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Some gritty(ish) mix anecdata

4 months ago

First post here. I've been lurking for quite some time, reading up all I can on potting mixes. I've been testing Al's gritty mix concept for a while, with FP 9-3-6 (of course), and both myself and my plants have been enjoying it very much.

I have been having fun with the gritty mix and I'd like to share a bit of that first.

I'm in zone 12b, Honolulu, Hawaii. My particular version of the gritty mix is:

1 part black volcanic cinder (instead of granite)

1 part pumice (instead of bark)

1 part napa 8822 (instead of turface).

Black cinder is very common here, and granite is not. For anyone on Oahu, if you want to make a gritty mix, you can get everything you need in one place if you want to use calcined clay instead of DE. Koolau farmers on Beretania has big bags of black cinder, pumice, and DuraEdge platinum (calcined clay). The duraedge has very little waste (maybe 5%) when sifted through 1/12 screen, with maybe 10% of that being over 1/8. From what I read that is better than turface mvp. Bags of duraedge are in the $45 range but at least the yield is good. I may switch over to using it instead of 8822. It is much more tiring to sift but the 8822 dust is really bad whereas the duraedge has almost none.

I have classifier sieves in 1/12, 1/8, and 1/4, the green ones that fit on a 5 gallon bucket.

Both lava and pumice get dried in the sun. Anything smaller than 1/12 is garbage, thrown in the worm bin for grit. 1/12-1/8 is kept for fine seed mix, 1/8-1/4 is used for the gritty mix. Both pumice and lava yield about 40-50% in the 1/8-1/4 size from a good bag. Most of the waste fraction is large particles (>1/4") with only a little between 1/12 and 1/8. I save this for other mixes (mixed 50:50 with coco chips that have seen a few turns through the worm bin and are full of goodness).

The last couple bags of napa 8822 i purchased had a very good yield in the 1/8+ size range, maybe 70%. It is expensive though at about $25 per bag and the dust is atrocious.

So far I've put a hawaiian ti plant, arabian jasmine, lemongrass, spearmint, dwarf minette basil, calamansi on c35 rootstock, rosemary, miracle berry, and a variegated monstera adansonii into the mix. All seem pretty happy though the hawaiian ti plant has much smaller foliage than when it was in peat mix. I'm actually happy with that. I also have some newly germinated adenium seedlings.

Here is where I'll get into some of the things that I haven't heard from anyone else here on the forum.

It seems most people who use gritty mix either water on a schedule, or use a wooden dowel. Normal moisture meters don't work in gritty mix because they actually measure conductivity. But what about soil tensiometers? Has anybody tried using one of them? They can directly measure the amount of osmotic potential necessary for roots to pull water molecules from the soil particles or soil humidity. I have a blumat digital which I've started playing with in the gritty mix. So far I can provide one bit of data.

The moisture level in my particular gritty mix tends to hang out around 27-30mBar of vacuum (this is a very low number, very wet) even though when I dig into the gritty mix it isn't sopping wet. The black lava particles feel fairly dry, the pumice might have just a hint of moisture, and the DE particles have that cool damp feeling when rubbed across your lips. The mix also seems to like to hang out around this moisture level. even a couple days after being watered. The DE and pumice probably hold a good amount of water together and keep the humidity high between particles. I've read many houseplants are comfortable up to the 100-120mBar range, and trees up to around 150mBar.

I'm also experimenting with blumat classics (not tropf, though I have them too) to help me keep my plants watered while I'm away. Most of my pots have 3 classics in them. Lets see how long the blumat digital indicates a resonable moisture level, and the plants don't wilt. Supposedly the pores in the ceramic cone are sized in a way to provide the most water around 100mBar.