It takes deluges of water, mountains of fertilizer and vast greenhouse gas emissions to grow corn crops on the scale they’re now being grown in America.

Not to mention a doting Congress.

In 2005, Congress – paying lip service to the environment and tribute to the farm lobby – delivered to corporate agriculture one of the greatest regulatory bonanzas in history. It did so by mandating that 7.5 billion gallons of the nation’s fuel supply come from ethanol by 2012.

Because U.S. ethanol is currently distilled almost entirely from corn, the law created an artificial demand for this grain that has driven up its price and muscled other crops out of farmers’ fields.

A bushel of corn sold for $2 in 2002; today one goes for $4. More than 93 million acres were under corn last summer, the most in more than 60 years.

That translates into fewer acres for vegetables, alfalfa, soybeans, etc. – and higher prices all around for consumers. Feed for cattle is costing more, which means that dairy products and beef cost more.

The price of groceries has been rising faster than inflation, some staples very much faster. Dairy products are up 14 percent over the past years; eggs are up more than 25 percent. The price of fuel is another culprit here, but it can’t be cleanly separated from the ethanol mandate: It takes a lot of natural gas and petroleum to grow corn.

Meanwhile, the expansion of corn acreage has had severe environmental consequences. Midwestern corn fields “leak” much of the fertilizer applied to them, dumping nitrogen compounds into waters that ultimately empty into the Gulf of Mexico. Fertilizer runoff has created a 7,900-square “dead zone” in the gulf where explosions of algae have depleted the oxygen and asphyxiated marine creatures.

One would think that Congress, having seen the harm done by its ethanol requirement, would acknowledge its error and stop distorting the market on behalf of corn growers.

One would think. Instead, the House and Senate have just approved – in an otherwise excellent energy bill – another vast expansion of the corn ethanol mandate: 15 billion gallons by 2015.

Don’t confuse this with the bill’s incentives to develop “cellulosic” ethanol, a fuel that could be produced from agricultural waste, switchgrass, even wood chips. This promises to be genuinely benign, earth-friendly fuel, and Congress should do what it can fast-track it into production.

Corn ethanol is another story. It harms the environment, produces little net energy and is now gouging the people who actually eat farm products – in other words, the American public.

Isn’t that the same public Congress is supposed to be serving?