Pile it on. Upper crust mulch: a cover to bark about. DENIZENS OF THE HEAP

CUT trees have an additional worth besides sawdust and lumber. The bark that gives trees lifetime protection can be returned to the soil as nugget mulch and soil conditioner to improve the soil. For many years, bark was a waste problem of the lumbering, paper, and plywood industries.

``But today we gather the bark at the scene of cutting operations, process, bag, and ship the product for marketing,'' says Edward F. Kelly Jr., president of Greenlife Products in West Point, Va.

Mr. Kelly says pine bark makes better nuggets for decorative or landscape use, while hardwood bark is better for shredding and use on slopes to help reduce erosion.

Both may be used for soil conditioning and are completely organic and weed free. They do not crust as peat moss can.

They also add nutrients (sawdust takes nutrients from the soil) and retain more than 2 times their own weight in moisture.

The ideal soil pH ranges between 5.5 and 7.2 for most plants. Generally, sphagnum peat moss will lower soil pH (make it more acid) while straw and barnyard manure will increase pH (make it more alkaline).

Tests conducted at the Plantation Experiment Station in Florida show bark contains 55 percent lignin, an excellent soil builder.

``Lignin is one of the three basic components of plant cell walls,'' says Kelly.

Lignin particles in the soil help keep rock and clay particles apart to provide better drainage, while retaining moisture and nutrients.

FOUR sizes of bark for garden use are:

Finely ground bark to improve clay or sandy soil.

Mulch grade, one-inch size for spreading around plants and trees to keep the ground moist, cooler in summer, and warmer in winter.

Micro-nuggets, about to 1 inches, for decorating indoor potted plants and for outdoor surface mulching.

Big nuggets, 1 to 2 inches, for landscape use over small nuggets to provide mulching and decoration.