Most mixes for interior plants contain a combination of organic matter, such as peat moss or finely ground pine bark, inorganic materials, such as coarse sand, vermiculilte, and/or perlite. More common mixes used for interior plants are the "so-called" peat-lite mixtures. These consist of sphagnum peat moss and either vermiculite or perlite. The following are some comments about these ingredients:
- Peat Moss This component is readily available as bagged sphagnum peat moss. Materials such as Michigan peat, peat humus, and native peat are not recommended as they are too highly decomposed to provide the necessary struc tural and water-drainage characteristics. Most sphagnum peat moss is acid with a pH ranging from 4.0 to 5.0. It usually has a very low fertility level. Sphagnum peat moss should not be ground too finely. Lime should be added to raise the pH to the 5.5 to 6.5 level.
- Vermiculite This is a sterile, light-weight mica product. When mica is heated to about 1800 F, it expands, much like popcorn, into a plate-like structure. Vermiculite will hold large quantities of air, water, and nutrients. Its pH is usually in the 6.5 to 7.2 range. Do not overwork vermiculite when it is wet. This can cause it to lose its struc tural properties and nullify the desired effects of using it. Vermiculite is avail able in four sizes. For horticultural mixes, the coarser textures are desired, usually 2 or 3. The coarser the texture the better, as the coarser materials provide better soil aeration.
- Perlite This, too, is a sterile media component which is produced by heating volcanic rock to about 1800 F. The result is a very light-weight, porous material that is white in color. Its principal value in soil mixtures is aeration. It does not hold water and nutrients as well as does vermiculite. The pH is usually between 7.0 and 7.5.
- Pine Bark Numerous commercial producers are successfully using a number of mixes that contain pine bark as the source of organic matter. These mixes, when properly prepared, have resulted in good root growth with a minimum of root problems. However, as with any mix, they can be overwatered. A soil- con ditioning grade of pine bark is available in most garden centers. The desired par ticle size is a little larger than sawdust. The coarse mulch grade of pine bark is not desirable for use as a potting mix ingredient. A general purpose mix that has proven successful is a 2:1:1 ratio (volume/volume) of pine bark, spagnum peat moss, and coarse sand or vermiculite. Another combination of one-half pine bark and one-half vermiculite has also proven to be successful. The same fertilizer ad ditives for the peat-lite mixes can be used for the bark mixes.
As perlite ages, it may produce fluorides which may cause fluoride burn on some foliage plants. Fluoride damage is usually seen on the tips of the leaves. The burns progress from the tip down the leaf. Fluoride burns can be reduced by adding about 1-1/2 times the recommended amount of lime when mixing the soil. The calcium in the lime binds the fluoride into a form that is non-soluble and, therefore, unavailable to the plant.