Domestic marijuana growers are harvesting a record $8.2 billion crop of homegrown pot this year, the National Association for the Reform of Marijuana Laws has estimated.

The 1981 marijuana crop is twice as big as last year's, making the illegal herb the nation's fourth most valuable farm product, after corn, soybeans and wheat, the pro-pot lobbying group claimed.

A spokesman for the Drug Enforcement Administration called the $8 billion figure "far-fetched" but said federal authorities recognize domestic marijuana production is increasing.

DEA estimated the 1979 U.S. pot crop was worth $1 billion to $1.5 billion and supplied about 10 percent of the marijuana smoked in America that year.

"We have no numbers for the 198l crop yet," said DEA press officer David Hoover. "But based on the tonnage and the total market, I don't see how the domestic marijuana market could be $8.2 billion."

George Farnham, political director for NORML, said the organization's estimates are based on law enforcement statistics, newspaper accounts and figures supplied by growers.

Like law enforcement authorities, NORML values marijuana based on the price it would bring if sold in small quantities, Farnham said. Depending on quality, homegrown pot sells for $10 to $150 an ounce, he added.

Much of the marijuana grown in the United States is smoked by the grower or friends rather than sold, so sales of domestic marijuana are smaller than the $8 billion crop value indicated, he conceded. NORML estimates between 1 million and 2 million Americans grew pot for personal use this year, but only one-tenth that many grew it commercially.

Though several states have removed criminal penalties for possession of small quantities of marijuana, growing pot is illegal in every state except Alaska, which permits cultivation for personal consumption.

The total number of marijuana arrests last year was about the same as the year before--342,000--but arrests for cultivation and sale jumped more than 25 percent, from 49,700 to 63,300, according to the National Commission on Marijuana and Drug Abuse.

Marijuana cultivation has spread all over the country from Hawaii and California, and 29 states produced crops worth more than $100 million this year, Farnham claimed.

With a crop valued at more than $1.5 billion, California is the leading marijuana producer, NORML says, followed by Hawaii at $750 million and Oregon at $400 million.

NORML estimates Virginia grew $200 million worth of marijuana this year and Maryland produced another $100 million worth. Virginia Beach police arrested 180 people one day last August for growing pot around apartment complexes, NORML noted, and Scott County authorities found a field with an estimated 15,000 marijuana plants.

Farnham said the growth of domestic pot production has been encouraged by the Reagan administration's decision to cut the budget of DEA and curtail federal aid to local law enforcement authorities.

The administration recommended cutting the DEA budget 12 percent, from $228.5 million to $201 million for fiscal 1982. The House and Senate versions of the DEA budget call for appropriations of about $231 million.

NORML officials say the government's decision to spray the herbicide paraquat on Mexican marijuana fields led to widespread cultivation of marijuana in the United States. Rather than risk paraquat poisoning, potsmokers decided to grow their own, Farnham explained.

Homegrown marijuana was once regarded as inferior to that grown in Mexico, Colombia or Jamaica, but that's no longer the case. Native American marijuana is a plant variety called cannabis sativa, the same hemp plant cultivated since colonial times for the fibers used to make rope. Now growers are cultivating cannabis indica, a Far Eastern variety that is richer in the intoxicating chemical tetrahydrocannabinol (THC). While cannabis sativa is a skinny plant that can grow 15 feet tall, the more potent indica strain grows like a bushy Christmas tree and can yield as much as two pounds of dried and ready-to-smoke marijuana.

To make the smoke more potent, Farnham explained, growers have learned to weed out the male marijuana plants and cultivate a sterile, seedless female plant known as sinsemilla, the Spanish word for seedless. Unable to reproduce, the frustrated female plants pour all their energy into producing THC resin.

It is this high-powered homegrown pot that sells for up to $150 an ounce, inflating the value of the domestic crop, NORML officials say.

The organization says wild animals and poachers are greater threats to marijuana patches than are law enforcement authorities.

To avoid being caught with the crop in their yard, growers have taken to planting their patches on public land. The U.S. Department of Agriculture reported earlier this year it has found marijuana being cultivated in most of the national forests.