The grim years of the Dust Bowl disaster of the 1930s, when the land dried up and blew away, are now only a distant memory for most farmers who till the fertile Nebraska prairies. Much of the land devastated by wind erosion during those years now produces bumper crops. But there are disturbing signs that the careful conservation practices that helped to reclaim the land are being abandoned.

All across the Midwest, the soil is being swept away by wind and water. In the last two months of 1977, about 1,430,000 acres were damaged by wind erosion. The experts report that the cropland is being washed away "at nearly twice the rate considered acceptable."

In western Nebraska, giant green circles of corn now cover the land where great herds of buffalo once foraged for grass. The corn's thirst for water is quenched by giant irrigation systems, which draw water from under the ground and spout it into the air in slowly rotating circles.

Agriculture Department soils expert Benny Martin is troubled over the increased soil-erosion problems that have followed the rapid development of the irrigated circles. He estimates that over 200,000 acres of this land should never have been put to the plow.

Vital tree stands to break the wind, landscaped after the Dust Bowl at the taxpayers' expense, have been bull-dozed down to install the irrigation systems. Cross-slope farming methods to reduce erosion are too often ignored.

The increase in irrigation, which has brought an economic boom to some Nebraska counties, has had other unwanted side effects. Some of Nebraska's drinking water is being polluted by the nitrogen fertilizers that are mixed with the irrigation water. Nitrates from the fertilizers seep through the sandy soil into the underground water pools that the irrigation systems tap. Doctors in the area have treated infants for nitrate poisoning.

A U.S. Geological Survey of the drinking water in Holt County, Neb., disclosed that 40 percent of the tap water contained nitrate levels exceeding federal safety standards. Some wells showed contamination three times the acceptable amount.

And with each irrigation system spewing out 1,000 gallons of water a minute over a 16-acre section of corn, the ground water levels in Holt County are dropping drastically. A Bureau of Reclamation spokesman told our reporter Hal Bernton that there is now a "critical ground water depletion." This means Holt County farmers face a shortage of water irrigation.

As a remedy, the Carter administration now wants to dam the Niobrara River and divert its waters to the underwater reservoirs in the hope of diluting the high nitrate concentrations and replenishing the water. Environmental critics fear this will only worsen the situation by opening the gate to more extensive irrigation, which will leach more intrates back into the water. The project would also cost the taxpayers $194 million.

The Nebraska Center for Rural Affairs estimates that at least 30 percent of the controversial irrigation has been financed by non-farm investors. Prairie acres once fallow have been bought up cheaply, plowed under and pumped full of chemicals to get in on the corn boom. Should the land wear out, it can be auctioned off.

From pioneer days, Nebraska farmers have known that much of their soil rests on a precarious foundation. Veterans describe areas of volatile surfaces as "blow sands" that pocket the state. Bernton was taken to a "blow sand" area near Tilden during his investigation.

Knowledgable family farmers had cultivated the sandy soil with care, planting their crops in narrow strips amid long stands of trees to serve as wind breakers. A proven system of crop rotation was followed to maintain soil fertility, and the land was never allowed to lie bare, exposed to the winds and rains.

A few miles away, the irrigation circles had taken over. Seventeen circles of corn had been planted, and each was marked with the towering sprays of irrigation water. A feed company had leased a segment of the land from a local farmer and "sod busted" the earth for corn.

Despite constant irrigation, erosion problems have arisen. The first sprouts of corn barely emerged when a prairie wind whipped the sandy soil into commotion and cut down the seedlings. The company's hired farm crew replanted a second crop, and the same thing happened. A third planting has now shot up, but no one foresees a good mature crop.

Stan Grubb, a local farmer, glumly predicted: "The company's leasing the land, taking the good out, and then when they give it back to the farmer, he won't have nothing left but a blow hole. When they leave, it'll take years to get back into productivity."

Jim Hall, an investment broker for the Wall Street firm of Loeb, Rhoades, Hornblower and Co., believes good corporate farm managers are as capable as their smaller neighbors in erosion control. He cites a corporate farm in Dundy County that has produced steadily increasing yields in each of the past five years by following sound soil-conservation practices.

But Hall agrees that some of the investors try to cut too many costs and "get into a lot of trouble."

Footnote: In the past two years, the spread of irrigated circles has slowed down. A spokeswoman for Valmont, a leading manufacturer of the irrigation systems, says their sales will drop off 25 percent this year. This slump is due mainly to the low prices of corn on the market. Those systems, incidentally, cost from $27,000 to $42,000 to install.