FARGO (AP) - The cost of nitrogen fertilizer is influencing the planting decisions of many North Dakota farmers this spring.

The price of nitrogen, which promotes plant growth and is necessary for big yields, has risen about 40 percent since late last year. A ton of anhydrous ammonia, a common form of the fertilizer, costs about $500. That can mean an increase of up to $20 per acre to a farmer's cost.

"The way nitrogen prices are, we'll have to take a close look at how much we use," said James Burbidge, who farms near Mohall in north-central North Dakota.

Nitrogen is particularly important for corn, which is in high demand because of the booming ethanol industry. The Agriculture Department says North Dakota farmers plan to seed 2.6 million acres of corn, up 54 percent from last year and the largest crop since record-keeping began in 1929.

Average U.S. corn yields have tripled in the past half-century. U.S. nitrogen consumption rose about 4½ times in the same period, according to the Agriculture Department.

Corn is a factor in surging nitrogen prices, because a larger corn crop drives up demand for nitrogen, said Lynn Dusek, agronomist at Maple Valley Ag Chemical in Tower City. Nationwide, corn acres this year are pegged at 90.5 million, up 15 percent from last year.

Higher nitrogen costs will not stop farmers from planting corn and applying fertilizer because corn prices are strong, said Dave Franzen, a soil specialist with the North Dakota State University Extension Service.

However, Franzen said, farmers might be tempted to cut back on nitrogen, especially in areas where moisture is short and the chance of a good crop is reduced. Cutting back too far would be a mistake, he said.

"Under-fertilizing guarantees poor yields," Franzen said.

He recommends farmers test their soil to see how much nitrogen and other fertilizers it already contains. That might lessen how much fertilizer they need to apply this spring.