American farmers are planting 324 million acres this year, up 4 million acres from 2007. Too much of the best land is waterlogged, however. Indiana and Illinois have been the worst hit, although Iowa, Wisconsin and Minnesota were inundated last weekend.

Bob Biehl, whose farm is near St. Louis, has managed to plant only 140 of the 650 acres he wanted to devote to corn. Some farmers in his area “haven’t even been able to take the tractor out of the shed,” he said.

United States soybean plantings are running 16 percent behind last year. Rice is tardy in Arkansas, which produces nearly half the country’s crop. “We’re certainly not going to have as good a crop as we had hoped,” said Harvey Howington of the Arkansas Rice Growers Association. “I don’t think this is good news for anybody.”

Harvests ebb and flow, of course. But with supplies of most of the key commodities at their lowest levels in decades, there is little room for error this year. American farmers are among the world’s top producers, supplying 60 percent of the corn that moves across international borders in a typical year, as well as a third of the soybeans, a quarter of the wheat and a tenth of the rice.

“If we have bad crops, it’s going to be a wild ride,” said the Agriculture Department’s chief economist, Joseph Glauber. “There’s just no cushion.”

As every farmer knows, trouble can come at any point before the harvest is complete. Danny and Karen Smith get up in the middle of the night at their wheat farm in Milton, Kan., whenever they hear thunder.

In a few weeks, the wheat they planted last fall will be ripe. A bad storm or, worse, a tornado could destroy it. Last year, the Smiths lost nearly all their wheat to a late freeze compounded by too much rain.