Pumice and Lava rock size?

[Dudesome]

Building my soil from scratch.
What size of lava rock and pumice grains should I order?
Here is whats available
Pumice grain size 0-4 mm
Pumice grain size 0-8 mm
Pumice grain size 0-10 mm

Lava grain size 1-5mm
Lava 2 - 8mm grain
Lava grain size 2 - 12mm
Lava grain size 8-16mm
Lava grain size 16 - 32mm

 

[G Noam]

Had no idea this came down to such an exacting science. I'd think you'd go with the smaller grain stuff for soil. I used to used larger lava rock grains for gravity hydro. I like your location. Actually we're all 'there'. And the earth and atmosphere creates the illusion we're not- sort of. Lol.

 

[vonforne]

I go for a 1\2 and under even throw in all the fine grain powder also.

 

[Dudesome]

So ive got the 2-8 mm lava rock. Should I crush it into smaller pieces or is it ok to use?
Cheers

 

[Carbon.Chains]

2-8mm seems fine to me.
I remember reading here that a variety of sizes is beneficial, and that the particles in the soil to be able to bump with each other.
I designed a soil for a friend of mine using the same source for the aeration, I used 0-10, 2-12 and 8-16. I thought of using 16-32 but it does seem a bit big.
Hope it helps.

 

[bigshrimp]

I go quarter inch or smaller, but that's cause i have a quarter inch classifier laying around.

Small enough that you don't feel like there is just a bunch of rocks sitting in your dirt.

Real useful i know...

 

[EclipseFour20]

Two of my posts from another thread regarding pumice & lava rock--

Here is some information that got me rethinking the value of pumice as a growing medium aggregate.

"Pumice possesses large pores and consequently its volumetric water content decreases sharply as water tension increases. (Boertje, 1994; Raviv et al., 1999). The water-holding capacity of pumice is relatively low compared with rockwool, perlite or organic substrates and may limit water and nutrient uptake by plants, especially in hot climates (Raviv et al., 1999)."

"Pumice has no buffering capacity and possesses a very low surface charge, derived mainly from impurities of carbonate and metal content ...However, caution is recommended when using new pumice material because high concentrations of Na are leached out at the beginning of use."

Source...UN's Food and Agriculture Organization....http://www.fao.org/hortivar/scis/doc/publ/8.pdf

Jump to the pdf file page 43...that is where the inorganic substrate discussion starts...and page 50 for pumice info.

Lets just say, my growing medium is now pumice free. Cheers!

Referring to the FAO file I linked to above, check out pdf page 52 under the section "2.5.1.5. Pyroclastic materials (tuff)"

Tuffs possess a buffering capacity and may adsorb or release nutrients,
especially P, during the growth period (Silber et al., 1999; Silber and Raviv, 1996).
The chemical stability of tuffs depends on their mineralogical composition.
Volcanic glass dissolution is very rapid while that of secondary minerals, such as
kaolinite and halloysite, is slower. Hence, non-weathered materials containing a
high concentration of volcanic glass, like black tuff are unstable and dissolve
easily in solution below pH 6, while red and yellow tuffs are more stable (Silber et
al., 1999). Introducing plants to black, or even red tuff, after equilibration with an
acidic electrolyte (pH below 5) may be risky due to Al and Mn toxicity

The CEC rates for the different types of volcanic rock are: black is 107, red is 285, and yellow is 601. (Table 8. Mineralogical composition of various tuff types.)

Seems that not all "volcanic rocks" are equal....just like most things in life. Hmmm, "yellow tuff" think we might have to find some and run some tests....side by side.

Keep up the unconventional thinking!

BTW--I no longer use pumice or lava rock.

 

[charlie_mops]

BTW--I no longer use pumice or lava rock.

So whats your aeration material of choise then? Or dont you use any?

 

[EclipseFour20]

So whats your aeration material of choise then? Or dont you use any?

Perlite #3, Fossil Shell Flour (Food grade DE), vermiculite and calcined clay (Oil Dri) are my primary porosity aggregates. I know, it sounded "so cool" to incorporate pumice and lava rock in the growing medium, but after doing tests and see the differences--"cool" was no longer "cool".

Cheers!

 

[charlie_mops]

Thanks for your answer..

I was under the impression that lava rock in a size between 1-5mm is a good additive. Can you elaborte a bit more why its not?
I have already known that pumice (and perlite) isnt that good because its quite light and can "swimm up" in your soilmix after repeated waterings.. but my "knowledge" is only theoretical...

Stay safe..

 

[EclipseFour20]

I see no real benefit--unless I am missing something, most of the volcanic rock I have seen are of the "black" and "red" variety--never any "yellow", which ought to be the preferred one; "yellow" tuff/volcanic rock is more stable and possesses a higher CEC rate than the "black" & "red" rocks.

If the purpose is to add minerals/elements--then there are probably more efficient options available...than volcanic rock.
If the purpose is to increase CEC--then sure, great option. My custom mix already has a very high CEC rate, thanks to bark, calcined clay, vermiculite, and compost (which I get locally and at real affordable prices...the volcanic rock is not).
If the purpose is to increase air/water porosity--then bad choice, as there are better options available.

From the 2nd quote in my above post--
"...non-weathered materials containing a high concentration of volcanic glass, like black tuff are unstable and dissolve easily in solution below pH 6, while red and yellow tuffs are more stable (Silber et al., 1999). Introducing plants to black, or even red tuff, after equilibration with an acidic electrolyte (pH below 5) may be risky due to Al and Mn toxicity.

The CEC rates for the different types of volcanic rock are: black is 107, red is 285, and yellow is 601.

Hope this helps, cheers!

 

[charlie_mops]

Thanks a lot!

 

[Dudesome]

Well I will try pumice/perlite/lava rock/foodgrade De
Diversity should win methinks

 

[EclipseFour20]

BTW...for years I poo-pooed Perlite. Why use it I? It has very little CEC benefits, it is derived from silica--but provides zero PAS (plant available Si), it migrates to the top after flushing, etc...we all have our reasons for not liking perlite.

But then I conducted extensive experiments with different aggregates (and combinations/ratios of each) attempting to dial in "air & water porosity" ratios of my custom grow medium...and that is when I discovered perlite gave me the best answer for one tiny problem--air.

When water is absorbed in the perlite air pockets, almost 100% of the water is available to the grow medium (increase in water porosity)...later when water is displaced by "air", there is an increase in "air porosity". Perlite possesses high capabilities of both "water" and "air" porosity.

Pumice & lava rocks do have air cavities--but their openings are much smaller. Water can not flow freely from the pumice/lava rock cavities to the grow medium...rather it has to be "sucked out" through the tiny opening (increase in water tension).

I pay $13 for 4 cuft bag of sterile perlite and my grow mix contains about 18% by volume (probably 20% if one includes the perlite contained in ProMix). As for pumice, I used to pay about $6 for 4 cuft--but then I had to sterilize it for 8 hours too.

Do a test using pumice/lava rock and then try substituting perlite and see for yourself. What I did was--after satisfied with the "porosity rates" I then jacked up the CEC ratios by incorporating bark, vermiculite, and calcined clay at goodly amounts--and then fine tuned the air/water porosity by adjusting the amount of perlite used.

Why jack up the CEC Rates?....$$$$ savings; less fertilizer/nutrients are required for grow mediums with high CEC capabilities! For me, I now use 25% less ferts/nutes and enjoy the nice bump in quality/quantity!

The pdfs below I found helpful in understanding the concepts of air/water porosity--and they explain ways to conduct your own tests.

fs812.pdf-- Fundamentals of Container Media Management, Part I Physical Properties (Rutgers Univ)
fs881.pdf-- Fundamentals of Container Media Management, Part 2 Measuring Physical Properties (Rutgers Univ)
ghsubfert.pdf-- Greenhouse Substrates and Fertilization (NC State Univ)

Cheers!

 

[Carbon.Chains]

If I may ask, what risk is there to use un sterilized rocks?
And what is that sterilizing method that takes 8h? Wouldn t heating at a high temperature for an hour or two suffice?
And since you already use perlite at 20%, at what rate are you using your vermiculite and calcined clay?
Thanks for all that info btw. Really helpful!

 

[EclipseFour20]

If I may ask, what risk is there to use un sterilized rocks?
And what is that sterilizing method that takes 8h? Wouldn t heating at a high temperature for an hour or two suffice?
And since you already use perlite at 20%, at what rate are you using your vermiculite and calcined clay?
Thanks for all that info btw. Really helpful!

During "those days" I was fighting Root Aphids and Symphylans...and the pumice contained symphylans. Funny, I never really found where the "Root Aphid" came from...but today neither are a nuisance in my life.

My grow mix contain equal amounts (18.2% by volume) of:
ProMix BX
Bark
Perlite
Vermiculte

6.8% for calcined clay (Oil Dri--auto department at Walmart)

The below pdf, Finding the Balance: Calcined Clay Rate Effects in Pine Bark Substrates (NC State Univ), is a study that concluded:

Plant weight and net photosynthesis both fit
curvilinear models when plotted as a function of clay amendment rate (Figs.1
and 2). A 39% increase occurred in total dry weight when the clay amendment
rate increased from 0% to 12% (Fig. 1). However, as the clay rate increased
from 12% to 20% top dry weight decreased 20% (Fig.1). Clay amendment rates
produced a quadratic relationship with maximum net photosynthesis and plant
growth at clay rates of 12% and 11%, respectively (Figs.1 and 2).


In my world, adding clay at 11-12% was excessive (perhaps due to other CEC components I used, but not used in the study) but when I adjusted it to 6-7%, things hummed. You might find different rates will work better.

Cheers!

 

[Carbon.Chains]

Thanks for the quick answer!
I'm designing another soil mix at the moment, so I really appreciate the info and pdf files, love to try new things; hopefully the results will be lovely too! haha
Sorry to get a bit off-topic.
Goodluck to the OP with his grow!

 

[LilMan72003]

Eclipse,

Thanks for the knowledge on aeration components. Also,
I have found adding calcined clay to thermal compost an effective way of boosting soil CEC

 

[EclipseFour20]

Eclipse,

Thanks for the knowledge on aeration components. Also,
I have found adding calcined clay to thermal compost an effective way of boosting soil CEC

Glad to help!

Yep...but too much calcined clay can be no bueno! My fav right now is OilDri oil absorbent (Walmart)--the worst product I tried was the cat litter variety.

Also, adding vermiculite (coarse grade) will also jack up the CEC ratio...and provide the soil with good water/air porosity. Can be $$$ if you buy it at the wrong place.

Cheers!

 

[dank.frank]

Fwiw - I stopped using the calcined clay / oil dry long ago because I kept getting an odd deficiency with it - and it was kind of smaller and heavier than I really liked...when rinsed really well, it was still a bit like having "river rocks" for a fish tank in my soil mix. I also found my soil was compacting to tightly over time if I used to much of it...

I went back to perlite - tried lava rocks - started trying all sorts of various sized combinations - then found permatill...

I told people about it many months ago - but merely called it expanded slate - perhaps I should have said "rotary-kiln fired expanded slate particle" - it's about the size of chunky perlite and a touch bigger, consistent, and doesn't crush or break down with repeated use. It's pH neutral, and sorta like lava rock is very porous, but on a much smaller scale...ie the pores are smaller.

I personally think it's the BEST aeration amendment I've found / tried. And I've really tried them all...

dank.Frank