Growing with Hard Water

A lot of growers have to adapt to hard tap or well water. I had to grow with well water on a limestone aquifer for years. We discussed a couple solutions to make it work successfully:

  1. Use a water softener → Reverse osmosis unit. I still change my filters every 3 months, but this ensures clean water for plants. If an RO unit is too slow or expensive, sodium from the water softener can go up to ~40 ppm without harming plants. Small amounts of sodium work positively as an electrolyte in plants and are distinctive in cannabis when “ocean grown” by the coast.

  2. Get a water test and use the calcium, magnesium and iron to your advantage. If you use Hydrobuddy, calculate the minerals and phosphoric acid as part of your fertilizer formulation. The pH in phosphoric acid makes the calcium and magnesium carbonates in hard water available to plants. If the pH still drifts up too quickly after phosphoric adjustment, citric acid works as a weak acid buffer, and is also a nutrient complexor (but not a chelator like AGT-50 Fulvic Acid) as a bonus.

How do you deal with water quality in the garden?

(Thanks, @Sermac!)


You can get a de-ionizer cartridge for your RO and get to 0 ppm TDS too…


Electrodeposition is a great way to clean up hard water for a grow if you have enough reservoir. Combined with a carbon filter and UV and your just about as clean as RO with no membrane cost or waste water.


When I was a good ol’ boy in Indiana, the water was 50 ppm out of the tap. I dropped fertilizer in and the pH was in range every time and stayed rock solid. Although it was a super-crime, that was one thing I never had to worry about.

Are you adding that calcium and magnesium back without spiking the N too badly like with hydro store cal-mag products?


I never did, and never had an issue. I did saltwater aquariums and had the water system for that, and just used it for a hobby grow a couple of cycles though.

My roomie had serious pH issues on tap that of course went away instantly with RO/DI.


I need to look into something to increase longevity on these RO membranes…. Replacing the filters on my hydrologic evolution just ran me $425.


I had to buy a RO skid from Hydrologic. 7500 gallon per day system. If you contact them directly and forward them a water test they should give you some options for pre treatment to give the membeanes longer life. Then buy the stuff from a regular RO/ plumbing shop to save money because hydrologic taxes like all hell on their commercial equipment.


Mine is 20-30 ppm right now in neighboring Illinois.


Thanks for the info @AgTonik . Currently it corns out at 350ppm out of the tap. Super hard water as I’m in limestone country. It works for great mash water and my dog and I love to drink it but my plants seem to not like it at all with the overload of calcium.


I love the water at the new place, better than most bottled water.



I would be very wary of feeding 40 ppm Na or recommending others do so without an obligatory YMMV warning: use at your own risk. While that can work fine for most crops in a greenhouse or outdoors, the way we push cannabis with very high PPFD and DLI, >75’F air temp, and frequent irrigation means 40 ppm Na is too much IMVHO. Otherwise, use 20-25% leachate run-off based on total daily irrigation volume.

I shoot for 0 ppm Na, but because that’s impossible with the grades of salts we use, I ensure Na stays below 1-2 ppm.

Also, note that Na acts as an elemental substitute for K, and Na can interfere with K uptake and xylem NO3-dependent systems:

Hydroponics: A Practical Guide for the Soilless Grower

There is considerable evidence that some nonessential elements can partially substitute for an essential element, such as Na for K, Rb for K, Sr for Ca, and V for Mo. These partial substitutions may be beneficial to plants in situations where an essential element is at a marginally sufficient concentration.


Coordinated Transport of Nitrate, Potassium, and Sodium

Sodium is taken up by many K+ transporters, and a large proportion of Na+ ions accumulated in shoots appear to be loaded into the xylem by systems that show nitrate dependence. Thus, an adequate supply of mineral nutrients is paramount to reduce the noxious effects of salts and to sustain crop productivity under salt stress. In this review, we will focus on recent research unraveling the mechanisms that coordinate the K±NO3–; Na±NO3–, and K±Na+ transports, and the regulators controlling their uptake and allocation.


I’m surprised no one mentioned UNH’s great AlkCal.

This tool has been available for 13 years online and 27 years as a spreadsheet. It will calculate the amount of acid (sulfuric, phosphoric, and nitric) needed to neutralize your alkalinity to a target pH or alkalinity. Where ya’ll been? LOL :wink:

All you need is your municipality water quality report to find your water alkalinity (often termed ‘total hardness’ as CaCO3 or HCO3). To find your municipality’s water quality report enter your city into Google plus, “water quality report.” If you don’t find the report, call your water treatment plant and ask them to email it to you. But sending a sample to a lab for water analysis is best practice. You have to send a sample for water analysis if you’re on a well. However, it’s best to send a sample for analysis every three months because alkalinity changes throughout the year (especially for well water).

RO water has zero alkalinity. But you can irrigate with alkalinity anywhere from 0 ppm to upwards of 40 ppm (I would keep it to 60 ppm, max). Soft water is 0-60 ppm CaCO3. So, if you have hard water, you don’t have to remove all the hardness. Keeping some alkalinity will help buffer against downward pH drift.

Thanks to @danielfp, using HydroBuddy is a great way to go if you formulate your own nutrient solutions because it accounts for the alkalinity causing maco nutrients (CaCO3, MgCO3, etc.). But if a grower is buying a commercial product (like Athena), there’s no need to use HydroBuddy. Just follow AlkCalc’s dosage using 35% or 50% sulfuric acid (diluted from 40% here or 98% here), and you’re done. :slight_smile:

A nice thing about AlkCalc is you can use it for in-line injection, so your water is already neutralized by the time it hits your storage tank.

Here’s an example using city water with a total hardness of 234 as CaCO3 ppm:

Note that total hardness includes a few different ions, so even though it’s listed as ppm CaCO3, you can assume it includes Ca, Mg, etc. Greater than 120 ppm CaCO3 is considered hard water. So, this example is very hard water; therefore, I’m assuming the pH is 9.0.

I set a target of 40 ppm CaCO3, which is soft water, and if the 234 ppm CaCO3 all calcium hardness, that means I have ~93.7 mg/L Ca. When using 35% sulfuric acid I would add 1.64 mL/gal, providing 62.72 mg/L S (sulfate). Note the final pH drops to 5.69 after neutralizing 194 ppm of CaCO3 (234-40 = 194):

Also, if you’re on a well, check the Fe concentration. To remove the Fe through oxidation, you can add chlorine using calcium hypochlorite, which is technically a mix of hypochlorous acid and hypochlorite depending on pH, with free chlorine at 1.2 times the Fe concentration (ppm). Then run the water through a greensand filter to remove the insoluble Fe. A bonus is that free chlorine will regenerate your greensand in the filter. Then, measure the free chlorine in the water with a chlorine test strip. Followed by adding ascorbic acid (vitamin C) at 2.5 times the free chlorine concentration (ppm) to neutralize the chlorine. A bonus is ascorbic will reduce the pH by a few points. Finally, test the pH and adjust it to below 7 as required using sulfuric acid (so you’re not adding P or N). Now, your water is ready for nutrients and final pH adjustment!


  1. Instead of using ascorbic acid for dechlorination, you can run the water through an activated carbon filter. Make sure to test for free chlorine (using test strips) afterward.
  2. If you’re using city water with chloramine (more potent than chlorine), you can use ascorbic acid to dechlorinate. Use the same dosage as used for chlorine.
  3. Don’t feed an RO filter water that has chloramines unless you want to replace your RO membranes a lot. Make sure you pretreat city water with chloramines by running through a filter designed for chloramines before your RO filter.

Yeah, de-ionization is excellent for lab work but not scalable or ecologically friendly for irrigation purposes. With a HyperLogic commercial RO system, the permeate is 2-3 ppm TDS without de-ionization.

Have you checked your city water quality report? If your municipality is adding chloramines, that’s your problem. Unless you’re filtering an insane amount of water, lol. But, in either case, installing a PreEvolution will extend your RO membrane lifetime.

Also, if you’re running cold water through your RO filter, you’re getting around 40-50% of the rated permeate flow rate (as GPD). Especially if your water pressure is on the lower end. However, if your feed water is 75-78’F at 50-60 PSI, you will achieve the rated GPD (permeate flow rate). That should help extend your RO membrane lifetime as well.

Are you using a PreEvolution before the Evolution? If not, install one yesterday. And swap out the Pleated Sediment Filter and the KDF85/55 catalytic carbon filter with two ChloraShield filters.

If your municipality isn’t adding chloramines, replace the sediment filter with the KDF85/55 catalytic carbon filter and add a ChloraShiled filter as the second filter.

You could even use two PreEvolutions before the Evolution RO fitler. Check out the Hydrologic website for install options, pumps, etc.


Do you have a water quality report? It sounds like you’re on a well. Is that 350 ppm as total hardness defined as CaCO3?

Here’s a ballpark solution based on an assumption of pH 9.5 at a total hardness of 350 ppm CaCO3 (6.99 meq/L alkalinity), with a target hardness of 50 ppm CaCO3 (1 meq/L alkalinity) using 35% sulfuric acid:

1.) Prepare 1 L of 35% sulfuric acid by diluting 40% sulfuric acid:

  1. Add 125 mL distilled water to a beaker
  2. Add 875 ml of 40% sulfuric acid
  3. Stir well
  4. Bottle in HDPE container

2.) Neutralize 300 ppm total hardness (CaCO3; 350-50 = 300):

  1. Add x gallons of water to a pale, drum, etc.
  2. Add 2.533 ml of the 35% sulfuric acid per gallon
  3. Mix

3.) Done.
Your very hard water (350 ppm CaCO3) is now soft (50 ppm CaCO3) with a pH of 5.64 (from an assumed starting pH of 9.5) and 96.88 mg/L of S (sulfate) added. Assuming all the hardness is from calcium (which I doubt), your water has 140.154 mg/L calcium (Ca).

4.) Add your fertilizers.
But, as you can see, with ~140 mg/L Ca, you won’t need to add much calcium. Assuming you’re running 180 mg/L N, with an N:Ca ratio of 1:1.1, you’re only adding an extra 58 mg/L Ca. Therefore, you could rely on NH4SO4, MKP, PeKacid, Haifa GrowClean, KNO3, K2SO4, KOH, MgNO3, and MgSO4 for your ammonium, nitrate, phosphate, polyphosphate, potassium, magnesium, and pH up; using CaNO3 and/or gypsum for the 58 mg/L Ca. No need to add sulfate salt for S because your acidification provides all the S you need (and then some), but you can use NH4SO4 and CaSO4·2H2O (gypsum) as noted for NH4 and Ca.



I published a blog post a couple of weeks ago about this exact issue, using the water from the city of Valencia, in Spain, as an example to create a one-part generic formulation for hydroponics. You can read more about this here :

As a general rule of thumb I have found that neutralizing around half the total alkalinity yields results that put you inside the pH range that is suitable in hydroponics. Provided your Na, B, Cl and heavy metals are low enough and your S high enough (S>25ppm) then these inputs can allow you to create pretty good one-part formulations for soilless growing of most plants.

Often compromises are needed - you are unable to run lower Ca or Mg than your input water allows you to - and your K will often be quite high to obtain enough N. Your P will also be controlled by the pH adjustment required (unless you have access to nitric acid).

But if that means avoiding all the energy costs associated with RO and injecting just one solution - very simple process - then that’s a huge plus I believe.


Good morning and thanks for this wonderful information. Yes I am on a well, I have not had a water analysis done as all of my funds have been spent moving but my neighbors both have about the same PPM coming out of their Wells and they say that their tests came back pretty hard with calcium and magnesium so I was just going off of their info . I was thinking of just going the route of RO since I don’t have to worry about chlorine messing up the membranes and I am on a septic tank with no regulations where I live so I can basically run that gray water down the hill off of the RO and not fill my septic tank up with that (we allow lagoons here) .

Edit: once I obtain a water analysis I am fully open to chatting with you on formulating something that would work in my favor to help not waste nutients!

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@Ralf You are absolutely right about sodium. I should have said plants “tolerate” more than trace amounts of sodium. I definitely wasn’t promoting using that much. Thank you again for the information resources like AlkCalc.

@Greenleafpro Looks like good water. I would just keep an eye on the root system health and lower leaves for necrotic spots for iron/manganese toxicity.

@Sermac Are you using commercial fertilizer or rolling your own? Ca/K is like a see-saw. While Ca is probably the most important element for plant growth, extra potassium will help balance when it’s THAT high.

In case you didn’t know, Coors Light is clean and crisp tasting from the mineral content in their rocky mountain water. Now that they are a macro beer, they probably add those minerals manually for batch consistency.


So I was taught most of the distilleries are down there in Kentucky and Tennessee because of the limestone water and it helps with the flavor of the mash. Don’t know exactly how true that is but I’ve definitely made some tasty stuff off of the water here. A lot better than when I was living in the city in colorado. I am buying premix nutrients right now I’m running front row AG but I’m going to switch back to Jack’s 321 because I felt like I was having better results with that. Just using up what I had left to try not to waste any money has moving to Missouri has definitely put a hurting on the bank and they pay a lot less here LOL.

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Good looking out!!
I have a feeling the iron was from the old pump and pipe, I need to have another test done since I’ve replaced everything from the well cap down.


You’re welcome :slight_smile: Going with an RO system would provide the best starting point, but it’s unnecessary. You could get excellent results and formulate an optimized nutrient solution using sulfuric acid to neutralize alkalinity. When growers use phosphoric acid or nitric acid for alkalinity neutralization, they are limited in formulation options and end up with a non-optimized, ‘good enough’ nutrient profile.

I’m happy to help you formulate an optimized nutrient profile and stock solution plan. We are fortunate at F4200 to have quite a few exceptionally well-versed members experienced in custom nutrient formulation. Including @emdub27, @danielfp, @AgTonik, me, and others - mainly because none of us are ego-driven. We all work well together for the greater good.

Is that well water or treated city water? Looking at your water report, I wouldn’t worry too much about the Fe and Mn concentrations because they are quite low at 0.024 ppm Fe and 0.005 ppm Mn. We want about 135-145 times the Fe and 140-170 times the Mn as chelated Fe and Mn in our nutrient profiles than in your water. Also, considering that Fe is likely ferrous iron (Fe+2) and Mn is likely manganese(II) oxide (MnO), they will readily oxidize and precipitate out of the solution. And, if you’re adding polyphosphate to your solution (like Hafia GrowClean), the ferrous iron will be sequestered by the polyphosphate, making it unable for plant uptake.


You ever run backwash cycle them? I rigged mine with a couple extra small lines and shutoff valves so I could just turn some levers and let it backwash the ro. I only even ran into needing this with hard well water, killed a membrane finding out. Cheap large sized y filters ahead of the ro helped a lot too.