Crushed Concrete as aggregate?

[Schmikah]

I've seen discussion about crushed bricks as a soil additive but I haven't seen the same for concrete. If this is a well trodden path, just point me in the right direction.

I know the issue is mainly that water retention is pretty low for high fired bricks, plus there doesn't seem to be the right micro structure for roots to really penetrate like lava/pumice.

But I know concrete is generally more porous. And since concrete undergoes micro fracturing over time that doesn't readily mend itself (for non-roman concrete at least) that could allow root penetration. Obviously it wouldn't replace akadama because the macro grain structure is different but I have a feeling it might be worth a good test.

The only major issues I see are these: 1) getting the right sized concrete. I've seen RCA being a pretty common thing aggregate companies offer, but I assume they usually don't come in 1/4 inch and smaller sizes.

2) the ph could be pretty high, plus the calcium content.

Any opinions?

 

[Wires_Guy_wires]

Calcium, magnesium and sodium. But modern concrete was developed to specifically be resistant to plant roots.
Concrete, when damaged and frequently watered, weathers quite fast and it turns into sand.

I have some concrete pots and they're not as porous as clay.

 

[Schmikah]

Calcium, magnesium and sodium. But modern concrete was developed to specifically be resistant to plant roots.
Concrete, when damaged and frequently watered, weathers quite fast and it turns into sand.

I have some concrete pots and they're not as porous as clay.

I'll agree concrete is not as porous as low fired clay. I also have some concrete planters and it's pretty obvious they're not as good st water absorption. But it would be more porous than chicken grit.

From everything I've seen, modern concrete is absolutely not root proof. Just at a glance I've found a whole bucket of research papers about root proofing concrete, and they bemoan the current fragility of concrete resistance to plants.

The erosion might be an issue. I don't think it would be more of an issue than akadama breaking down though.

This part is just me waxing philosophic about chemistry, so ignore if you want.

Calcium is a concern because of the calcium hydroxide. Then once the pH settles out, you still have water soluble calcium in potentially high amounts. For those of use without hard water, that's less of a concern, but if I undergo this experiment, I'll have to watch out.

Sodium I don't think is a real issue. It's about 0.6% by weight at max. Plus, anecdotally, there seems to be no lack of seedlings growing in the cracks of my driveway where they are constantly soaking up the concrete filtered water.

Magnesium is also something that doesn't concern me. Most Magnesium in concrete it MgO, and is limited in weight percent because it causes deterioration. However, it does react with water to produce magnesium hydroxide, but that preferentially binds to metal ions (hence its used in ground water mediation). That might cause issues with iron deficiency in the soil, but that might be a long term issue rather than acute. That might actually help with sodium excess but I don't have any proof for that.

Long story short, I need to do an experiment and get some water tested.

 

[milehigh_7]

I would worry about the PH, to be honest.

 

[rockm]

pH is a big problem for concrete in soils. That problem gets bigger depending on the species that's in it. Acid-loving species, azaleas, holly, beech, apple, etc. will probably not do well at all in a mix with this ingredient. Might even kill some of them, if you're using a lot of crushed concrete. Concrete, from what I've seen has a pH level of 10-13, making it extremely alkaline., The Mid-range of pH is 7. Acid loving plants like pH below 7.

 

[Colorado]

Why on earth would you want to grow bonsai trees in concrete?

There are plenty of great soil options available. Concrete? Sheesh…..

 

[Paradox]

Why on earth would you want to grow bonsai trees in concrete?

There are plenty of great soil options available. Concrete? Sheesh…..

Most common reasons:
"Bonsai soil is "expensive"."
"Don't want to pay money for "dirt""
"What's the cheapest soil components/alternative"

 

[Paradox]

Double post cause phone being stupid

 

[Michael P]

Some years ago a client wanted to do a gravel driveway. Part of the drive passed over the root zone of a Shumard oak. The two materials under consideration were crushed recycled concrete and decomposed granite. I was concerned about the pH, so had both tested. To everyone's surprise, the crushed concrete had a lower pH (7.3) than the decomposed granite (7.6). Of course pH might vary with different batches of crushed concrete depending on source and age of the concrete. But since the pH of our soil is about 7.8 on average, the concrete was unlikely to cause any harm. in this case.

In bonsai soil I think you could easily correct any raised pH from crushed concrete with other components of the mix. And since we water bonsai so much, the pH of the irrigation water is probably the biggest factor affecting pH in the pot.

 

[rockm]

FWIW, I would avoid it. The pH, from what I come up with on searching around, is highly variable. Just like I wouldn't use crushed limestone in a soil mix, I wouldn't use crushed concrete. Just because you MAY be able to get by with it, doesn't really mean you will...

 

[Schmikah]

Most common reasons:
"Bonsai soil is "expensive"."
"Don't want to pay money for "dirt""
"What's the cheapest soil components/alternative"

Not really in this case. I mean, if I found something cheap that worked well I wouldn't cry about it, but I think it's kinda bad to to look down on people that don't want to spend an entire paycheck on bonsai soil. Also, sometimes it's a question of availability.

Seems like an elitist attitude.

 

[Michael P]

I'm on a budget and don't use akadama for that reason. Plus the fact that shipping soil components half way around the planet is not sustainable.

 

[Schmikah]

Some years ago a client wanted to do a gravel driveway. Part of the drive passed over the root zone of a Shumard oak. The two materials under consideration were crushed recycled concrete and decomposed granite. I was concerned about the pH, so had both tested. To everyone's surprise, the crushed concrete had a lower pH (7.3) than the decomposed granite (7.6). Of course pH might vary with different batches of crushed concrete depending on source and age of the concrete. But since the pH of our soil is about 7.8 on average, the concrete was unlikely to cause any harm. in this case.

In bonsai soil I think you could easily correct any raised pH from crushed concrete with other components of the mix. And since we water bonsai so much, the pH of the irrigation water is probably the biggest factor affecting pH in the pot.

Good to know. I see the 10-13 pH get tossed around alot but that's for freshly poured concrete as it is curing. I would assume that there might be some affect on pH in the micro sphere close to the concrete surface, but who knows.

 

[Paradox]

Not really in this case. I mean, if I found something cheap that worked well I wouldn't cry about it, but I think it's kinda bad to to look down on people that don't want to spend an entire paycheck on bonsai soil. Also, sometimes it's a question of availability.

Seems like an elitist attitude.

It's speaking from experience.
We see those statements/questions all the time.

You don't have to pay an entire paycheck on soil but you can kill several paychecks worth of trees by not using good soil components.

You want to keep the costs of soil down, keep only the number of trees you can support and have time to work on.

Most newbies bite off more than they can chew in the beginning. How do I know that? I was one that did.

Been there, done that. I also tried the cheap and easily obtained local soil components. My trees suffered for it.

 

[yenling83]

Hmmm for refined trees in a Bonsai container, you can't beat some type of Akadama & Pumice/Scoria/RiverSand. I would spend the money-it's well worth it here.

If you absolutely can't get Akadama, I'd go w/ Pure Pumice. If I could not find Pumice, i'd go with Large sized perlite mixed with Coco coir.

I def would not use crushed cement or brick, if i'm in the US.

 

[Schmikah]

It's speaking from experience.
We see those statements/questions all the time.

You don't have to pay an entire paycheck on soil but you can kill several paychecks worth of trees by not using good soil components.

You want to keep the costs of soil down, keep only the number of trees you can support and have time to work on.

Most newbies bite off more than they can chew in the beginning. How do I know that? I was one that did.

Been there, done that. I also tried the cheap and easily obtained local soil components. My trees suffered for it.

Now I understand what you meant.

I'm mainly just curious. I was trying to see if anyone had any direct experience since I couldn't find anything on the forum readily. I can't even find anyone on a Google search so I was kinda surprised.

I don't even have, nor have I looked for, a supply of RCA in a size that would be usable, so this is all academic.

If I ever find a source, I'll probably give it a shot as a soil amendment for early development trees. I'm definitely not brave enough to use concrete in a bonsai mix on anything even approaching pre-bonsai.

 

[Schmikah]

Hmmm for refined trees in a Bonsai container, you can't beat some type of Akadama & Pumice/Scoria/RiverSand. I would spend the money-it's well worth it here.

If you absolutely can't get Akadama, I'd go w/ Pure Pumice. If I could not find Pumice, i'd go with Large sized perlite mixed with Coco coir.

I def would not use crushed cement or brick, if i'm in the US.

That's my experience so far. My developed mix is basic something close to a 1-1-1 mix. Or I've got the Kanuma mix (I think that's the name).

But I'm always wondering about new soil amendments, especially for our friends outside the US/areas where imports are more difficult.

I'm not brave enough to experiment on developed trees, especially with an untested soil addative.