What Are the Ingredients in Gravel Mulch?

Source

• Gravel, despite its humble nature, comes from enormous glaciers that once dominated the northern regions of the prehistoric world. As the glaciers melted and receded, the water runoff swept lighter soils away. Heavier stone, gravel and sand remained behind in pockets. These pockets are called glacial outwash and occur in specific geographical areas, such as ancient lake beds, deltas and moraines. Basic erosion also contributes to gravel formation as wind, water and temperature fluctuations break mountainsides into smaller and smaller particles.

Composition

• The majority of gravel consists of local rock, and the rock types are as varied as the region allows. Some stone types may be fairly exotic, carried by water or ice to new areas, but the vast majority of gravel is the same material as the surrounding geography. Most gravel consists of gray, relatively round pebbles of granite, dolomite or limestone. Gritty, tan decomposed granite is also a type of gravel popular in the Southwest. The particles of decomposed granite are not smooth, and their jagged surfaces pack tightly together to make a firm but water-permeable layer.

Debris

• Gravel companies screen gravel to remove larger rocks and other foreign objects. Screening is a mundane but important step: It sizes gravel, allowing the company to target the gravel or aggregate -- mixed gravel and sand -- to specific end uses, such as fill, concrete or gravel mulch. Gravel mulch, depending on how it's stored and distributed, may have blown leaves, twigs, weed seeds and other debris mixed into the load.

Concerns

• Some rocks are alkaline and have a high pH. The calcium in limestone, for example, leaches into the ground and gradually raises pH levels in underlying soil. Moderately high pH levels aren't necessarily bad, but high pH does tie up existing iron and causes iron chlorosis in maples, blueberries and other vulnerable plants. Test your soil annually if you have concerns about gravel's influence on your garden's pH. Also keep in mind that gravel eventually works its way into the soil and, because it does not decompose, makes your garden bed rocky and difficult to dig.