Farmers have enrolled another 5.8 million acres in the federal Conservation Reserve Program (CRP), putting it more than halfway toward its goal of getting up to 45 million acres of highly erodible land out of production by 1990.

According to Agriculture Department figures, enrollment reached 22.9 million acres after the fifth sign-up round in less than two years.

The program, created by the 1985 farm bill, pays farmers an annual "rent" to remove their most erosive land from crop production. The idea is to protect soil and at the same time prevent further buildup of commodity surpluses.

Congress set a goal of 40 million to 45 million acres for enrollment by 1990, which represents about one-tenth of the nation's cropland. Farmers who are accepted into the program must agree to plant their land in soil-saving grasses or trees and to keep it out of production for at least 10 years.

While the latest sign-up figures were cheered by conservationists and department officials, a University of Nebraska economist warned that the program actually may be contributing to forcing farmers off the land.

Extension economist Lynn Lutgen said that in areas such as eastern Nebraska, where farms are numerous and much land subject to erosion, young farmers who rent land and are trying to establish themselves in agriculture may be hurt by the program.

Lutgen said that the government, by offering 10-year payment contracts, has in effect become a competitor with farmers who rent, which has set off a scramble for enough land to make family farming operations economically stable.

"As an example, in a neighborhood where milo {grain sorghum} ground would normally cash-rent for $40 to $50 an acre, Uncle Sam has accepted bids of $60 to $70 for the acreage," Lutgen said.

As a result, he added, the "artificial prop" on land values offered by the Conservation Reserve may stimulate more demand for farmable land by renters and force higher rental rates in a bidding war.

Another caution about the CRP came from Kenneth A. Cook, a senior associate of the Conservation Foundation, who warned that a public investment in the program that could hit $13 billion over the next decade might be undermined without further measures by Congress.

Cook said that Congress should protect the investment by making conservation requirements a part of farm-credit legislation that the House will take up after its recess.

The Conservation Foundation, the National Audubon Society and other soil conservation groups have urged Congress to require the Farm Credit System and other lenders and insurance companies to follow the same conservation rules required of the Farmers Home Administration in the 1985 farm bill.

The House Agriculture Committee, in adopting a Farm Credit System bailout bill before the August recess, rejected the organizations' plea for a full conservation umbrella over all lenders. The groups are expected to renew their effort when the Senate Agriculture Committee takes up the bill.