The Water Retention & Air Porosity of Soilless Potting Mixes

Peat Moss

• According to the University of Illinois Extension, soilless mixes are composed of two parts – a mineral part and an organic part. Sphagnum peat moss is an organic component found in most soilless mixes. It is water-retentive, it decomposes slowly and it is fairly inexpensive. However, adding too much peat to potting mixes can inhibit soil drainage because it holds too much water. Porosity is improved by blending peat moss with larger particles, such as bark.

Bark

• Bark is the main ingredient in most commercial soilless potting mixes. The University of Arkansas Cooperative Extension Service recommends using softwood bark over hardwood bark. Softwood pine bark is preferred because it decomposes slowly and contains fewer leachable toxic compounds than most hardwood barks. Pine bark does not retain water or nutrients as peat moss does, but it provides a more porous medium for increased aeration. Although pine bark is used in mixes for ornamental landscape plants, fir bark is used in mixes for orchids and other epiphytes.

Vermiculite

• Vermiculite is the particle ingredient in soilless mixes that looks like shiny flakes. It is derived from a mineral in the mica family that is heated until it expands. After expansion, it forms accordion-like folds that are air-, water- and nutrient-retentive. If handled roughly or stored too tightly, vermiculite compresses and loses its porosity. It is available in different horticultural grades, with finer grades used in seed-starting mixes and coarser grades used for stem cutting propagation.

Perlite

• Perlite is not used interchangeably with vermiculite. Although they are both minerals that are heated until they expand, their applications are the opposite. Perlite is volcanic rock that pops under high heat. In potting mixes it has the appearance of tiny popped corn. Whereas vermiculite holds water and nutrients, perlite provides excellent drainage because it does not retain water. It is commonly used in succulent plant potting mixes and seed-starting mixes to reduce root and seedling rots.