Soaring fertilizer costs have left America's three main crops - corn, soybeans and wheat – neck and neck in the race to win acres for next year's harvest, and have a chance to rebuild depleted supplies.

On revenue alone, the summer rally in farm commodity prices has left corn the clear victor, especially on land where it has been rotated with other crops, giving higher yields, research from Purdue University showed.

Farmers with high productivity land can expect revenue of $926 per acre on this basis, one-third more than soybean growers, and 57% more than peers sowing wheat.

However, rising fertilizer costs, which have seen ammonia prices in the key US farming state of Illinois up 41% since April according to US Department of Agriculture data, mean the race for profits is much closer.

Beauty parade

Corn's need for heavy nutrient applications gives it only a 9% advantage in profits terms over soybeans on high quality land, once fertilizer and other variable costs, such as sprays and seed, are factored in.

For average fields, the advantage is only 3%. On low productivity land wheat is the best option, the university said.

"While prices for corn, soybeans and wheat are up from this past spring, farmers will need those higher returns to offset a spike in variable crop production costs," said Bruce Erickson, Purdue's director of cropping systems management.

"Much of the projected cost increases are tied to a recent surge in fertilizer prices. After years of incremental movement in fertilizer prices, the market has been much more volatile since 2007."

Inventories fall

The comments follow data from PotashCorp, the world's biggest fertilizer group, on Monday showing prices of all three fertilizer types – nitrogen, phosphate and potash – on the rise, albeit with potash showing only a marginal increase.

Prices are being spurred by inventory which has fallen among major North American producers to well below average levels, sapped by strong demand from farmers attempting to cash in on strong crop prices, at a time when wholesalers have run down their inventories.

"There's not a lot most farmers can do to soften the blow of higher production costs," Mr Erickson said.

He added that fertilizer was increasingly a "world market", with producers often heading outside the US to countries with lower costs of energy, a key input in particular for nitrogen nutrients.

The race for acres is seen as particularly crucial in next spring's sowing season, with stocks of all three crops being run down, thanks in part to revived demand, after the emergence of the world from economic crisis, but also by the loss of Russia from grain export markets, following drought.

Purdue's calculations are based on futures prices at the end of October, minus a discount to mimic cash prices.