Regarding Turface fines, do I understand correctly that they would be helpful for in-ground trees in flat or otherwise insufficiently drained areas? I do have some trees like that and would love to put the Turface fines to good use. It depends on what the soil is like and whether or not they are used in a raised bed. If you have an area with heavy clay and you incorporate them into the soil, they might actually increase the amount of water that collects in the soil surrounding your plants' roots. This is known as the "bathtub effect". Amending a garden where the soil is clay just causes the garden area to hold more water unless the garden is on a slope or there are tiles or a ditch to direct water away from the area. THEN, it can be very helpful.
Whenever there is an area where percolation of water through the soil is slow, amending it in the hope of creating more air space results in an additional measure of water filling the newly created pores until the water naturally percolates through the soil. In the case where they are used in a raised bed (RB), water will move horizontally away from the RB across the top of the clay soil, where it will evaporate much faster than water below the surface of soils not exposed to air movement. When water evaporates from the surface of the surrounding soil, it 'pulls' additional water from RBs via capillary action. That's why clay soils below RBs should not be amended with organic material or sand as it creates thew bathtub effect and holds more water in the amended areas unless there is a plan in place to direct subsurface water away from the area.
I couldn’t seem to locate uncomposted pine or fir bark fines ........ Will exceeding 3/8” on the bark fines be a big problem for the gritty mix?
April through Memorial Day is the easiest time of year to find good bark products for making media. I suggest that you should until you find the right ingredients to make either the 5:1:1 mix or the gritty mix; this, because if you 'try to make something work', there is a good chance you'll end up unimpressed and give up. If you get it right the first time, it will be a game changer. Too, once you find a supplier, keep looking for other suppliers. I've found I can always find it somewhere near me, but operations go out of business or might decide to stop carrying a product some of us find very useful. What I'm saying is, it's better to have more than one basket to put your eggs in. I'm on my last bag of fir bark screened 1/8-1/4, so I'll be needing to screen bark again come next spring.
Also, do you prefer/recommend gypsum or Foliage Pro 9-3-6 more for trees in pots? Your fertilizer will dictate what liming agent is appropriate. I always lime the 5:1:1 mix with dolomitic (garden lime) to bring the pH up to a more acceptable level. If I'm using the gritty mix + Foliage-Pro 9-3-6, there is no need to use a liming agent. If I'm using the gritty mix and a fertilizer without calcium or magnesium, I'll use gypsum (CaSO4) as a calcium source and Epsom salts (MgSO4) as a source of magnesium as neither compound has an appreciable effect on pH. Note: I haven't had to supplement Ca/Mg for a long time - since I started using Foliage-Pro 9-3-6. If someone asked me what hort product I would consider to be the most indispensable, it would be the FP 9-3-6.
Al
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