Many ingredients are suitable for potting mixes, as is obvious from the wide variety in use. The choice is based on availability, cost, personal preference, etc. However, a satisfactory potting mix must meet certain criteria. It must support the plant, provide a suitable environment for the roots so that they can absorb water and nutrients, and must not produce or attract any toxic substances. Most potting mixes have adequate supporting properties once the orchid has developed a reasonable root system. More important are the physical properties over the life of the mix. These will vary with the choice of materials, their preparation, the cultural practices and the interval before re-potting. Generally an interval of two to three years is required, and therefore the components of the mix must remain essentially stable for this time. Barks, gravel, wood shavings, rice hulls etc., are useful ingredients but composted leaves and manures decompose rapidly and are thus unsuitable.
Raw materials generally are unsuitable without some pre-treatment. This applies especially to organic components such as bark and wood shavings. In their raw state they contain compounds that must be modified or removed before healthy root growth is possible. Composting microorganisms break down these toxic compounds, but to work effectively they require an adequate and readily available source of nitrogen. The addition of a small amount of old compost will suffice to introduce the composting organisms. However a nitrogen source, such as urea or ammonium nitrate, greatly accelerates the composting process.
Nitrogen is the most essential additive, although lime is often also mentioned. Lime will change the colour of bark and remove the raw pine smell but it does not assist composting and indeed its presence may be detrimental by raising the pH excessively. If the addition of calcium is desired, it should be incorporated in the form of gypsum (calcium sulphate), which does not alter the pH. It is best to allow about six weeks for composting, by which time the nitrogen draw down should be minimal. Ageing of a potting mix simply allows natural processes to proceed over time with no active stimulus; it is slower and less effective than active composting.
Chemical additives may be included in the composted mixes as specific food sources for the plant. These additives include low solubility substances such as IBDU for nitrogen, lime and dolomite for calcium and magnesium, superphosphate for phosphorus and fritted trace elements. Slow-release fertilisers, such as Osmocote® and Nutricote®, in which a membranous resin coating controls the rate of release of the enclosed chemicals, are also used by some growers. Others use animal manures but I believe these substances interfere with the physical characteristics of the mix, and are unsuitable. We compost our bark fully and then rely entirely on liquid feeding, as it provides the best control over growth and flowering.
Of particular importance is the air-holding capacity of the potting mix and its ability to provide constant water contact with the roots. Roots require a constant supply of oxygen to remain healthy, the root tips being solely responsible for water and nutrient absorption. The components of the potting mix must be of a suitable particle size to ensure that an adequate amount of air, water and dissolved nutrients are available when required.
Many different growers get excellent results in a wide variety of potting mixes. There is no special secret ingredient that must be present. It is simply essential to follow the principles discussed above. Healthy root growth results in healthy leaf growth and this is only possible in a suitable potting mix.