All right, I reached out to John Trager with some questions, and I got a reply today.
The questions:
- Is there a specific size of pumice you shoot for? I've got access to sizes between 3/16" to 3/8".
- From other members emails with you, it sounds like "forest humus" is a mix of shredded fir and redwood bark. When you measure out the 80/20 ratio in your growing mix, is that by weight or volume? Maybe this is nit-picking, but shredded bark mulch is quite compressible and so I imagine measuring by volume could get you very different results depending on how densely its packed.
- Is the "forest humus" a specific commercial product?
- With mulch chips and shredded bark having a low pH, do you use any buffering agent when watering?
- And lastly, do you take a different approach between North American and South American species?
And his reply:
Hi Zac,
Your mix is not much different than ours in terms of percentage of organic to inorganic. We use 80% pumice and 20% organic. We shifted away from the Forest Humus product for a mix made by a local soils and potting mix company that offers the convenience of mixing it in bulk for us. The pumice we use is the 3/8” size. Here is a link to the organic component. We use their nutrient mix with peat and perlite:
https://www.ofwolfinbargerinc.com/soils/.
The organic component can be whatever is available to you locally. LGM planter mix would be good if you can find that. It is largely oak leaf mould. The Forest Humus was a well-composted pine and fir bark medium. It worked well enough but became quite hydrophobic when dry. The same will happen with just about any organic component (like peat) so we try to keep our mix fairly damp and sometimes moisten it before use as needed.
Acidification of water and fertilizer has been shown to be beneficial. I refer you to the attached article from the Cactus and Succulent Journal 80(5): 245-250 (2008). pH seems to fluctuate significantly in real time so simply striving for slightly acid seems to be sufficient. We use Liquinox liquid fertilizer which is acidic by virtue of added phosphoric acid which seems to bring us within the desired pH range. When just watering, we have experimented with the addition of distilled vinegar to adjust pH. However, during the growing season, we generally pre-water with just water to saturate and then displace that by following with fertilizer solution at about 2/5 strength.
We do this for virtually everything we grow but have considered the addition of gypsum (MgS04) for some gypsum endemics but we do not use it as a regular practice.
I hope this information helps your plants thrive.
All the best, John
I personally found this reply to be really helpful. Particularly in the sense that the approach is straightforward and doesn't vary between genuses. It's also simple in the sense that even with the relatively acidic forest products as the organic component no correction or buffering is needed. I'm fortunate that my town's water supply has a mineral content such that when I mix up a batch of
Mike & Steve's Fertilizer Regimen the pH of the solution ends up being between 5.5 and 6 without any adjustment on my part.
One thing that this thread and John's reply revealed is that I think I'm misunderstanding or wrong about my conception of what exactly "soil" and "soilless" mean. It seems pretty straightforward that wood chips or shredded bark are "organic," but clearly not "soil." But then I look at the LGM Planter Mix or the Nutrient Mix With Peat and Perlite and it looks like, well, dirt. Plus looking at the components there are descriptions like "composted." At what point do these organic components cross the line and become "soil?"
Lastly, for my NorCal compatriots, I was over in Santa Rosa this weekend and was able to swing by
Sequoia Landscape Materials. I got a few bags of their Double Grind Redwood Bark and just because it looked good I got some of their 1/4 Inch Minus Fir Bark. Gimme a little bit to actually get around to playing around with them and I'll report back.
