Experimenting with Soil Mixes - a formula to guide me
I've experimented with container mixes in the past, but never in a scientific manner. Starting with the 5-1-1 I've generally adjusted it to suit my purposes solely according to "feel." Taking a step back, I'd like to approach it in a more deliberate manner. To do this, I've developed a rudimentary formula for establishing the drainage characteristics of a proposed variation. This is only meant to provide very general guidance.
Considering the main components (peat, pine bark fines, granite, Turface, perlite) I've assigned a numerical value corresponding to the relative drainage characteristics or each. Granite and perlite rate a 1 (most free draining), PBFs gets a 2 and Turface and spaghnum peat get a 3 (most retentive).
Using this formula, the drainage profile of the 5-1-1 mix is a 2. ((5*2)+3+1)/7 parts). The gritty mix also gets a 2: (1+2+3)/3 parts. A heavier mix, let's say just for example 2:1 mix of PBFs and peat would rate a 2.3.
I freely admit, there is a lot more to substrate components than this single factor. At the same time, it's quick and easy and provides a very general rule of thumb for experimenting.
Please, feel free to comment and poke holes.