Less than a year after the summer drought of 2012 baked the U.S. grain belt, farmers in the region have been deluged by rain.

Aside from the threat that weather might pose for a second year to the U.S. harvest, the heavy rains may help fulfill of a prediction by the National Oceanic and Atmospheric Administration: A swath of the northern Gulf of Mexico that each summer turns into a dead zone, drained of oxygen and devoid of life, will be larger than usual.

The science behind this phenomenon is well understood. So are the remedies, the most practical of which would require changes in farming policy and practices.

The dead zone starts innocently enough. Each year, when the snow melts and spring rains fall on Midwest farmland, millions of tons of nitrogen-based fertilizer that was applied to barren fields the previous autumn are washed into Mississippi River tributaries.

In years when there is more rain, more nitrogen ends up in the water -- and vice versa. Last year's drought is considered the main reason the 2012 dead zone covered only 2,889 square miles in the Gulf, the smallest in several years, and down from 6,767 square miles in 2011. If conditions are right this year, the dead zone might occupy an area the size of New Jersey, or 7,800 square miles. Researchers usually take an official measurement in July.

Because the Mississippi has been dredged, straightened and channelized to control flooding and accommodate shipping, the river flows faster than it once did. Excess nutrients, instead of being absorbed and filtered during a meandering journey, are blasted into the Gulf in a manner that some have likened to a fire hose.

Once the Mississippi's waters reach the Gulf and the warming sun, the nutrients cause huge algal blooms. While the algae are blossoming, they suck oxygen from the water, and again after they die and fall to the bottom to decompose, where bacteria further deplete the water of oxygen. Fish either die or head farther from shore.

A state-federal environmental task force in 2008 set a goal of reducing the amount of nutrients in the Mississippi by 45 percent by this year. By all accounts, little progress has been made.

The culprits behind the dead zone are many, but one deserves special attention: corn. Unlike, say, soybeans, which can grow without fertilizer, corn can't grow without it. It takes 195 pounds of fertilizer to grow an acre of corn.

And the U.S. grows a lot of corn -- more than any other country. What's more, 40 percent of the U.S. corn crop is devoted to making ethanol, which fuel companies must blend with gasoline under a congressional mandate. The Gulf dead zone is yet another reason for Congress to kill that mandate.

Farmers can also mitigate the nitrogen loss from their fields. One step would be to end the practice of applying fertilizer in the fall after crops are harvested, and switch to applying it in the spring. This would allow them to use less fertilizer -- they often over-apply in the fall because they assume a certain amount of loss to runoff -- more of which would be absorbed by growing plants.

The enemy here is force of habit. Farmers are accustomed to laying down fertilizer in the fall, when harvesting is completed and they have time to spare. In the spring they are often under pressure to sow their fields during a brief window when conditions are right.

Farmers also need to be encouraged to limit how much water is drained from their land. Millions of acres of the most fertile fields in the Midwest are laced with underground piping known as tiling, which shunts water into local waterways and allows plants to flourish. Putting gates on these drainage systems during non-growing seasons would allow more water to percolate through the soil, rather rushing into the Mississippi's tributaries.

Here, too, tradition poses a challenge. Mark David, a professor of natural resources and environmental science at the University of Illinois at Champagne-Urbana, recalled an experiment in which a gate was installed on one farmer's tiling system. Even though the farmer had agreed to be a test subject, he couldn't stand to see his field flooded. He ended up opening the gates.

To contact the author on this story:
James Greiff at jgreiff@bloomberg.net