How much gravel for two 585l grow beds

[gavinl]

Hi again

I will soon be starting a new system with 2 x 585L growbeds and was wondering what the rough weight of the gravel/blue metal might be for this configuration.

Thanks

gav

[Murray]

Gavinl,
Takes just over 1/2M3 per grow bed. Depending on which gravel you use it can weigh up to 1100kg per grow bed when full of water together with the gravel.
Using 20mm drainage gravel it takes 170 ltrs of water to fill a grow bed.

See edition #2 of Practical Aquaponics for Everyone for step by step building grow bed supports from concrete block and sleepers.

[gavinl]

Hi Murray

I'm pretty bad at conversions. If I have this right...two grow beds are roughly 1 cubic metre - or a bit over. Also, if the growbeds weigh around 1100 kgs (depending on media) minus 170 litres (or 170 kgs) then I'm looking at about 930kgs x 2 or about 1900 kgs of gravel. Hopefully two tons should do the trick when ordering. Normally, I order crusher dust in tons or loads - but they might take cubic metres as well.

Does this seen about right Murray?

Thanks
gav

[Murray]

Hi Gavinl.
Most landscape/gravel supply places sell gravel by the cubic mtr these days. (around here anyway).
Gravel is much heavier than water... one table says that dry gravel weighs 1650 kg per M3 so 585 ltr grow bed will have 965 kg of gravel and 180 kg of water when full.
total weight = 1,145 KG
It is heavy, make sure you do a good job of the bases for the grow beds.

[gavinl]

Thanks Murray.

It looks like 1m3 or 2 tons will hopefully do the trick.

I'm still on the old system here....you head to the quarry...weigh your truck first (or in my case a mate's tiptruck)...they dump the load...and then they weigh you again roughly.

I just want to make sure that I get a bit over rather than a bit under.

[Martin A1]

Hi Everyone,

I am also starting a second system with 3 of Murray's large grow beds so stay tuned for a new system thread from me soon . Your calculations for gravel quantities are consistent with what I have estimated.

I have another question that is relevant to this thread. When shopping around for gravel today I have found my local sand & soil mob can supply recycled concrete. It looks like smashed up bricks and concrete etc and is even cheaper than blue metal.

My question is will this be OK as a growing media? Apart from it being really dirty, my main concern is that this material may leach lime or other nasties that create an alkaline problem. Is this a valid concern?

I like the fact that this stuff has a lot of little holes, cracks, crannies etc to harbor the all important bacteria. I was even thinking of putting a 2 inch layer of volcanic rock (scoria) at the bottom of each grow bed to improve the bacteria growth rates in my grow beds. What are your thoughts on this?

I have a gravel delivery coming at the end of this week one way or another so any feedback on the recycled concrete is needed urgently.

Thanks folks and as usual your wisdom is welcome!

[Murray]

Hi Martin,
I personally have not used the crushed concrete so I cannot comment really, although gut feeling tells me it may be a problem....Too much lime I would think.
When I was first cycling my system it tended to want to have high PH, so it would have been a real job to get it down if there was crushed concrete in the system.
Others , please comment.

Old bricks etc may have been painted with waterproofing compounds in their previous life and this could pose a risk.

I have heard others who have used Scoria and are very happy with it.

Personally I would stick with drainage gravel or similar. Around here that is the cheapest product avbl. It may be different where you are.

[GaryD]

Hi Martin,

I'd opt for the blue metal. It has less clay and other garbage in it than drainage gravel (I have both in my grow bed). The only problem with blue metal is that the dust will push your pH through the roof initially.

Flood and drain the grow beds a few times and pump the water onto your trees.....they'll love it.

Test your water (ph will probably be about 8.0) and adjust it down to 7.5 or thereabouts with lemon juice or vinegar.

What's your tank capacity?

Gary

[Martin A1]

Thanks!

Always go with your gut feeling.... nine times out of ten it is the best thing!! OK maybe 7.5 in the early days

I'm gonna do the 20mm blue metal with a layer of scoria (not sure if thats how you spell it?)

Gary, I am using my original 1800ltr gal tank for this 2nd system. I have made progress today installing 2 of my 3 new grow beds. It was very hot in Sydney today and then another massive storm front in the arvo (there was messages of golf balls from the sky just south of me ). I could just see those going through Murray's fibreglass beds before the gravel was laid! We have had sooo much rain the last couple of weeks!

In the end no harm done to the virgin grow beds

Thanks for the tips and look out for my new system thread (incl. photo's). I an holding back as I find myself changing things all the time. The photo's from lunchtime are already out of date