A crisp winter Sunday is an ideal time to go prospecting for the gardener's gold: horse manure. For Washingtonians, the mother lode is at the Rock Creek Park Horse Centre, just off Military Road, between Connecticut Avenue and 16th Street NW. There is a big sign that says "stables," and it's hard to miss the road turning into the woods.

I went there last Sunday afternoon, equipped with shovels and bags and garbage cans. It was a maiden errand for a van I had just bought; I think it's good luck to start a vehicle's career with a load of manure.

Picking up loads is always easier in company, so I invited my friend Ben Vandegrift, a lawyer downtown and an ambitious gardener in Spring Valley.

"Where could we pick up some manure?" I asked a bearded giant of a man clearly in authority. He was guiding a small child sitting a bit unsteadily on a pony. "Just drive around this pile, back up as close as you can get to the bottom of it and take all you want," was his genial response.

I introduced myself and told him that I planned to write about my experience. "What happens if readers mob your manure pile?" I asked.

"You promise?" he said, with the widest of grins.

The genial giant's name is David Douglas, and for 10 years he has been driving through the park in a surrey--a horse-driven coach. He is also a gardener in Lanham, Md., where he has a quarter-acre back yard with six fruit trees, for which he uses 100 pounds of horse manure a year. He applies manure to his acid-loving plants as well, such as azaleas. He is pleased with the results. "Horse manure is wonderful," he said.

Douglas explained that the Rock Creek stables currently have 62 horses--half of them boarders, the other half available to the public for instruction ($15 for half an hour) and trail rides ($8.50 an hour). "We feed them well and they keep producing the stuff," he said. "They never stop. So we want people to come and pick up. Tell them to come on down. It's great stuff, and the price is right: It's free."

The pile was enormous, steaming here and there, and spilling out into the woods. Douglas showed us where the oldest stratum was, and the stuff turned out to be airy, dry, well-rotted manure laced with sawdust, the color of old gold. Our shovels sank into the pile with the lightest of pressure. It took the two of us no more than a leisurely quarter of an hour to fill up four large trash cans and 10 polyethylene garbage bags. We found that the bags tore easily and spilled, but we could pack a lot into the trash cans.

"You don't feel greedy when you take this stuff away," Vandegrift said, and pointed out that the two of us didn't make a dent in the pile.

Douglas said most people come for manure in the spring; few in the winter.

But fall and winter are the best times to spread manure, particularly if there is no danger of a runoff--or if you find a balmy day to dig up the soil and work in the manure.

"The excreta of agricultural animals, along with stable litter, constitutes one of the oldest and most effective fertilizers known to man," says "The Encyclopedia of Organic Gardening," published by Rodale Press in Emmaus, Pa. "The rise of fertilizers in the twentieth century has led to a decrease in the amount of manure utilized by world agriculturists." The encyclopedia rates horse manure as the best farm-animal manure and suggests a thin layer spread directly on the ground, to be disked or plowed under, or added to a compost pile en masse.

Elizabeth Crowley, D.C. agricultural extension agent, highly recommended the Rock Creek Park stables. "Horse manure may be low in actual nutrients--nitrogen, potassium and phosphorus--but it's an excellent soil improver," she said. "Its greatest single asset is that it loosens up our tight clay. But never use raw manure next to plants because it burns them. And make sure you complement horse manure with extra nitrogen, particularly if the manure is fresh."

Rick Heflebower, a Montgomery County extension agent, said that "as far as the major nutrients are concerned, particularly nitrogen, you do better, pound for pound, with chemical fertilizers. But horse manure is rich in micro-nutrients, such as manganese, zinc and iron, that you don't get in chemical fertilizers." He cited a textbook that says the nitrogen content in manure is between 2 and 8 percent, whereas the range in chemical fertilizers is between 5 and 30 percent.

Heflebower warned against spreading fresh manure in the winter because of the quick loss of nitrogen content even if the manure is worked into the soil. "Don't dig up the soil if it's too wet," he said. "And don't put manure on frozen soil because then the nitrogen easily leaches out."

Is there such a thing as too much well-rotted manure?

"If you exceed 15 tons per acre," Heflebower said, "the chemicals introduced may make it difficult for the plant to soak up the moisture from the soil."

"It takes a lot of manure to get to the point when it's too much," Crowley said. "Just keep working it in."

My friend Vandegrift and I agree that it's lovely stuff. We both intend to build a few raised beds of manure and compost. We'll make that trip to the stables again.