A few points I would like to add to Peter's notes, pertaining to container cultivation: 1. Plants with fine root systems (irids especially) are better suited to a mix of finer particles overall. Coarse-rooted bulbs (e.g., most amaryllids) prosper in a mix of mostly coarser particles (pumice, perlite, even bark). All of these ingredients come in various grades to allow control of moisture levels; fine perlite (what is normally available) functions very differently than the coarser grades. 2. More important than the components of the soil mix are the grower's watering habits. Plants grown in a very coarse mix with much air space can still rot under conditions that are too wet and/or warm or cold. No combination of ingredients is a good hedge against heavy-handed watering. Allowing drying between waterings is beneficial if not critically important for most potted plants. In my case this means allowing the *surface *to dry to the point that there is little or no visible evidence of moisture. 3. A mix that is low in organics and consists mainly of inorganic sand, pumice or perlite will last longer than one with a substantial % of organics. This can be an important consideration in terms of time and labor. Bulbs ensconced in a low organics mix can grow happily for five years or more without repotting. 4. Keeping plants pot-bound also helps moderate any shortcomings of a particular soil mix. Pot-bound plants with healthy roots dry out more quickly, making the components of the mix a less critical consideration than timing and extent of watering. This requires more labor perhaps but results in happier plants. Dylan Hannon On 24 October 2012 10:57, Peter Taggart <petersirises@gmail.com> wrote: > I think that the basic principle for growing mixes is being glossed over in > this discussion; that growing mediums consist of a THREE way ratio of AIR > : WATER : SUBSTRATE > > Increasing the particle size will increase the proportion of air in the > mix, provided that the larger particles are a high enough proportion of the > ingredients so that they touch each other in order to have air gaps between > them, - the threshold for this to work is about 1/3. > > To use moisture retaining particles will make a damper mix, (I use leaf > mould for this job - it helps stop sudden dryness which causes premature > dormancy in many corms). Coarse leafmould, large particles of perlite, > larger pieces of composted bark, (?pumice)... will all raise the quantity > of air and water in the mix and reduce the proportion of substrate (and > reduce the weight). > > Small particles of moisture retaining ingredients will increase the water > content, reduce the air content and increase the water even more as well as > the amount of substrate, and therefore the weight will go up. > > Small particles of non water retaining ingredients such as fine sand, clay, > loam, rock dust.... will increase the water (due to capillary action) and > substrate content, make a denser and heavier mix, and reduce the air > content. > > Larger particles of non water retaining ingredients will make for a mix low > in water, high in air, low in substrate and low or high weight depending on > what the particles are made of (- usually gravel which is heavy). > > considering this then take the ingredients you have and mix for the air : > water : substrate proportions which are desired, remembering the other > properties of those ingredients such as weight, nutrient retention, PH... > and your personal growing conditions. > > > Peter (UK) > _______________________________________________ > pbs mailing list > pbs@lists.ibiblio.org > http://pacificbulbsociety.org/list.php > http://pacificbulbsociety.org/pbswiki/ >