A lot of thought went into how to best put the gritty mix together.
It first had to provide long-lasting structure that couldn't be appreciably diminished by breakdown of the organic fraction (pine or fir bark). To protect the long-term structure, the organic fraction/ bark is limited to no more than 1/3 of the medium's o/a volume.
Second, how much perched water a medium can hold is based primarily on the size of the particles that make up the medium. Since particles of a uniform size lose their ability to hold perched water as the size of the particles becomes larger than about 1/10", the particles should be screened so particles aren't small enough to support perched water, and not so large the medium tends to stratify/ form layers, or water retention is diminished to the point it requires too much effort to water.
Third, it needed to be adjustable for water retention w/o increases in it's ability to hold water coming at the expense of introduction of perched water as the reservoir by which watering intervals are extended. It is designed so all or nearly all water is held in a microscopically thin layer of water on the surface of soil particles, within internal pores of those particles with internal porosity (bark and Turface), and at the interface where soil particles contact each other, leaving all the spaces between soil particles filled with air and water vapor.
Fourth, the adjustability in water retention needed to be a function of changing the ratio of two ingredients (Turface and grit), one of which has no internal porosity (to facilitate water retention) and the other of which has a great deal of internal porosity. By not changing the size of particles, water retention can be increased without introducing a perched water table, undoubtedly the best reason to use the gritty mix.
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Approximately 2 weeks after repotting
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