in less than two years.

In this week's series, "Vanishing Land," Post reporter Ward Sinclair described a nationwide trend that is already cutting into agricultural production. Soil erosion is unusual among economic and environmental concerns because no one is arguing that the problem doesn't really exist, or that if it does it isn't really serious, or that it needs more study before action. Nor is there any mystery about how to prevent erosion or how to stop it once it has begun. Selective planting (adapting crops to soil types), keeping the most vulnerable land in timber or pasture, contour farming, terracing and crop rotation are practices almost as old as agriculture. Newer techniques like minimum tillage, where the rubble of harvested crops is left undisturbed to protect the soil, are also widely known. The needed answers are economic and social, not technical.

However, American farming is in the grip of a system that cannot easily be interrupted at a single point. The intensive use of heavy machinery, fertilizer, fuel and chemicals that dramatically raised crop yields also dramatically raised costs and therefore reduced the per acre profit. This pushed farmers to plow up marginal land, which in turn led to larger crops and therefore lower farm prices and therefore lower profits. The appetite for agricultural exports, now a crucial part of the national balance of trade, creates further pressures for more production. Escalating land values has meant more absentee ownership, more farming under short-term leases, and more speculation in agricultural land. All of these mean an emphasis on quick return rather than long-term productivity. Land that could produce indefinitely with proper care is now being ruined in as little as three growing seasons.

All of this is changing, but slowly. Last year's farm bill recognized for the first time the need to keep good land in farms instead of suburbs and shopping centers. New and newly appreciated farming methods are spreading. But there are still plenty of short-sighted and ignorant farmers, often encouraged by federal policies that underwrite farming on erosion-prone land. Many good farmers cannot afford the soil conservation steps they would like to take. Federal and state conservation funds are being substantially cut. Fifteen tons of topsoil washes out the mouth of the Mississippi River every second. So far, erosion is winning the race with ease.