Under the Conservation Reserve, farmers are paid an annual rent in exchange for idling environmentally sensitive land for 10 or 15 years. About 34.8 million acres are enrolled in the reserve, the largest U.S. long-term land-idling program ever, for an annual rent of $1.67 billion.

Contracts expire on 16.1 million acres in 2007, 6.1 million acres in 2008, 4.3 million acres in 2009 and 2.2 million acres in 2010.

Separately, the administration said it would hold a sign-up for the Conservation Reserve beginning Aug. 30. It has a target of enrolling 800,000 acres, roughly double the amount of land due to leave the reserve in 2004 and 2005.

Land is selected on the basis of rental cost and impact on soil erosion, air and water quality, long-term benefits and wildlife enhancement.

A spokeswoman said USDA would request in early August public suggestions on how to improve the Conservation Reserve and how to manage the large amount of land that could leave the reserve in 2007 and the next few years.

Those suggestions will help determine how much land may be considered for re-enrollment, she said.

"President Bush's directive to offer early re-enrollment and extensions of existing contracts ... underscores a commitment to full enrollment of (the reserve) up to 39.2 million acres," USDA said.

The White House said since the Conservation Reserve was created in 1985, it had reduced soil erosion by 40 percent nationwide, led to restoration of 1.8 million acres of wetlands and prompted farmers to plant 2 million acres of trees.