Functional difference between pumice and lava

[DrTolhur]

Obviously APL is a very common blend for bonsai soil, but I'm curious about the functional difference between pumice and lava in this mixture. In my understanding, it seems like they both provide pretty much the same functions: drainage, aeration, and preventing compaction. Additionally, they're both inorganic. So what's the value of using both of them? Would there be much/any difference between APL and just AL or AP?

 

[Glaucus]

I think you are making a mistake in applying definitions. Pumice and lava are not technical terms defined by their horticultural properties like drainage and aeration. In fact, lava rock is a very general and ordinary language term.
On top of that, pumice refers to a mechanical process through which the rock is created. It doesn't include the chemical composition of the mineral.

Lava rock would refer to igneous rock, usually extrusive. Pumice is a specific type of extrusive igneous rock that was ejected out of a volcano under conditions with a lot of gas/water vapor, so that it is a mineral with a spongy structure, ie containing air pockets.

I wouldn't say lava rock and pumice have the same properties. Lava rock can be quite dense and thus heavy and is not porous.
Since lava rock usually is just a piece of rock with a rough surface, it drains quite well. But actual drainage are properties of the size of the particles, as well as the surface roughness. Large smooth pebbles (that could be extrusive in origin) from a river would drain the fastest.
While small uneven rough particles of pumice would drain quite slow.

I am not sure what APL is.

 

[pandacular]

I am not sure what APL is.

Akadama, Pumice, Lava. This is a commonly used mixed in the US. You make a very good point about being loose with definitions; I feel that folks don’t grasp how complicated and regionally diverse of a subject the geology involved is here, even before we start talking about putting it in a pot.

Lava in this case is probably more accurately referred to as scoria, but I am far from a geologist. Generally, the pumice in my area is rhyolitic volcanic rock, in largely a grit size.

…but I don’t know enough about the topic to add more besides recommending this resource.

Introductory Soil Physics

Here is the second resource developed from the thread "Introductory Soil Physics". It is a document explaining how the important physical properties in soil and substrates combine to control the amount of air and water in the pore space of soils...

www.bonsainut.com

edited to add
here’s a DNR report on Washington’s pumice that I was reading yesterday. Would be a good read if you would like to find a very thorough answer yourself.

https://www.dnr.wa.gov/publications/ger_ri15_pumice_pumicite.pdf

 

[DrTolhur]

I'm just speaking at the general level where people recommend a mix of akadama, pumice, and lava. So basically, from this perspective: how do pumice and lava function differently in the mix? Of the research I've done, the descriptions of pumice and lava in bonsai soil seem to suggest pretty much the same functions.

Maybe the difference is fairly subtle and just AP or AL would be sufficient as a general mix or less developed trees? Maybe APL is ideal, but you can get 90% of the same results with just two?

It seems like often times advice given is for developed trees that people are trying to get every bit as good as they can and put in shows or whatever. But I think there's often different advice that would be appropriate for lesser trees or trees that you're not trying to get into the best shape possible, especially when cost is a factor.

 

[markyscott]

Pumice and scoria have nearly identical water holding capacity and cation exchange capacity. Scoria is denser and using it will result in a slightly heavier pot, but this difference is very marginal. Use both or either according to your taste - aside from color, I don’t think it makes much difference.

Scott

 

[Russ1]

Scoria and pumice take vesiculation to the extreme. Scoria is an extremely vesicular basaltic lava with very small (< 1mm) vesicles. You can find scoria all over North America: The red variety of scoria (it also comes in black) is commonly used as landscaping pebbles at Taco Bell. Landscapers know this rock as lava rock.
Pumice is a froth of felsic volcanic glass. It is rock foam with so much air in its structure that it often floats on water. Close examination of fresh pumice shows its glassy nature. Older, weathered pumice looses its glassy appearance (volcanic glass rapidly breaks down when exposed to water), but it is still lightweight and feels abrasive against the ski
Quoted from this source:

 

[Weta]

My novice 2 cents.
I use scoria on top to hold pumice down and slow evaporation.
When it comes to repot pumice is easy to remove, no sharp edges to damage roots.
I do use a scoria mix in my palms for extra drainage.

 

[Eckhoffw]

Since @DrTolhur asked about differences between pumice a lave, I’d have to assume he’s talking about the products you find named as such in the retail bonsai world.

In my experience, the “lava“ I’ve purchased (through Amazon or online bonsais stores) is slightly less water retaining and heavier than the “pumice“ I’ve purchased for bonsai through like sources.

 

[pandacular]

It’s worth noting that the use of this mix came about due to local availability, particularly in the Bay Area. Local availability certainly varies, and may change what makes sense to use.

Related; https://crataegus.com/2023/03/10/is-lava-good-for-bonsai-media/

 

[Kievnstavick]

I think the bigger question we should address first is exactly why do you want to know the specific differences? Do you have access to one without the other? Worried about the cost? Or simply just wish to dive into the deep-end of the specifics on the substrate science. (I am not wholly knowledgeable to really address that last one)

Swapping one component for another (specifically scoria vs pumice) won't really have a drastic change in the management of the plant in a container.

At a very basic level, both pumice and scoria both increase the drainage of a mix simply because they are inorganic (rocks) and don't hold much water within their porous space. Scoria is extremely hard and more than likely won't break down in your lifetime (or a few others added on to that). Pumice in the other hand is softer and can break down fairly readily (easily crushes with enough force via hands or pliers) although weathering can still take several years to break it down (unlike a few years for akadama in certain climates).

Getting more into the weeds here, scoria and pumice have different CECs (I believe scoria is close to zero) and hold different amount of moisture and air within their pores. Do you really need to know that information and make sure you have a optimum substrate mixture? Probably not, unless you are working on trees at the highest level and pushing them to their limits. Adding on to that, the optimal substrate mixture with differ largely between species and between trees of the same species.

Swapping Akadama out can change your management practices and is a subject of a whole other discussion that I don't really want to delve into.

I know some people in the tropics who use 100% scoria or primarily scoria with some organics simply due to availability. There are others that go with 100% pumice.

I am of the mind set of if your substrate mix works for you and your trees are healthy than it is fine. As long as you have a logical reasoning to use a certain substrate (Great CEC, drainage, water retention, attractiveness) and the mix works for you then you should be good.

For the most part, I often hear people say to use akadama on more refined trees simply due to the expense. If cost is a concern, just use the components you can get on hand.

 

[Glaucus]

I'm just speaking at the general level where people recommend a mix of akadama, pumice, and lava. So basically, from this perspective: how do pumice and lava function differently in the mix? Of the research I've done, the descriptions of pumice and lava in bonsai soil seem to suggest pretty much the same functions.

Maybe the difference is fairly subtle and just AP or AL would be sufficient as a general mix or less developed trees? Maybe APL is ideal, but you can get 90% of the same results with just two?

It seems like often times advice given is for developed trees that people are trying to get every bit as good as they can and put in shows or whatever. But I think there's often different advice that would be appropriate for lesser trees or trees that you're not trying to get into the best shape possible, especially when cost is a factor.

Pumice is porous while lava rock is not. This makes pumice way lighter, have better water retention and aeration. The chemistry of the mineral could be exactly the same, leading so similar CEC.
Akdama is a form of pumice. If you have a substate mix with no pumice-type material and also no organic material, you need to be aware that your soil drains rapidly, has low water retention, and gets dry quickly. It could be good in areas that are very rainy and not too hot. But in drier and hotter parts, you want more pumice-type material. It of course also depends on the species you are growing and the size of the tree. On top of that, if you are skilled at watering and you can water 3 times a day if needed, you can go for a substate mix higher in air and lower in water. Meaning no or low in pumice and no organic matter.

 

[River's Edge]

Bonsai often has an aesthetic side to decision making. Pumice is usually light gray to white in colour. Lava is often black, red or brown in colour. Akadama has a yellow to light brown colouration and unique properties of it own. If you can combine the material and come up with an aesthetically pleasing combination of particles that function similarly , perhaps that is a winning combination. Three volcanic sources of substrate that complement each other and function well for bonsai.
On the other hand if you prefer one colour? Wish to save money, or simply do it your way, then by all means give it a go.
Personally I have a tough time placing trees in white/ gray substrate. At least those that are developed Bonsai and tending towards refinement.