Pumice vs. Vermiculite

Source

• When tiny gas bubbles are trapped in volcanic lava before it cools, the result is pumice, a naturally lightweight rock. Most pumice used in the United States is mined in Oregon. Vermiculite is a kind of clay silicate that forms a series of thin, parallel planes. Under heat treatment of 1,500 to 2,000 degrees Fahrenheit, it acquires a pleated structure, which aids in water retention in potting mixes.

Vermiculite Characteristics

• Vermiculite contains nutrients – potassium, aluminum, iron, calcium and magnesium -- that are released to plants in water. It is lighter weight than pumice and works in a different way. While pumice creates spaces in potting mixes for water and air to flow around and through it, vermiculite holds water and then releases it. This makes it valuable for seed-starting mixes and propagation mixes because it helps the soil stay constantly moist but not wet. Vermiculite has a neutral to alkaline pH and comes in four sizes, or grades.

Pumice Characteristics

• The Latin word “pumex,” from which the name "pumice" is derived, means “foam.” Pumice contains tiny bubbles of air, and it floats in water. It also is porous, allowing water to flow through it. It is usually white or pale gray, but, depending on the minerals in the area where it is mined, it also can be pale yellow. It also comes in four sizes, from fine to pebble size.

Horticultural Use

• Adding one-third pumice to potting soil creates a well-draining mix suitable for succulents and other plants that require fast drainage. Some growers root cuttings in pure pumice, which is sterile and weed-free. In the nursery industry, where many plants are grown in bark rather than soil, pumice often is added to increase porosity. Vermiculite is mixed with potting soil to aid drainage and is used as a top dressing for seed-starting flats.