# Notes on watering a mixture of rain water and tap.



## garysan (Aug 6, 2016)

Read a couple of articles that mention mixing a percentage of tap/city water with rain or RO water in order to get a more balanced watering (adding back calcium and/or magnesium to 'pure' rain/RO water). I'd be curious to hear of people's thoughts and/or recepies for such while I get my new regime for watering and feeding dialled in properly. Thanks


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## Ozpaph (Aug 6, 2016)

Rainwater + fertilizer, done.


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## abax (Aug 6, 2016)

The mixture depends on what type of plant I'm watering.
For Phrag. species I use rain water and for Paphs. and
species Phals. I use tap primarily. Fertilize once a week
with K-Lite and tap.


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## Stone (Aug 7, 2016)

You could use straight tap water and possibly adjust the pH if necessary.
You could mix rain and tap to get your Ca.
You could use straight rain water provided it was clean and your fertilizer contained sufficient Ca S and Mg.
You could use straight rain water and add dolomite or lime to your mix.

All these are done and all are successful.


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## Ray (Aug 7, 2016)

If your fertilizer contains calcium and magnesium, why would you need to add more via tap water?


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## JAB (Aug 7, 2016)

I suppose it is human to try and make a blanket, easy statement about one thing versus another. Their are a number of factors that should go into such a decision. For instance rainwater is great... unless you live down wind from a processing plant, or in a place like say... Beijing! Tap water varies even within jurisdictions (look at Flint). 
I think the best approach is to get a PPM/PH meter and constantly test the various water you are going to use. Supplement where needed but not more so. 

Just my two cents
JAB


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## garysan (Aug 7, 2016)

Worth measuring so here we are:

My rain water is currently PH: 6.8 and TDS: 13
With OrchidFocus fert added at 1/2 strength it changes to: PH: 5.9 & TDS: 139

Consequently, my tap water is PH: 7.2 & TDS:366.



JAB said:


> I suppose it is human to try and make a blanket, easy statement about one thing versus another. Their are a number of factors that should go into such a decision. For instance rainwater is great... unless you live down wind from a processing plant, or in a place like say... Beijing! Tap water varies even within jurisdictions (look at Flint).
> I think the best approach is to get a PPM/PH meter and constantly test the various water you are going to use. Supplement where needed but not more so.
> 
> Just my two cents
> JAB


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## phraggy (Aug 7, 2016)

Hi Gary. Your rainwater plus fert is absolutely perfect . If you have enough rain water I wouldn't consider using water from the tap. I use RO water plus Akernes Rain Mix and the day after I leach with RO plus a little Seaweed extract, I water with this regime every day, I can do this because all my multis are on heated sandbeds ( kept moist ) with strong air movement and planted in a very loose bark and perlite mis and also planted in baskets. I water in the morning and they are almost dry the following day
Ed


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## gonewild (Aug 7, 2016)

garysan said:


> Worth measuring so here we are:
> 
> My rain water is currently PH: 6.8 and TDS: 13
> With OrchidFocus fert added at 1/2 strength it changes to: PH: 5.9 & TDS: 139
> ...



Do you know the content of the OrchidFocus fertilizer?
It is N2.2 : P1.3 : K2.1 : 2.5Ca
Very dilute fertilizer, when you purchase you pay mostly for water.

The calcium content is greater than the nitrogen content. Based on that alone there is no need to add tap water.

2.2 : 1.3 : 2.1 is the same NPK ratio as 22:13:21 in a standard blend fertilizer.

When you apply at your half strength you are adding 126ppm of fertilizer.
So 126 x 2.2% = 2.7ppm nitrogen supplied when you fertilize your plants.
That is basically nothing. 
OrchidFocus does not even have enough magnesium to give it an position on the label.

What all this leads to is that they use tap water to make the liquid they sell and whatever is in their tap water makes up the bulk of the minerals you are applying. 

The answer to your question is don't add tap water.
The reality is that the OrchidFocus is so weak it probably does not matter if you use RO or tap water or a mix of both.


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## C. Rothschild (Aug 7, 2016)

I have heard that rainwater's the best but realistically whatever bucket or however you collect it in will have dust or residue in it. Might not matter though. If you do mist your plants I'd say rainwater.


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## Redtwist (Aug 8, 2016)

gonewild said:


> Do you know the content of the OrchidFocus fertilizer?
> 
> What all this leads to is that they use tap water to make the liquid they sell and whatever is in their tap water makes up the bulk of the minerals you are applying.
> 
> ...



This is interesting. I don't usually get too scientific about the water, but as I use OrchidFocus, I now wonder if it is the rain/tap water content and occasional epsom salts/dolomite lime that is feeding the plants, rather than the feed itself.
(At least they seem to be doing ok at the moment, whatever it is!)


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## gonewild (Aug 8, 2016)

Redtwist said:


> This is interesting. I don't usually get too scientific about the water, but as I use OrchidFocus, I now wonder if it is the rain/tap water content and occasional epsom salts/dolomite lime that is feeding the plants, rather than the feed itself.
> (At least they seem to be doing ok at the moment, whatever it is!)



It's probably a combination of both. The rain water may have some nitrates in it too.
It's only my opinion but in a situation like yours where the plants grow well with so little nutrients I think there is a high population of nutrient producing living organisms. The organisms decline as certain nutrient salts increase.

The OrchidFocus has a high calcium content which likely means the nitrogen source is from calcium nitrate. It also has no UREA which for me is a good thing. 

Your question was about mixing tap and rainwater which I gave you my opinion about. If you are thinking of ways to improve growth your first step could be to simply increase the strength you apply the OrchidFocus at.


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## Redtwist (Aug 8, 2016)

Hi Lance
I think you are mixing up myself and garysan, who started this thread. I was just commenting because I also use OrchidFocus. But thanks for the info - its all 'food' for thought! oke:


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## gonewild (Aug 8, 2016)

Redtwist said:


> Hi Lance
> I think you are mixing up myself and garysan, who started this thread. I was just commenting because I also use OrchidFocus. But thanks for the info - its all 'food' for thought! oke:



Are you sure? :wink:


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## Redtwist (Aug 8, 2016)

'I was confused, but now I just dont know.' :wink:
Chris


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## garysan (Aug 8, 2016)

All information is good. I'm getting really good root-growth and new growth from both my paphs and phrags. I just want to do the best job by my plants and wondered if there was room for improvement. My paphs all seem very happy and lots of them are/have flowered. Some of the phrags though are still licking their wounds from whatever was going on when I used the RainMix previously; others seem unaffected. I'll stick to what I'm doing currently - if it ain't broke....




gonewild said:


> It's probably a combination of both. The rain water may have some nitrates in it too.
> It's only my opinion but in a situation like yours where the plants grow well with so little nutrients I think there is a high population of nutrient producing living organisms. The organisms decline as certain nutrient salts increase.
> 
> The OrchidFocus has a high calcium content which likely means the nitrogen source is from calcium nitrate. It also has no UREA which for me is a good thing.
> ...


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## naoki (Aug 9, 2016)

gonewild said:


> When you apply at your half strength you are adding 126ppm of fertilizer.
> So 126 x 2.2% = 2.7ppm nitrogen supplied when you fertilize your plants.
> That is basically nothing.
> OrchidFocus does not even have enough magnesium to give it an position on the label.



I think this fertilizer is liquid, so your calculation is not right, Lance. oke:


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## gonewild (Aug 9, 2016)

naoki said:


> I think this fertilizer is liquid, so your calculation is not right, Lance. oke:



Why? If it's wrong correct it please.

garysan increased the ppm of his irrigation water by 126ppm by adding nutrient salts.
All of the increased PPMs came from the fertilizer solution.
According to the fertilizer label the salts he added contained 2.2% N.
Therefore 2.2% of the PPM increase is nitrogen.

Makes no difference to the math whether the source of PPMs is liquid or dry.
Does it?


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## consettbay2003 (Aug 9, 2016)

Personally I would not use a fertilizer that did not contain a decent amount of ammonium or urea as a source of nitrogen.


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## naoki (Aug 9, 2016)

gonewild said:


> Why? If it's wrong correct it please.
> 
> garysan increased the ppm of his irrigation water by 126ppm by adding nutrient salts.
> All of the increased PPMs came from the fertilizer solution.
> ...



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).

Maybe an example may help. For simplicity, let's say that we start from 22% N solid fertilizer. You put 100g of this and fill it up to a liter of water. Now this is the stock "liquid" fertilizer with 2.2% N (I'm using weight/volume % here). But when you only look at the solids in this stock, 22% of "solid weight" is N (not 2.2%). So you should have done 126ppm * 0.22 (=22%) = 27ppm. But we don't know what was the original composition. So this is just an approximation.

Or here is alternative way to think. TDS of of this hypothetical stock (with 2.2%N) is 100,000 ppm. So to get TDS of 126ppm, you need 793x fold dilution. If you dilute 2.2%N with 793x dilution, you get 22000ppm (=2.2%N)/793 = 27ppmN. 

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).


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## garysan (Aug 9, 2016)

naoki said:


> 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).


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## gonewild (Aug 9, 2016)

naoki said:


> 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.


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## Brabantia (Aug 10, 2016)

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.


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## naoki (Aug 10, 2016)

garysan said:


> 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.


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## gonewild (Aug 10, 2016)

Brabantia said:


> 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. 
Checks and balances!


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## myxodex (Aug 10, 2016)

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


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## myxodex (Aug 10, 2016)

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


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## gego (Aug 10, 2016)

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.


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## myxodex (Aug 11, 2016)

gego said:


> 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 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.


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## Ray (Aug 11, 2016)

gego said:


> Consider that the Ca from your tap is coming from calcium carbonate, which is not readily soluble.




If it's in the tap water, it obviously IS soluble...


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## gego (Aug 11, 2016)

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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