Fertilizer costs could tip the balance between US farmers drilling their fields with corn or soybeans this year, academics have said ahead of a key planting report.

The volatility in commodity markets was prompting farmers to take planting decisions right to the wire, a report from Indiana's Purdue University.

"Not only have crop prices been changing rapidly, but so have some of the input costs such fuel and fertilizer," Chris Hurt, a Purdue agricultural economist, said.

Current market prices of a little under $4 per bushel for corn and roughly $8.50 for next season's soybeans meant it was a close call which crop to choose for unplanted fields.

"The return for corn and soybeans is very close right now, but those returns favoured soybeans by $20-50 per acre for much of the winter," Mr Hurt said.

The deciding factor for farmers may be their stocks of fertilizer. While corn yields more than three times as much as soybeans per acre, it requires greater quantities of nutrients to achieve its potential.

"If farmers have been able to line up reasonably priced nitrogen supplies this year, that may cause them to favour planting more corn," Brice Erickson, director of Purdue's cropping system management, said.

"If their fertilizer costs are high they are likely to plant more soybeans."

The report comes ahead of the release on April 1 of the US Department of Agriculture's first estimate of US crop acreages this year.