That’s the new logic dominating trading in agriculture futures today, after a swath of cold weather stretched across much of the United States and threw the 2007 crop into question. Wheat prices rose to a one-week high in Chicago, and corn prices ticked up slightly, as temperatures plunged below freezing across the Midwest and meteorologists called for continued cold and rain into the coming weeks.

The news is a good reminder for traders, who have been so focused on the recently released USDA acreage estimates that they’ve forgotten the other big factor in crop prices: yields. You might plant a million more acres of corn, but if spring is delayed, that won’t necessarily mean bigger harvests. In fact, because added acreage is often marginal acreage, it can be more susceptible to imperfections in the weather.

For those new to the game, cold weather is particularly bad for wheat, where the winter crop is approaching harvest. Cold, wet weather is bad for corn, which can’t germinate in cold soil and rots as seed if things get too wet.

The larger point is that no one dynamic dictates pricing. Corn prices are down 20 percent since reaching an all-time high of $4.50/bushel in late February, but could recover if a cold snap sets in. Then again, sustained lower prices could dissuade farmers from planting all those acres they promised in the first place. After all, the USDA survey was just a measure of expectations; most of that corn is not yet in the ground.

The UN Food and Agriculture Organization expects 2007 to be a record year for global crop production, with total grain harvests rising 4.3 percent, led by large corn crops in South America and the U.S. Still, it pays to keep an eye on the thermometer. And, of course, don’t count your kernels until they are ... well ... harvested.