Gravel for grow beds

[dingorunner]

Has any one used lime stone in there grow beds as I do have access to lime stone.The gravel is a round type of stone I will have to grade it to the sutiable size with a couple of screens before washing

[GaryD]

Hi Dingorunner,

Limestone gravel may present an issue when it comes to stabilising the pH of your system.

Having said that, I used blue metal in my grow beds and, while the pH started off at 9.0, I was able to get it down to 8.0 with water change and down to 7.0 with a litre of lemon juice. It's now fairly stable at 7.0.

How big do you plan to make your grow beds? It might be worth a trial before you commit to a large set.

I like the idea of round gravel.....it's much easier on the hands than blue metal straight out of the crusher.

Gary

[sillyoldfart]

Limestone gravel may present an issue when it comes to stabilising the pH of your system.

Agreed Gary, limestone being Calcite is a naturally occuring form of Calcium Carbonate.

Alkaline by nature. Would work as a great buffering agent if you're starting with a neutral or acidic water source and from what I've read be beneficial as a system matures and tends towards acidicy with ammonia conversion.

But if the startup water is itself alkaline by nature wouldn't the added buffering of the limestone work against the bacterial growth as you try and cycle your system?

[GaryD]

Hi,

Nitrifiication peaks in the ph range 7.2 to 9.2. so I wouldn't have thought that high pH (of the sort that could be expected from using limestone gravel) would be too much of an issue.

I'd be much more concerned about ammonia toxicity associated with high pH.......10 times more toxic at 8.2 and 100 times more toxic at 9.2....than at the optimum aquaponics pH of 7.2. Of course, all of that assumes that live fish are being used in the cycling process.

If nitrogen dosing (fishless cycling) is being used then high pH doesn't matter....in fact it's preferable because it will allow colonisation of the beneficial bacteria at the optimum rate. Once the system has cycled, the pH can be adjusted down and the fish added.

Gary

[gavinl]

Hi Gary

Just wondering if the blue metal rock you are referring to is also called 'crusher dust'? If so, it is around $250 for a truck load (8-9 tonnes) up where I live. This would be a very good option. Because the soil where I live is pretty ordinary, we use it to plant grass instead of topsoil - heaps of nutrients. What size would you suggest for the rocks? Most of the stuff here is pretty fine - about 4-6mm. To wash it, do you plantput the gravel into the beds...let it cycle for a day or so, then get rid of the murky water and start the cycle again?

Thanks mate

gav

[Murray]

Hi Gavinl,
You need the grow bed medium to be fairly course. If it is too fine then it will block up after a while. I use 20mm drainage gravel. I have seen some folk use as small as 10mm gravel, but I would be worried about going any smaller than that.
There is no doubt that there are lots of minerals etc in crusher dust, but to use that only would lead to difficulty with poor water flows before long.
I have grow beds that are now almost 2 years old and they are just fine. The old roots from previous crops are quickly dealt with by the resident worms. There is not a hint of blocking.
I do not wash the gravel when I add another grow bed to my system. Recently I added 4 x 250 ltr grow beds to the main system. 20mm drainage gravel from the landscape supply place went straight in to them. The water in the fish tank went a bit murkey for a few days, but after a week...beautiful water again.

When first starting it took about 5 weeks for the water to clear. I guess it clears more quickly now because the system is much larger and able to absorb changes more easily.

I now have 4 x 585 ltr grow beds, 1 x 40 plant NFT table and 4 x 250 ltr grow beds running off my 2300 ltr fish tank.

[GaryD]

Hi Gavin,

Crusher dust is far too fine to be use as a growing medium.

As Murray suggests, gravel of 10mm - 20mm is the best option.

You don't need to wash blue metal before putting it into the beds.....the rock dust is very fine and it remains in suspension for several days. A quick water change will resolve the discolouration and your plants and trees (external to your system) will love you for it.

I've also used drainage gravel and that contains a lot of sand and clay. Given the choice, I'd prefer to wash it before placing it in the bed.

Gary

[dingorunner]

I was looking at starting with 1 or 2 bath tubs for grow beds to get started and yes I will have fish in the main tank from the sound of it I probably be better of using blue metal the same as what you are using.My main tank is about 2000 - 2500 ltr.All I need to get now is the plumbing sorted

[GaryD]

Hi Dingorunner,

Blue metal is good for use in Aquaponics systems but you'll need to sort the pH out before you put too many fish into the tank.

Blue metal (complete with dust) may cause your initial pH readings to be up around 9.0. You'll get that down to 8.0 if you flood and drain your beds a few times and then pump the water out onto your garden (you only need to have a few inches of water in the tank for this part of the operation). Your plants and trees will love the rock dust that is suspended in the water.

To get the pH down to the optimum pH 7.0 to 7.2, you can use diluted hydrochloric acid (always remembering to pour the acid into water to dilute it.....never pour water into acid). Add the diluted solution in small doses to gradually adjust the pH down.

We used lemon juice (about 500ml) to reduce the pH in about 800 litres of water.

Gary

[gavinl]

Thanks Gary and Murray.

That has cleared things up a fair bit.

gav