The Army at Fort Belvoir has found the ideal lawn mower.It leaves the grass closely cut and neatly trimmed. It saves money, eliminating the need for 10 helpers nine months of the year. What's more, it can't accidentally throw off a spark that will make the landscape explode - a fearsome, if remote, danger.

It is a living mower.

It is a herd of 35 goats.

There was a look of satisfaction on chief groundskeeper Garrald B. Phillips' face yesterday as he surveyed the neat grounds of one of the post's two ammunition dumps.

Around him, waiting for a handout of wilted lettuce or cabbage heads was the herd.

"We don't have to do no cutting, no trimming, no nothing," said Phillips. "The goats do it all."

For 15 years Phillips has been using goats to maintain the appearance of the ammunition dumps and the grounds surrounding the post's five water towers.

He notes that a spark from a gasoline-powered lawn mower could ignite one of the ammunition laden magazines scattered around the dump.

Furthermore, he said, the goats are thorough, chewing blades of grass and weeds right up to the foundation and steps of each magazine. "We can't have any vegetation growing around the buildings," Phillips said, "because of the possible fire hazard."

At this point in the conversation, Pride, the 10-year-old billy goat who Phillips said had been running the herd for years, stamped his feet. After some apparent differences of opinion among the other billies, the herd wandered off to . . . greener pastures. "Pride is getting old," Phillips is in charge of almost 10,000 acres - 3,000 of which have to be cut nine months of the year.

For this monumental task, he has only 11 workers and his goats. The goats can be used only on about 100 acres that are fenced - the ammunition dumps and the water-tower grounds - because, otherwise, they would have to be chained. Phillips doesn't want to do that because he'd be afraid of bringing down the wrath of humane societies on his head.

"I haven't been told (not to do it)," he said, "and I don't want to be told."

Phillips' goat power at the military post in Fairfax County has become known far and wide at other military installations. "I've been called by at least 25 other posts and asked, 'do you have any goats,'" he said. Recently he shipped a herd of 15 - offspring from his Belvoir herd - to the Quantico Marine Base in Prince William County.

For the nine-month growing season, the goats require no food other than the vegetation they eat. Phillips keeps them supplied with salt licks and water (kept in salvaged latrines), and during the winter, they are fed hay and horse and mule feed.

Once a year, the goats see a veterinarian, or, rather, the veterinarian comes to see them because, as Phillips says, they're not at all keen about going to see the doctor. Some of the billies are castrated to keep the herd manageable.

At night, the goats sleep in shelters that have been fashioned out of old ammunition cases or built by post carpenters. Patrol dogs police th grounds at night, but they and the goats get along fine, Phillips says.

"The government would have to pay $7 an hour to get this job done for me," Phillips said. "I think we're making out like a champ."