Plant Growth Medium

A proper growth medium is essential to giving seedlings or cuttings the right start. Prolonged, vigorous plant growth requires a good nutrient balance, achieved through soil additives, as detailed below, or by way of fertilizers. Compost can play an important role in the early life of cutting or seedling. Knowing what to use, when, how, is what this section is about.

Potting Soil And Garden Soil

Most plants require a growth medium that contains at least some soil; most grow happily in ordinary potting soil. However, some benefit from richer soil mixes, which may be purchased separately, or be prepared using a richer compost, for example, plants that are particularly avid growers. 

Straight garden soil is not appropriate for starting plants, or for the growth of houseplants: it is poorly drained, poor in nutrients, and often contains lethal nematodes, fungus, etc. Sterilized garden soil may be mixed with peat, compost, pine or fir bark, and filler to make a good potting soil, with whatever additives are desired, as below. Mix well, and keep fine, light and fluffy. The same ingredients, minus the garden soil, also make excellent potting soil.

Soil mixes frequently contain variable amounts of "filler," light and loose stuff that prevents compaction, thus helping with aeration and drainage. Vermiculite and perlite are commonly used, and despite their appearance are both natural products.

Perlite and vermiculite both help retain moisture, especially the latter. Vermiculite further slowly decomposes to provide potassium and magnesium. Perlite is inert and has no nutrient value, but is stable and therefore more reliably prevents soil compaction. Finally, sand may be used, for very rapid drainage.

If garden soil or compost are to be used at any stage, or if plants will be moved to the garden, it may be worthwhile to have both of these tested for their nutrient values and relative pH.

A good starter mixture for ordinary seeds and cuttings consists of equal parts of compost or even topsoil, peat and filler, or of peat and filler, provided this mixture is loose, fine and crumbly. Composted fir or pine bark is a useful additive that helps prevent damping off. Potting soil itself may be too rich in nutrients and may burn young seedlings. Soil MUST be truly sterile, and is preferable to peat in most instances.

Bake up to eight pounds/four kilograms of moist soil at 180F/80C for 30 minutes to adequately sterilize your growing medium, or at 135F/57C for 30 minutes to simply prevent damping off. Or heat a pound/one-half kilo-gram of slightly moist soil in the microwave, set on high, for 7 minutes. The more soil is baked, the less suitable for growth: avoid over-baking if you can.

Special needs

A mixture of ordinary soil mix and rough builder's sand is ideal for cacti and succulents, as prolonged moisture at their roots kills them. Similarly, alpine gardens require rapid drainage, and benefit from solid helpings of sand, gravel and compost. Sand should be coarse, and free of sea salt.

Bonsai also require good drainage, as well as adequate moisture reten-tion. In general, a good mix of equal parts of soil and coarse sand will suffice. Flowering species, however, require relatively more rich humus, up to three quarters of the total mixture. Conifers, especially pines and junipers, love a light mixture composed mainly of sand (just over half of the mixture for most conifers, up to three quarters for pines). Finally, acid-lovers like Azalea or Cotoneaster thrive in soil mixed liberally with peat, then cut with the same amount of sand.

Orchids are grown in various media, but most commonly Douglas fir bark, which is light, porous, and drains very readily. Ordinary soil will keep oxygen from roots: most orchids are epiphytes whose aerial roots require free access to the air. Plants should be repotted before the medium decomposes. Special feeding needs are discussed separately, in the Fertilizers section.

Another orchid growth medium is Osmunda (a genus of ferns), a now uncommon organic substrate that requires no additional fertilization. If inorganic media are used, both nutrients and fertilizers should be provided.

Sphagnum Moss, Peat Moss

Sphagnum moss is the precursor to peat, and is often referred to as peat moss. Note however that true sphagnum moss is the better growth medium. Both are used as filler for any growth medium, but are also useful on their own, or mixed with perlite and vermiculite.

I find these most useful, mixed with soil, with acid-loving plants. They are also the growth medium of choice when reproducing by air layering. However, their nutritive value is low, and once peat moss has been allowed to dry out, it becomes about as useful as cardboard. Wet thoroughly before use.

Manure, Nitrogen, And Potassium

Manure is a rich source of nitrogen, and an inch of cattle or horse manure, or half the amount of poultry, sheep, goat, or rabbit manure, is a good ad-dition to the garden. Use only well-composted manure at least six months old, or the sterilized manure sold commercially.

So-called green, or plant-derived, manure is also rich in nitrogen; see the separate entry on this subject. Both forms of manure will also restore potassium to the soil, slowly.

Bone Meal And Other Natural Additives

Bone meal is an excellent source of phosphorus, essential to plant growth, and also of calcium. This phosphorus is water-insoluble in neutral or alkaline soil, can precipitate out quickly, and therefore should be mixed into the growing medium, where plant roots have direct access.

The same applies to rock phosphate, which though cheaper must be used more frequently, may be harder to get, and may also contain significant amounts of uranium. Superphosphate will not contain radioactive material, and may be used; it is also rich in sulfur and calcium.

Blood meal is an excellent source of nitrogen, as are fish meal and hoof meal. Wood ashes and potash (potassium) sulphate raise soil potassium levels. Wood ashes also raise the soil pH, for more alkaline soil, and potash lowers it, for a more acidic soil. Use both with caution, if at all.

Ground or crushed dolomitic limestone may also be used to raise a soil's pH. Mix in very thoroughly. Note that quicklime (burned lime) and slaked lime should NEVER be used.

Do not use too much of any of the above additives: too strong a concentration of the active ingredients will adversely affect plant growth. Use no more than two or three pounds per 100 square feet, about one kilogram per 10 square meters, once per year. Add no more than a handful to the compost pile.

Have your soil tested, or test it yourself. Kits are cheap and readily avail-ble, and should guide what you add, and how much. This applies especially to calcium and magnesium compounds: adding calcium lowers magnesium levels, and vice versa. Note also that if used in sulphate form, these will render the soil more acidic.