YOU ARE familiar with carbon in many forms, from diamonds to the graphite in pencils. It is the basis of all organic matter, including the carbon-based life forms we call humans.
But until recently, no one thought of carbon as a potential cash crop, good for the environment of places like the Chesapeake Bay as well as for farmers' incomes.
It's beyond the theory stage. Last week, I followed Jeri Berc's U.S. Department of Agriculture team as it inventoried a central Maryland cattle, grain and vegetable farm.
Did the farmer plow each spring, or drill seeds into the soil with a "no-till" planter? Did he rotate his crops? Did he mulch? Did he use manure?
It all goes into a mathematical model called CQUESTR -- short for sequester, as in sequestering, or storing carbon in the soil through good environmental practices.
The model being refined by the USDA will let any farm quickly calculate its potential for storing carbon by building up its soils' organic matter, which is 50 percent to 60 percent carbon.
Farmers who enlarged their carbon stores might get per-acre cash payments under the next national Farm Bill, up for Congressional reauthorization next year.
Many of the measures that would increase carbon in soils are also prescribed to reduce agricultural pollution of the Chesapeake Bay.
Examples include planting winter cover crops, limiting plowing through "no-till" farming, substituting manure for commercial fertilizer, planting grass and forest buffers to intercept runoff, and converting cropland to pasture.
More organic matter in soils also improves their drought resistance and productivity.
"We should be doing all of these things anyhow for soil health and water quality," said Rattan Lal, a soil scientist at Ohio State University and an expert on agricultural carbon storage.
Carbon sequestration has become a hot topic since the 1997 Kyoto Protocol, an international pact that commits industrial nations to attack global warming by reducing the buildup of CO2, carbon dioxide, in the atmosphere.
Ultimately, that means curtailing the burning of fossil fuels such as coal, gas and oil -- by far the largest source of the 6 billion tons of CO2 produced.
But significant potential to reduce CO2 also lies in enhancing so-called "sinks" for carbon, mainly forests and soils.
In the near term, building up the sinks would be cheaper and easier than big cuts in sources of energy, says David Donniger, who headed the U.S. Environmental Protection Agency's global warming efforts until recently.
These can permanently bind carbon that would otherwise combine with oxygen in the air to form CO2, absorbing perhaps a third of the total reductions the Kyoto agreement seeks from the United States.
Excess carbon dioxide is trapping enough of the planet's heat to unnaturally change Earth's climate, many scientists fear.
Lal estimates that with proper management, U.S. farmers and ranchers could store an additional 105 million to 208 million tons of carbon each year in the nation's 370 million acres of cropland.
Another 30 million to 110 million tons could be sopped up by better management of America's 523 million acres of grazing land, he has said in a report to USDA.
Lal says the nation's agricultural soils, since the original forests were cleared and the virgin prairies broken by the plow, have probably lost 50 tons or more per acre of their carbon stores.
"We can't restore all of that, but I believe we could eventually put back 20 to 30 tons an acre," he said in a phone interview this week.
Another scenario for farmers would have them receive credits for carbon stored. These might be sold in a market, created by industry's demands for "offsets" to the extra carbon their emissions have put into the air.
The idea of rewarding farmers for storing carbon is part of a broader issue -- the health of the Chesapeake Bay.
Federal payments to farmers, which have been averaging almost $20 billion annually, are heavily biased toward maximizing crop production. Many people think it's time for payments to shift toward rewarding good stewardship. It would benefit states like Maryland, where farmers are pushed by water-quality goals to set new environmental standards.
Currently, federal payments per dollar of farm produce sales run 12 cents to 27 cents on the dollar for big, Midwestern grain-producing states -- vs. 2 cents to 4 cents per dollar for states in the bay watershed.
The Midwest needs to re-emphasize water quality, too, because agriculture there has created a large "dead zone" in the Gulf of Mexico from fertilizer flowing down the Mississippi River.
Storing more carbon permanently involves some uncertainties for farmers. Plowing, for example, is a major no-no. Within minutes after spring plowing, carbon begins combining with air to form CO2, a process that continues for weeks, Lal says.
Many farmers plant without plowing for a few years at a time, but most plow some years to help manage weeds and pests. Organic farmers depend heavily on plowing to manage weeds without herbicides.
But that offsets most of the gains in carbon storage, Lal says. Other carbon-building, water quality-improving techniques, such as winter cover crops, just cost extra money.
So farmers will need compensation. But anything that combats global warming and cleans up the Chesapeake Bay ought to be worth something.