Notes on watering a mixture of rain water and tap.

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garysan

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Estimated TDS (126ppm) from EC is not really the actual TDS, so it is just a rough approximation without knowing how many ml of OrchidFocus per liter is used (it is difficult to understand what "half" strength means as Ray always points out).

For the record and not sure if relevant as most of what naoki said in above post went straight over my head at 01:15 this morning but I'm using 5ml per 4l of water (half strength according to OrchidFocus label).
 

gonewild

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Sorry, Lance, I didn't mean to be cryptic, but it does make a difference. In the liquid form, you are forgetting that water is already in the stock solution (water isn't considered as "solid" in Total Dissolved Solids).

You are right I forgot to consider the water weight for the 2.2% number.
Without knowing the actual density of the solution its not accurate. But definitely higher than I said and still a very low nutrient level.
 

Brabantia

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Just a remark about measure of salts content base on TDS.
The value of TDS is based on a conductivity measure using a coefficient for the conversion. This coefficient is variable from one manufacturer to another.
Conductivity measure is based on instrument calibration using a solution of a salt which is never present in our fertilyser solution ( normally KCl 0.005M = 706.5 µS). The most accurate method to state the real conductivity of a fertilyser solution is to take into account the molar conductivity of each salt present really in solution and adding each values of conductivity in fonction of there concentrations. No body do that but it exist computer programs which do that. Usually the values calculated are lower than the values measured by the conductivity meter.
 

naoki

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For the record and not sure if relevant as most of what naoki said in above post went straight over my head at 01:15 this morning but I'm using 5ml per 4l of water (half strength according to OrchidFocus label).

Sorry... The dilution thing can be confusing, and I also get confused easily, too. But now we know your dilution; you are giving 27.47ppmN.
 

gonewild

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Just a remark about measure of salts content base on TDS.
The value of TDS is based on a conductivity measure using a coefficient for the conversion. This coefficient is variable from one manufacturer to another.
Conductivity measure is based on instrument calibration using a solution of a salt which is never present in our fertilyser solution ( normally KCl 0.005M = 706.5 µS). The most accurate method to state the real conductivity of a fertilyser solution is to take into account the molar conductivity of each salt present really in solution and adding each values of conductivity in fonction of there concentrations. No body do that but it exist computer programs which do that. Usually the values calculated are lower than the values measured by the conductivity meter.

Yes that is very true. I just use the meter to make sure the salt content is close to where it should be. Mostly to watch for drastic changes to warn of a potential problem. An exact TDS reading is not needed for our use because we don't even really know exactly what TDS is best for orchids. We don't even know within 100ppm how much nitrogen should be in fertilizer solutions!
What's correct 125ppm or 25ppm? It's all just reference points and should be used as a guiding tool and not as an unbreakable law.

In the post above I made a math error and said the N content was really low. If I had mixed up the fertilizer thinking that and then checked the final mix with a TDS meter it would have alerted me about my calculation error. because my error was off by 10x. The TDS reading would have told me I was actually mixing a stronger dosage than my calculator told me I was. :eek:
Checks and balances!
 

myxodex

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I mix tap water with rain water routinely.

When I first started doing this I was using a fert that had 25% N as NH4 and no Ca so a source of Ca was necessary. But there is another thing going on here that depends on the fert and particularly how much NH4 you are feeding. At the time I was getting acidification problems and initially I was correcting this with powdered dolomite ... then I switched to tap water because it was easier to control pH without too much alkalinisation. That was a long time ago.

I now make up my own feed which is designed to accomodate a partial addition of my tap water. There are some issues with this and I can use my own strategy as an example to explain.

Without and with tap water I would be adding in ppm (aaN is total N from amino acids) ... the second row is with tap water (4% of total volume).

Ntot - 18.7, NH4 - 2.5, aaN - 3.1, NO3 - 13.1, P2O5 - 4.0, K2O - 6.7, Ca - 7.1, Mg - 4.9, SO3 - 0.9
18.9, 2.5, 3.1, 13.3, 4.4, 6.8, 12.1, 5.1, 2.8

So the big additions from my tap water (London) are Ca and S, so you need to check your local water analysis... mine has 120 ppm Ca but only 4 ppm Mg. So without tap water my fert Ca/Mg ratio is too low (by design). The tap water is also providing 11 ppm HCO3 which is 4.4 X the NH4 and so just enough to balance the acidification from this. If I increase the feeding I also increase the tap water in proportion, ... up to a limit... usually not more than 2.5 X the above, because my tap water also contains Na, Al, Cl, and I don't want these increased too much. So there are issues with adding tap water which can only be known from an analysis. So I agree with JAB's post above.

For a few years I used an all NO3 feed and no tap water, this was sort of OK but not all my plants were happy, ... and I noticed a significant improvement in going back to the mixture of rain and tap water and a NH4/NO3 feed despite the fact that I'm only using a small amount of tap water. So I wonder whether the improvement is all about pH and hence HCO3, or using a mixture of NH4/NO3, or whether Na and/or Cl are beneficial in small quantities or even the extra S ? All I am sure about is that the addition of tap water has improved things for me and provides a stable and flexible system for pH management. So for example with the limestone paphs I can increase the tap water % and with the barbata types I can decrease it a bit ... but I would not use any tap water with an all NO3 fert, especially in hard water areas e.g. south east UK, because in the absence of some acidification from NH4 the higher pH and Ca will just encourage scale build up in the long term.

My rain water, based on figures for the UK generally, contains at least a few ppm N and S and some of the other nutrients ... so there is a significant difference between using RO and rain water particularly if you're feeding at low rates. So for the figures above I've not included
 

myxodex

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Damn, I tried to make a little table of the figure but I haven't cracked how to get decent formatting ?
 

gego

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myxodex, thanks for sharing that very interesting information. I may one day have my fert solution analyzed. I have water info from our local supply here and looks like yours is very similar to ours.

Looking at your numbers, the increase in available Ca is very significant, 7.1 to 12.1 ppm, about 70%. Consider that the Ca from your tap is coming from calcium carbonate, which is not readily soluble. A lil bit of acidity may help in freeing those Ca and maybe the result of adding NH4.

Just like yours, Na and Cl are present in high amount so I had the same question.

I experimented making a solution close to the available calcium carbonate of my tap by dissolving calcium carbonate from shells in RO. I then add the same ratio as adding tap.

I can't see any difference in result yet but maybe because the amount of other stuff we add back are just not significant.

But I would say that the increase in Ca in your case means a lot.

It means lot in mine.
 

myxodex

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myxodex, thanks for sharing that very interesting information. I may one day have my fert solution analyzed. I have water info from our local supply here and looks like yours is very similar to ours.

Looking at your numbers, the increase in available Ca is very significant, 7.1 to 12.1 ppm, about 70%. Consider that the Ca from your tap is coming from calcium carbonate, which is not readily soluble. A lil bit of acidity may help in freeing those Ca and maybe the result of adding NH4.

Just like yours, Na and Cl are present in high amount so I had the same question.

I experimented making a solution close to the available calcium carbonate of my tap by dissolving calcium carbonate from shells in RO. I then add the same ratio as adding tap.

I can't see any difference in result yet but maybe because the amount of other stuff we add back are just not significant.

But I would say that the increase in Ca in your case means a lot.

It means lot in mine.

Just a little clarification about the Ca. Both Ca and Mg can exist as bicarbonates ( HCO3 ) in solution. My tap water has 120 ppm Ca of which a proportion (up to 74 % max) will be as bicarbonate (total HCO3 is 264 ppm). Unlike K, Na and NH4, all of which have highly soluble bicarbonates that also exist in solid crystalline form, Ca and Mg bicarbonates only exist in solution. If a solution of Ca bicarbonate is evaporated the residue will be CaCO3 (one CO2 will be lost). This is why so often hardness in water is expressed as CaCO3 in mg/l even though CaCO3 is insoluble.

There are some folk out there that have started a health fad, their claim is that Mg bicarbonate is a wonderous magical substance that will transform your life. There are recipes for making it ... essentially you can take either MgCO3 or Mg(OH)2 (milk of magnesia) as a finely ground powder suspended in cold water and bubble CO2 through the water (dry ice would do the trick). The carbonic acid formed will slowly dissolve the carbonates by converting them to bicarbonates (at least up to saturation). This will work with CaCO3 as well but it is a little slower.
 

gego

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Thank you. You are right about the bicarbonate. But I did mention carbonates and as the water evaporates (fast if ventilation is ideal) what's left is carbonates. So that Ca may not be available for so long. And pH shoots up.

But back to what I'm implying, your increase of Ca available to the plant is what I believe makes the difference. I have proven it for myself and have heard from other growers/hobbyist here in this forum.

In fact you can hear/read calcium nitrate in a sentence here all the time. Even the famous K-lite, if you look at it in a different angle is the availability of Ca and its ratio to the other cations. I don't like to name names but there's one that only adds calcium nitrate to his tap water. We look at N all the time, even when we make our solution.

Just my two cents.
 
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