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. We indicate where a special type of growth medium is required.
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 kilogram 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.
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 retention. 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.
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