Using crushed granite as a growth medium!!!

This deserves attention.

SableZen said:

I know with aquaponics that limestone = problems w/ pH (too high). Pea gravel often has a lot of limestone in it unfortunately. Crushed granite (not decomposed granite) is supposed to be neutral'ish and usually available in bulk from landscaping places.

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So I'm talking about crushed granite which is not decomposed.

The correct term should be "granite aggregate".




I want your opinions and thoughts about using granite rock as a growth medium as an alternative to coir or hydroton clay.

Unfortunately I can find almost 0 information about this. It seems to be something that people don't have experience with.

The biggest pro for using granite aggregate for me seems to be the low price of it.

If hydroton clay costs €450 per 1000L...

... then coir costs a bit less - €275.

But hear me now. Granite aggregate would cost me 50 euros for 1000L !!!

So if one wants to use 1000L of growth medium for his plants, then the price difference would be immense.

This leads to the main question. Can granite aggregate be used as a cheaper alternative for coir and hydroton clay?

What might be the pro's and con's?

The pH of a body of water is affected by several factors. One of the most important factors is the bedrock and soil composition through which the water moves, both in its bed and as groundwater. Some rock types such as limestone can, to an extent, neutralize the acid while others, such as granite, have virtually no effect on pH. - http://www.watersheded.dcnr.state.pa.us/what/ph.html

Everything I have came across seems to be leading to the same conclusion, that granite is inert and does not alter PH of water.

You gotta ask yourself ' ..why is it crushed...? and eventually you will discover ,...is because it originally comes as a by product from a mine, where the rock is crashed to extract even more precious minerals, gold, silver lead, even quartz, with gold, and the most profitable, onve its crushed it mixed with heavy arsenic and or cyanide to get the metals to cling together then easily extracted, few mine companies would waste precious arsenic by leaving it in the waste but rather recycle to save bucks, but the by products aren't washed well, and bits to escape to the waste, ask any local greenie!

Tho is a good idea that Helmut79 provides, ...............DO WASH THE SHIT REAL WELL...!

This leads to a question of what would be a good way to wash it. Any thoughts?

I can't remember which poison turns you green maybe arsenic ...? but if you do ...better get to the doc quick, the other poison I think makes you smell like almonds ...bit hazy here, to wash the stuff, you would need some way of detecting the chemical in the first place, so you can test for and afterwards for residue, ..................sorry to piss on your fire

Hot soap and water...? the real question is to ask " How much residual poison would the average plants absorb, then ....how much poison can you absorb from the plant, and does it accumulate in the body? (I think it would)

Good point. Definitely something to think about.

look into the crushed granite and where it comes from cause its not always left over..... not sure where vostok got this idea? I'm sure there is crushed granite thats leftover from the mining industry but there is no way that landscape granite aggregate has arsenic in it. if it does, people would have to pay hell.

there are alot of mines around the world that produce granite slabs and sheets and blocks and all that good stuff... its called a quarry.... now granite is very very valuable as is so I'm not sure why you would crush it then add arsenic..... (Which they add the arsenic to pulverized stone IE *powder* so it has a chance to get to all the metals... not aggregates) at these quarries all kinds of smaller chunks are made either by accident or in the extraction process of the other blocks. afterwards a skilled tradesman eyeballs the chunks and cuts them to be the biggest nicest square piece possible (Might be automated by now) in this process alot of end pieces and just plain scrap gets created that i could see crushing up to sell as landscape aggregate....

i guess what I'm saying is your probably safe.. i know your in EU so you might wanna just get it checked to be sure. maybe ask the retailers how it was produced? get into the manufacturer and check out their methods? i just highly highly doubt any granite aggregate is contaminated with arsenic... i could be wrong but I'm feeling pretty strong on this one.

Thank you for the reply! I will contact the manufacturer right away, I will ask about it and post here for you what they said to me.

Colander is a great way to wash gravel or aggregates. The only problem with granite is it will not absorb water and nutrients into it's matrix as it is very dense. So you may have to increase watering intervals because that type of medium would dry out quicker because it cannot absorb water.

Interesting idea.. I work with crushed and decomposed granite a bit.. I have never thought to use it.
Most of what I get is not consistent in size... 1/4in minus.. but I have seen some "mountain granite" that was grow media sized..

I like expanded shale.. It is pretty damn cheap too.

I have washed with a colander... Another good way to (pre)wash large amounts of media is in a wheelbarrow/container with a strong leaf blower/shop vac.. gas powered, little elec bullshit won't do.
Move the rock around with a shovel while blowing into the wheelbarrow.. If you have a good strong leaf blower/shop vac it will clean very very well
Blow it clean, you blow out a bit of dust that when wet is more difficult to separate from your clean media.

From there.. wash with lots of water.

I'm asking myself the same question right now.. cheaper grow media?

When you buy rock by cubic yard or by the ton you pay a much much lower price than buying rock bagged.
It's the same with soil, compost, mulch, rock, whatever.

I am setting up 4 large 200 gal aquaponic systems right now..
We are also trying to find something from the stone/soil yard that will work.

The clay pebbles are great but like the granite they have 0 % organic matter. The clay pebbles are cooked and expanded so they are light and roots can penetrate but I would not plant in 100% hydroton either. I had one guy give me his dying plants he planted in decorative white rocks, none survived. Your watering and fertilizing will be crucial without any O.M. - you are looking at an unforgiving medium.

First off, I agree with Fr33MASON....It won't hold water. So you would have to run a constant drip style or DWC maybe. BUT, I would be concerned about the micro nute concentrations as I use crushed rock powder for those in making organic "supersoil". Yeah you get some from the Kelp meal, but you add the rock powder for those too! Maybe a possible overage there!

As far as it being mined. I KNOW they mine it in Canada outside Calgary in Kananaskis County in Alberta. I've fly fished the Bow river right in front of the mine (even though they blow a warning siren, you still jump when they blast). The mine is for Granite only, no chems used in proccessing! It's ground down to the size's required and simply water washed!

Don't know if that's true in Europe. You should check that out.

FR33MASON said:

The only problem with granite is it will not absorb water and nutrients into it's matrix as it is very dense. So you may have to increase watering intervals because that type of medium would dry out quicker because it cannot absorb water.

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But if one chooses granite aggregate that consists of smaller pieces, then there would be more surface area for the water too. By the way, doesn't hydroton clay have the same "problem" ?

TexasHank said:

Most of what I get is not consistent in size

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I get to choose between many different size. The smallest in terms of diameter for one piece/granule is 2-5mm (0.078 - 0,2 inches)

A fountain hydro system works great..
I have built manifolds with a feeder to each plant.
Let me get a pic...

24/7 "fountain" hydro. manifold with an irrigation fitting "1/2 elbow" to each site/plant.
This is actually an aquaponic system.. but it works with nutes.. just have to change the water out over and over.
Aquaponics are set and forget.



Some plants aren't happy with the fountain thing.. Some manage.. some do really well.

Helmut79 said:

But if one chooses granite aggregate that consists of smaller pieces, then there would be more surface area for the water too. By the way, doesn't hydroton clay have the same "problem" ?


I get to choose between many different size. The smallest in terms of diameter for one piece/granule is 2-5mm (0.078 - 0,2 inches)

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Nice.. yeah.. options are totally local..
I get CHEAP granite here. It doesn't have to travel far. I should look for some agregate(sp?). I'm sure it i available somewhere.
The mountain granite I saw is just HD bagged stuff.

Yeah, watering more times a day is an easy fix to fast draining media.

Good luck with the granite.. hope it works out well for you.

Helmut79 said:

But if one chooses granite aggregate that consists of smaller pieces, then there would be more surface area for the water too. By the way, doesn't hydroton clay have the same "problem" ?


I get to choose between many different size. The smallest in terms of diameter for one piece/granule is 2-5mm (0.078 - 0,2 inches)

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For my freshwater aquariums and also my marginal plants for my pond, I use 1/8" crushed and screened gravel. I found that the plant growth was way more vigorous with the smaller screen size so I would say the smaller, the better. The only issue is still the lack of absorption of water and nutrients for hydroponic use.

I haven't tried this personally, but I've thought about cutting in a little lava rock to break up the substrate a bit - might give a little bit more water retention as well. What kind of system are you using by the way?