Backyard Aquaponics
A place to discuss aquaponics

hi I want to add magnesium to my system to try to increase the size of my chillies. I want to grow Hungarian sweet peppers, cultivation notes suggest higher magnesium increases fruit size. I was considering adding Epson salts but im unsure as the amount or wether it would be toxic. I have a 10000 lt f tank and 16 grow beds. Does anyone have any info they can share.

Molecular weight of MgSO4 = 120, of which Mg is 24, ie 1/5.
However, it is likely to be hydrated unless kept under hot dry air, so probably MgSO4 . 7H2O, so Mg will only be 1/10 of the weight of the crystals.

So, if you add 100g from the box to 10000l of water, Mg will be 1ppm, since it is very soluble.

As to what is an appropriate concentration, I don't know, but there will no doubt be some in the water already.

I'd be more inclined to use dolomite, to add Ca and Mg.

Thanks heaps I had a major fish kill when i first started into aquaponics so i am very wary of adding anything to my system. how do i find out the correct ppm to add? Dolomite sounds good where can i obtain some info regarding application and its effects on fish?

I've added Epsom salts to my system with no fish deaths

I know what you're saying though...I was so wary adding salt, lime etc etc it just didn't seem right but the fish and plants haven't seemed to mind my methods thus far!

what is the pH and hardness (KH) of your system ?

DLF,
not sure as im not home atm i will get my lovely assistant to take readings and post them soon. Other posts of read here recommend a foliar spray of sea sol.

just if you are running a higher pH or already have high Ca additives may be an issue
(in which case the foliar feed option).

Seasol will give you a basic dose but if you want extra mg then suspect you have to look at something specific.
Quite a few sites talk about simply watering it on/foliar for Tomatoes & Peppers. (Tablespoon per gallon)
Guess basically like adding potash to get a K boost.

If adding to system then becomes case of what is likely to be most efficient to apply - pH will factor in that.
(I tend to water onto grow bed and not add to water generally)

[edit] I have always had an issue with small fruit on capsicum so am interested in where this goes.
no issues getting fruit but they don't seem to get beyond 7-10cm.
The hydroponic guys must have something up their sleeves.

There is an important relationship between Magnesium, Calcium and Potassium, which must be taken into consideration if you intend on adding any one of the three in reasonable quantities. Adding too much of one will cause an imbalance in the system, making the other two less available to the plants.

All three are important for good fruit development, so adding a combination of them is probably the best approach, but I would wait until you've established what the system pH is before deciding in which form and ratios to add them.

I would also be very cautious about how much you add. If you don't have a deficiency in the system then only a very small amount would be required to boost the levels to a point where it may have a beneficial affect on the fruiting, but you will probably find it will be on the number of fruit the plant will develop, not the individual fruit size... This is where plant management techniques come into play.

I would avoid using Epsom salt, especially if you will be adding a reasonable amount, I have induced Sulphate toxicity in plants in my system, in a very short period of time, with the regular use of Espom salts. Guna has the right idea with the Dolomite.

I am currently adding a combination of Calcium carbonate, Calcium Magnesium carbonate, Potassium bicarbonate (Nate Storey would be getting light headed about now) and Calcium hydroxide. This is primarily for pH control while keeping all three elements in a decent ratio with each other... the excellent plant growth is just another benefit.

Mr Damage,

Would you say your ratios of those minerals are 65% Calcium, 15% Magnesium, 4% Potassium, and 1% to 5% Sodium
Of course some of that 65% Calcium would be in the form of Calcium Hydroxide to adjust the pH

The reason I am asking was that I was reading this article and started thinking how this can be applied to AP.

Dr. Albrecht came up with these ratios where the minerals are highly available for the plants to take up.

I have absolutely no idea... I long ago gave up hurting my brain with such stuff. Where it’s necessary to calculate such things I prefer to utilise other people’s grey matter these days.

As part of my horticultural apprenticeship I had to show how to calculate and apply such ratios and I later applied it a little in practice in my hydroponic pursuits. In hydroponics, especially on a larger scale, it is definitely applicable, but I've found that with aquaponics, using exact ratios is very impractical, for a number of reasons.

Hydroponics has very few variables, but the Nitrification process that exists in aquaponics throws in quite a few. As does the fact that in aquaponics we tend to grow a wide variety of plants and vegetables in the one system, whereas in hydroponics it tends to be mono-culture, or at the very least plants with very similar nutrient requirements.

With aquaponics I still observe certain nutrient ratios where required, or at least use them as a starting point, but I’ve found exact ratios very rarely work and need to be varied according to conditions, such as how much Nitrification and therefore carbonate depletion is taking place, what pH buffering and altering products are being added, as well as what sort of plants are being grown and what stage they are at in their development.

I have a friend that is a very knowledgeable chemist who specialises in developing and manufacturing hydroponic nutrients. A few years back I was having issues with the products I was using for pH buffering/control (ie: Potassium bicarbonate, Calcium hydroxide, Magnesium sulphate). He calculated a suggested ratio for me, which I used as a starting point, and which worked well for a while, but then I ran into Sulphur toxicity issues, so I halved the MgSO4 I was putting in, which worked for a while. I then ran into an Mg deficiency issue, probably attributable to a couple of issues, the reduced MgSO4 rate and a very large chilli plant that had started fruiting profusely, so I tweaked the rate once again.

The system ran well for a few more months, but then I ran into a Calcium deficiency issue due to a couple of large, healthy Silverbeet plants zapping it from the system, so had to adjust the ratios yet again. Around the same time I stopped using MgSO4 and started using a couple of other products, ie: Dolomite and plain old Calcium carbonate… By this point the combination of products I was using and my ratios were so far removed from his original one that it wasn’t funny.

I now run Calcium carbonate, Calcium Magnesium carbonate, Calcium hydroxide and Potassium bicarbonate. I’ve tweaked the ratios of these a couple of times in the last 12 months or so, but I’m currently running them at around a 4:2:2:1 ratio and it’s working very well... I’ll let you do the math on that ratio if you feel inclined.

It’s just been a matter of nutting out which products serve my purposes best and in what ratios. For example, at one point I tried Nate Storey’s recommended method of not using any carbonates to control pH, only hydroxides... The system was close to its bio-load limit at the time, consequently there was a lot of Nitrification taking place... and it only took 5 days, and an increasingly out of control system, for me to realise I had to go back to using carbonates as a matter of urgency. In fact I add a lot of carbonates now and it hasn’t affected the plant growth or fish health... What seems to work for him, definitely doesn’t work for me.

Long story short... For me anyway, there are no absolutes in aquaponics, especially in regards to nutrient ratios. There is no one ideal product, or combination of products, and there is no one ideal ratio of these products, it’s constantly changing.

It’s a matter of starting with a recommended ratio then watching your plants for indications that the ratio is working, or needs tweaking... I let my plants tell me what to add and when. If you keep a keen eye on your plants and react reasonably quickly to deficiency symptoms when they first start to show, you can maintain healthy plant growth.

Thanks very much for the explanation Mr Damage. That was realy helpfull.

Mr D, it would seem that you are adding a lot of calcium to your system. Are we talking a system located in Belmont or somewhere else ? I am in South Perth-Manning area so the scheme feed is not excessive in Ca and at present getting rain water anyway. That would presume you are running a slightly acidic system (pH in 0.65-0.75 range) with scope for Ca additives ?

How would your situation work if you had a scenario naturally high in Ca - which would be a lot of places say north of river ?

Is either Calcium Magnesium carbonate or Magnesium carbonate easy to obtain ?
Presume you may not want Ca in that situation.

We are concluding (from above posts which appears sound) that the Mg in epson salts is good but the SO4 is not.
And that carbonates are 'safe' because presumably their by-products are only C H and O (probably C goes to carbon dioxide in some form).

Also how do bicarbonates versus carbonates go ? Or carbonates simply because you get more Mg per unit mass.

[edit] can answer first bit because just realised dolomite is calcium magnesium carbonate, CaMg(CO3)2....
and Dolomite Lime is readily available in garden centres.

"Dolomitic lime is an attractive rock. It's calcium magnesium carbonate. It has about 50% calcium carbonate and 40% magnesium carbonate, giving approximately 22% calcium and at least 11% magnesium."

Again... With aquaponics I try and avoid unnecessary, head hurting, on-paper calculations thanks.

Everyone’s situation is going to be different, depending on source water, top-up rates, stocking levels, plants in the system, and products used etc. Anyone that claims they can do an on-paper calculation and come up with the perfect ratio has their hand on it IMO.

The system in question is in the shop in Belmont and I wouldn't have a clue about the Calcium content of the source water sorry. The Calcium being added may sound excessive, but I was having a hard time getting enough carbonates into the system and we are all somewhat limited to what products we can use. But the ratios are working well and the system obviously needs it, the plants are looking just about as healthy as I've ever had them.

The Dolomite I'm using claims to have a typical analysis of:

Calcium as Calcium carbonate = 14%
Magnesium as Magnesium carbonate = 8%

The straight Calcium carbonate I'm using is plain old crushed limestone, sold as Garden lime, with an analysis of:

Calcium as Calcium carbonate = 32%

But to me, those percentages, atomic weights, ratio calculations etc have very little relevance. I give my plants what they tell me they need and I try to work it in with, or around whatever products I'm using for pH buffering/control at the time. If my pH were fine I'd be more inclined to foliar feed for most nutrient deficiencies Bluefin, ie: To boost your Magnesium get a good quality Cal-Mag product from your local hydro shop and apply it to your chillies as a foliar spray once a week.

If you lived in the Northern suburbs of Perth Dlf, where I believe the scheme water is primarily ground water and possibly higher in Calcium, and you were only worried about pH, not a general Calcium deficiency like I've had in my system, then you may be able to do away with one or more of the Calcium based products, or even all of the products, including the Potassium bicarbonate, but again, it would come down to trial and error.

I live in the Northern suburbs and my kid’s little barrel system has never had any of those products added to it, and it’s nearly six years old. It’s only ever had Seasol added to it and the top-up water treated with hydrochloric acid occasionally. This is primarily due to the evaporation rate from the little system and the amount of top-up water required. On the other hand, I have a customer with a large system in the same suburb, who I know is having to add pH buffering and raising products, especially right now with little evaporation happening and his trout getting quite large.

Wow Mr D they are great looking veg

Thanks Bluefin, the system is in the front of the shop, so I have to try and keep the plants as healthy and presentable as possible.