Farmland in Loudoun County is disappearing about twice as fast as it did in the late 1980s and 1990s, according to figures in the U.S. Census of Agriculture released last week.

The comprehensive survey of American farming, which is performed by the U.S. Department of Agriculture every five years, offers fresh details about semirural Loudoun's transformation into the nation's fastest-growing county.

The new tallies on a host of agricultural indicators cover the year 2002. They demonstrate how the forces that have beset American farming for decades have accelerated on the fringe of the nation's capital, a suburban frontier where demand for new home lots has made the land itself a key commodity.

That has meant that even as many of the county's farm owners have continued to struggle to keep their farms going, many have found eager buyers for their increasingly valuable land holdings.

The average market value of farmland and buildings more than doubled between 1997 and 2002, according to the new figures, jumping from an estimated $4,746 an acre to $10,807 an acre. That in part reflects the sharp increase in the value of land countywide, as demand for homes has continued to outstrip supply.

Agriculture census agents determined there were 164,753 acres in farms in 2002, down more than 20,000 acres from the 1997 total of 184,988. Loudoun lost about 10,000 acres of agricultural land between 1992 and 1997 and about 11,000 acres between 1987 and 1992, the figures show.

The agriculture department defines the category "land in farms" as primarily agriculture land used for crops or grazing. It also includes rented land.

Although the figures on "land in farms" provide an important reference point, they do not tell the entire story of agriculture in Loudoun. Much of that land is not used for farming in the traditional sense. Many owners of estates or large parcels produce just enough on their property to qualify for agricultural tax breaks.

The size of farms in Loudoun has continued to shrink, while the total number of farms has increased, the census figures show. There were 1,516 farms in Loudoun in 2002, compared with 1,032 in 1997 and 942 five years before that. The average size of farms dropped from 179 acres in 1997 to 109 acres in 2002.

Part of the increase in the number of farms can be attributed to the rise of small, so-called farmette-type operations, such as those offering organic or other higher-priced vegetables.

More than half of all farm operators in Loudoun also worked outside their farm, the figures show. More than 40 percent of operators worked off the farm for at least 200 days in 2002.

The number of beef cows in Loudoun dropped from 16,667 in 1997 to 13,246 in 2002. But the number of horses and ponies rose, from 4,135 in 1997 to 6,162 five years later, putting Loudoun behind only neighboring Fauquier County in equine population statewide.

Fauquier, which had 6,231 horses in 2002, also saw its farmland total decline over the past five years, from 239,034 acres in 1997 to 238,135 acres in 2002.