The most important thing is size. The stones/gravel/pebbles/grit should be close to 1/8 inch. If you can screen out stuff that's 1/4 inch or bigger and the stuff that passes through an insect screen, and still have a reasonable amount left, you're good to go.

Hi Stefano,
Like Ohiofem stated, the size is what's important. I was talking with Al recently about how to sift pea gravel because I can't find grit.

Here's a quote from out emails..

" you could keep/use what passes a 3/16" screen but doesn't pass an 1/8" screen."

Now,
Finding a 3/16" screen is tricky, i'm still looking for that.
Bonsai screens are in metric, and the closest to the 3/16" would be...
Al said a metric 12/64 = 3/16", the closest I found is a 13/64, so far.

I hope some of this helps.

JoJo

Thanks for the help.
fractional measurements in inches are so counterintuitive. hard to imagine what 13/64 is like and compare without mental calculations! 1/8" is almost 3.5 mm think that gravel is more or less like that. some could be larger, Why is it so important to screen out the larger ones? couln't be an approximation? If a mix is not exaclty the same wouldn't just be sufficient to adjust the watering accordingly?

I also found fine bark at Gowanus nursery but is fine like a slightly coarse soil. Is that way too fine? At HD I found chunkier pine bark.

stefano

If you have 1 ingredient that varies significantly in size from the others, that ingredient tends to separate from the mix and you're left with a mix that retains the physical properties of the 2 more similar-size components. We want to keep the size range of the particles from just slightly under 1/8" (because that's where the PWT disappears) to about 3/16". The bark can be a little larger to allow for break-down over time. As particle size increases, the soils ability to hold sufficient water decreases. Also, if the bark and grit fractions are large, the Turface tends to settle to the bottom and hold too much water there, so you can see why size is important.

I would skip any marble gravel in the gritty mix because of its high Ca content.

Al

Tapla thanks for the reply. Would the pea gravel be OK?

As I said I found fine bark at the Gowanus Nursery as suggested on the thread where to find the materials for the gritty mix. It does not look like wood chips.

Could I mix just Napa Floor-Dry XXX 8822, some bark and some Miracle Grow orchid mix (it is a mix of some peat moss and fine bark, not a real orchid mix but way chunkier than potting soil) and / or perlite? I want to repot my plants on a grittier mix but I can't get huge bags or travel hours. I live in Brooklyn NY. Not much variety in gardening supplies and I live in an apt. I put some plants on the roof and fire escape in the spring / summer/ part fall.

If I can't get the perfect gritty mix I'd like at least have a better mix of what comes prepackaged.

I grow hot peppers (habaneros), ficus, mangos, lychee from seed. Have coffee plants, some tropical aroids.

S

I saw that at the petstore they have silica based gravel and another called spectrastone. They are for fresh water aquariums and the size is right. $3 for 5 pounds. Would that work?

stefano

In terms of sizing the gravel, I understand why we want to avoid granite fines, so the 1/8 minimum size requirement is clear. But do we really need to be so precise about the larger size dimension? 3/16 inch as a cap on size sets a very narrow size range. If the bark is 3/8 maximum size, and the turface is around 1/8, does it really matter if the granite (or some granite substitute) are 3/8? That doesn't seem so disproportionately sized to the other ingredients.

really old thread you've dredged up.

3/8" as an upper size is specific to Pine Bark, due to the normal geometry of pine bark being flat chips - in general most people screen bark (pine or fir) to between 1/4" and 1/8" using 1/4" and 1/8" screens, which aren't difficult to find or make. 3/8" screens seem to be harder to find - i don't have one anyway.

As sizes diverge so will the ingredients when you try to mix them - bigger particles will tend towards the top of the pot and smaller ones to the bottom, which is exactly what you don't want to happen. It's quite difficult to get different sized particles to mix well, try and you'll see.

Also, the smaller particles tend to take up the pore space between the larger particles.

Daniel

Turface is 1/8" and smaller particles. So if we are mixing with 3/8" bark we already have a size discrepancy don't we?

My problem is I don't have a good sense (yet) for the kind of size discrepancies in different ingredients that a gritty mix will tolerate well.