REMEMBER THAT crew you saw down the block the other day, trimming trees around the power lines? Their truck was full of wood chips, correct?

Almost. It was full of potential garden topdressing. What used to be trash, in today's ecologically minded world, has become eagerly sought mulch. Fifty pounds of mulch, shredded or chips, costs from $1.50 to $2.50 if you buy it bagged at your garden for hardware center.But it's free for the hauling at some government centers.

Local governments and utility company contractors in metropolitan Washington accumulate quite a few hundred truckloads each year or organic "waste" - mostly leaves and wood chips.And some are more than happy to let somebody else put the stuff back into the ground.

If you're strong on patience, luck and shoveling, that somebody might be you.

Home composters and cooperative or club gardeners would benefit most from the various freebles available - namely unscreened wood chips, raw manure and plain old uncomposted leaves. The chips and uncomposted leaves, often available only by the truckload, may be more than the individual gardener can handle.

Gardeners who don't mind toting their shovels to Rock Creek Park may find two bargains therein, courtesy of the National Capital Parks-West and the Rock Creek Park Horse Center.

The park's stables, home to more than 100 horses, allow interested gardeners or composters to "take all you want" from the operation's stock of manure behind the stables, says horse center president Bob Douglas. But please, he asks, no trucks.

The park management itself goes one step further. Using the horse center's manure and leaves collected in Northwest Washington, the park has maintained a composting operation for the past three years.

A certain amount of the mixture is dumped every Monday and Friday near the parking lot at the park's headquarters, for public consumption (the rest is used by the park itself), says park manager Jim Redmond. The compost, which Redmond describes as "very good stuff for mixing with soil or mulching," i s available to anyone with shovels, bags or baskets. But again, Redmond says, no trucks allowed.

Leaf mulch is also available in Northern Virginia from Arlington County, which operates a mulching porgram more advanced than most in the area. The county's collected leaves, says transportation department aide David Timbie, are "wild-drilled" on county property at Four Mile Run and Columbia Pike, and Yorktowne Boulevard at N. 26th Street.

Those interested in the mulch should call the county transportation department (558-2551), Timbie said - particularly if as much as a tandem truckload is needed. The county will deliever that truckload free, he said, if a truck is available.

Wood chips produced by Arlington County's own tree-trimming work are used almost entirely by the county itself, and therefore not generally available to county residents. The same holds true in Alexandria, Fairfax County and Maryland.

The District of Columbia does, however, allow residents to take as much as a truckload from its pile of wood chips at the city's nursery (600 Howard Road SE). General tree foreman John E. Stapleton says it's best to show up between 7 a.m. and 3 p.m.

Fairfax County and Alexandria, until this year, had leaf operations similar to that of Arlington. Alexandria's has been temporarily curtailed.

Fairfax County's leaf mulch is no longer available to the public because of property changes near the county landfill, said county-engineer Hugh Murtagh.

Leaf mulch or compost is not produced - for free public consumption, anyway - by the District of Columbia, Prince George's or Montgomery Counties.

Montgomery County will do the same, says transportation services official Greydon S. Tolson, if the delivery site is reasonably close to the central taxing district (Damascus and Gaithersburg, for example, are "out"). Tolson said most county leaves are brought each year to Leafco Inc., a privately run composting firm operating on public land in Wheaton.

Under a contract with the county since 1971, Leafco offers more leaf by-products and soil mixes than any local government - but the products are not free. They're for sale, primarily wholesale. Their prices are generally lower than area commercial outlets ($10 for a cubic yard of dark, leaf compost; delivery is extra).

Wood chips, while hard to get from local governments, are given away in abundance by the tree service contractors of the Potomac Electric and Power Co. (PEPCO) and its Virginia counterpart, VEPCO. But saving money on this popular type of mulching material has its problems. You can wait years for a requested truckload to appear on your property.

Harry Pullman, district tree trimming supervisor for VEPCO, says as many as 100 requests are often received for a single truckload of chips. And the contractors keep no priority list of requests - the chips are dumped at the requested site nearest to the tree crew when their truck is full.

Contractor for PEPCP (Asplundh in D.C. and Montgomery County; Nelson Tree Service in Prince George's County) operate likewise. They will take your name, address and dumping site - in writing - and will expect you to be patient.

Where to write, call or go:

WOOD CHIPS: D.C. Nursery, 600 Howard Rd. SE; Asplundh Tree Expert Co., 8121 Georgia Ave., Silver Spring; Nelson Tree Service, 5975 Rockhold Creek Rd., Deal Md. 20751.

LEAF MULCH: National Capital Parks-West at Rock Creek Park, Military and Clover Roads NW; Arlington County Transportation Department, 558-2551.

MANURE: Rock Creek Park Horse Center, Military and Glover Roads NW