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PostPosted: Nov 2nd, '18, 17:16 
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How much Calcium, potassium and magnesium needs to be added when PH is low ?


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PostPosted: Nov 2nd, '18, 17:24 
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Depends on your water. For most systems you don't need to add any calcium or magnesium because there is enough in the water. Potassium might be needed so look for signs of deficiency which are normally seen as yellowing margins on the older leaves of the plant.


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PostPosted: Nov 2nd, '18, 19:33 
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I've been following Mr Damage's recipe at viewtopic.php?f=1&t=23594 for a few years.

Have a look at his posts toward the top of the thread .

I'm no chemist or agricultural scientist. Don't fully understand all of this stuff... but it seems to work pretty well for me.

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PostPosted: Nov 2nd, '18, 20:05 
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I've tipped over 100kg of K2CO3, KOH, Ca(OH)2 and dolomite into my big system in the past few years, plus a few kg of shell grit, all to keep the pH in the 6s, so I guess all those hundreds of trout have been busy making lots of ammonia for the various bacteria to work on ;)

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PostPosted: Nov 3rd, '18, 15:41 
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There's been a lot of water under the bridge since I posted in that thread Dave linked to and I've changed the way I buffer/raise pH (Don't change if it's working for you Dave). I did a fair bit of playing around with the ratios and even with the products used, and have been using a slightly different formula and range of products for a few years now, which I've found works really well in the majority of situations, bearing in mind that water parameters will vary for different systems, source water types, seasons, crops.

I stopped using the Magnesium sulphate (Espom salts), as my lettuce in particular started to show signs of what I came to believe was Sulphate toxicity, which I'd never seen before, so actually took a while to figure out.

I now use the following products: Calcium carbonate (Garden Lime), Calcium Magnesium carbonate (Dolomite Lime), Calcium hydroxide (Brickies Lime) and Potassium bicarbonate... in a 6:2:2:1 ratio. The ratio is by volume, not weight and I use proper measuring spoons.

All of the carbonate based products are added first and I add them directly into the water, so into your FT or ST etc. I give them about half an hour, then I add the Calcium hydroxide (pre-mixed in a cup etc) into the GB at the start of a new flood cycle, so it has time to mix with a good amount of water and dilute down before it enters the FT/ST.

The Calcium carbonate, which is basically crushed limestone, looks just like beach sand and will settle to bottom of your FT/ST and build up over time, so a dosing tank plumbed into the system somewhere is on the cards for my next system. All the other products dissolve, although some components of the Dolomite lime do take time.

I use this mixture of products for a couple of reasons, the first being that they help keep the Ca-Mg-K ratio in the ball park. The second being, they release their carbonates at varying rates. The Potassium bicarbonate gives an almost immediate effect, the Dolomite lime takes a little longer, and the Garden lime (crushed limestone) is slow release, so gives a lasting effect. I use the Garden lime because I found adding lumps of limestone to be ineffective, the same with shell grit etc. They do have an effect when first added, but within a short period they became ineffective.

As I said earlier, that ratio has proven to be effective in the majority of situations, but there have still been occasions where my plants have started showing symptoms of Calcium, Magnesium or Potassium deficiency, usually crop related, and I've had to temporarily adjust the ratio accordingly. Once the deficiencies are sorted I've reverted back to the original ratio. This is why it's a good idea for AP'ers to familiarise themselves with the most common deficiencies in AP, which tend to be: Iron (mostly in new systems with a pH of 8+), Calcium, Magnesium and Potassium.

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PostPosted: Nov 3rd, '18, 15:52 
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In regards to the original question of how much... It will vary, so use the ratio and start small, figure out how much your system requires.

When the system pH starts it's natural decline, if you can catch it at around 6.7-6.8 and just maintain it there, it will require very little in the way of inputs. If you let it get all the way down into the low 6's before you act, then it will require much more, especially bearing mind that you shouldn't really raise your pH by more than about 0.3 in any one 24hr period.

If you let the pH, and therefore the carbonate level, get too low before you act, you could raise it by the max recommended 0.3 and find by the next day it has dropped by the same amount, or close to it, so you'll have to play catch up for a few days, or even weeks.

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PostPosted: Nov 3rd, '18, 15:57 
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The old recipe does the trick for my pH.

Plants seem OK - I occasionally do a foliar trace element spray too. Perhaps overkill... like humans taking high doses of vitamin C and the other water soluble vitamins

I'll be googling sulphate toxicity shortly. Will know what to do if I have it.

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PostPosted: Feb 21st, '19, 18:24 
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Thanks Mr D,
I allowed pH to drop to minimum measured by API kit so probably below six. I found adjustments were for 24 hrs at best (as you pointed out above) and I have been struggling to get it to hold above 6.2 or 6.3.
I was using garden lime and shell grit, but will now follow your ratio above.
Exactly what I was looking for.


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PostPosted: Feb 21st, '19, 20:14 
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I think it totally depends upon the water.

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PostPosted: Feb 23rd, '19, 13:37 
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+1

Yep, there are no perfect ratios that work for everyone. I don't think anyone here thinks otherwise, this is a starting point to go from and figure what your system needs. The main thing is watch your pH and watch the plants for deficiencies and you'll get it figured.


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PostPosted: Feb 23rd, '19, 17:49 
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3000L x 5 IBC growbed system , during our warm / stinking hot seasons (now)(fish eating heaps) I use 4 parts lime 2 parts dolomite 2 parts calcium hydroxide 1 part potassium , I mix up enough for 4 weeks , added to the mix is 4 teaspoons EDDHA Iron @ 1 cup of mix a week keeps my system at about 6.5ph

So far this has worked for me with a mix of tomatoes , cucumbers and sweet chillis and a bunch of corn to use up nitrates .

During our colder seasons (fish don't eat much) generally only need potassium and iron , our top up water is usually around 7ph.

Just have to find out what works in each system

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