Watering Orchids with Tap Water: a lesson on pH, Hardness, and Nutrient Availability You too can become a water chemist ;)

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Adjusting the pH of your water is a way to improve your orchid culture and squeeze the last 10% out of your plant’s potential. From phalaenopsis and oncidiums, to phragmipediums and paphiopedilums, pH might not be something you have to mess with, but in this post I’ll cover the many reasons why you might consider it.

Before I get into the thick of the details, there’s a graphic that really sets the stage for why pH adjustment is valuable to orchids. It illustrates nutrient and mineral solubility and the relevance to orchids. I’d like to instill this caveat: not all orchids need the exact same pH. Depending on the species of orchid you’re growing, it will have adaptations to deal with different pH. However, in general most orchids we collect are epiphytes and will fall into the range noted in the graphic.

Orchid pH Chart & Nutrient Availability Graphic

pH has no direct effect on plants; it is important only in its influence on biological activity and the availability of phosphorus and trace elements.
Northeast Organic Farming Association

What got me into researching pH and tap water for Orchids?

I hated the topic of pH…the info available lacked a clear connection to orchids, it was overly complicated (because the relationship between water and minerals is complicated), and I couldn’t see the line of logic or proof as to why someone should actually pH-adjust water. Frankly, I couldn’t even find someone who was pH-adjusting their water and could tell me why they did it or how effective it was for them.

When I turned to YouTube, I found a few orchid people talking about pH but their videos were over-inflated and loaded with false statements—things like, “hard water would kill orchids”, “calcium is something you have to add to your water”, and “pH is the only way to get firm leaves and lots of roots.” At the time, I was growing my all orchids with “bad” hard/alkaline tap water (7.5pH, 250ppm total dissolved minerals)—and I had been doing it for many years. So, hearing these statements and seeing my contradictions, compounded with an inability to find good info online, it made the topic of pH adjustment seem wishy washy and I largely thought ALL of it was BS.

The problem was…I was passively curious about pH and I desperately wanted to find “the secret” to more flowers, bigger leaves, and sexier plants. Then one day I got a Miracle Berry; not an orchid, but a plant that needs a very-acidic pH of 4. Suddenly pH mattered and I needed to understand why and how.

Video: Why I started pH adjusting and a quick lesson

In the spring of 2018, I started an experiment and began pH-adjusting my tap water for select orchids. My test group included a few epiphytic orchids, 2 compots of ~20 phalaenopsis seedlings, and any other orchids that were showing signs of chlorosis (yellowing often caused by nutrient deficiency) under my bright LED grow lights. After 2 months of experimenting with pH, I saw enough improvement that I started acidifying my tap water for the rest of my orchids. Now, after about 8 months, I feel strongly enough about the value of it and I’m documenting my insights and connecting the dots for others.

The bottom line

Adjusting pH worked for me and I saw an improvement in many of my orchids

 

Photo of my oldest phal
(who was grown in both the high pH AND the low pH times of my orchid career)
Photo of phal seedlings growing out of flask – being watered with pH-adjusted tap water
Some reaching blooming size in just 13 months

 

To understand why pH adjustment was helpful, let’s look at the science of it all…

Water Chemistry & Orchids: pH, hardness, alkalinity, and calcium carbonate

There are two primary components of water that you need to understand (pH and water hardness), and those two concepts cascade into the how and why adjusting the pH matters for orchids.

pH – Acidic vs. Basic

The acidity or basicity of your water is determined by the pH. Acidic water is under a pH of 6.5, basic water is over a pH of 7.5. There is a relationship between pH and “water hardness”, but it depends on the mineral composition of your water.  This kind of dates back to elementary school, but it quickly gets more complicated when you add the aspect of water hardness in relation to pH. Here is a really good video for the aquarium trade that effectively summarizes the aspects of water chemistry:

Video: Understanding pH, Water Hardness, & Calcium Carbonate

Note: basic water =/= “alkaline” – alkalinity is the reading of carbonate and bicarbonate ions in the water. They are closely related, but beware when saying “alkaline water” we mean both hard water and high pH.

Water Hardness – Soft vs. Hard:

As mentioned in the video above, there are two components to water hardness: General Hardness (Magnesium) and Alkalinity (Carbonate).

The hardness or softness of your water is determined by the total dissolved minerals in your water. Soft water is deemed to be 0-60 mg CaCO3/L, hard water ranges in 121-180 mg CaCO3/L, and Very hard water is over 180 mg CaCO3.

Find out your tap water hardness You can often find out your city water rating by googling [Your_city] and “Water Hardness Report” – if you’re in Canada, this site will tell you. My tap water has a pH of 7.5-8 and ranges from 180-250ppms depending on the time of the year; that is VERY HARD. If you cannot find a source from your city, try checking local aquarium societies; the fish peeps care about this stuff too; here’s a great reference for Calgary’s water that breaks down all the minerals.

Calcium Carbonate (CaCO3) / Scale / Lime

If you have hard water, this is the junk that builds up on your sinks and shower walls. It’s a common calcium-based compound that is fixed by many crustaceans and moved through a calcium cycle in our environment though geological erosion, rain, and biological processes. Eggshells are also made of calcium carbonate. It’s the component in water that is largely responsible for hard and alkaline tap water in homes of those who live near the mountains. As acidic rainwater runs through the limestone and basalt rocks of mountains, it dissolves it and takes the calcium carbonate with. If you want to knock yourself out with the details of calcium carbonate, limestone, and the calcium cycle, this is a really thorough resource.

There’s one important thing about calcium carbonate that is super relevant to orchid care:

What happens to Calcium Carbonate (CaCO3) in Alkaline Water when you acidify it?

You get usable Calcium.

Calcium Carbonate reacts with acids, releasing carbon dioxide (technically speaking, carbonic acid, but that disintegrates quickly to CO2 and H2O) :

CaCO3(s) + 2H+(aq) → Ca2+(aq) + CO2(g) + H2O (l)

 

Video: Usable Calcium From Lime: Calcium Carbonate (CaCO3) + Acid

Calcium carbonate can react with water that is saturated with carbon dioxide to form the soluble calcium bicarbonate; however, the reaction with calcium bicarbonate and acids yield either Calcium Chloride (CaCl2) or Calcium Phosphate (Ca3(PO4)2) both of which are reportedly also usable by plants through the process of biological interactions (bacteria and fungus). 

CaCO3 + CO2 + H2O → Ca(HCO3)2

If you want to read more about the chemistry of calcium carbonate, head over to wikipedia. The takeaway is that acidifying hard water can yield usable calcium for your orchids.

 

 

Valuable Facts about Water, pH and it’s relationship to Nature

Fun Acidic Water Fact:

Rain water pH is about 5.6 pH because it reacts with Carbon Dioxide and it’s nearly 0ppms (very soft). This is SUPER important to understand because low pH AND low total dissolved salts means rain water is kind of like a nutrient sponge. As it travels across leaves, down a tree, and through the moss to the roots of an orchid it quickly picks up any nutrients, sugars, and minerals it encounters. This is why most people assume, “pure water is best for orchids” but they forget that an orchid is part of a larger ecosystem. Birds, reptiles, bugs, fungi, dead leaves, bacteria, and all the fauna in the forest are excreting junk all over the rocks, branches and leaves. When it rains, and all of those nutrients and minerals are picked up and carried straight to the orchid’s roots…a poop-nutrient Slurpee. This is why wild orchids don’t get nutrient deficiencies and die (like they sometimes do in our homes…not in my house of course 😉 ).

Another fun acidic water fact:

“Black Water” is a term used to describe water from some sections of the Amazon that are…black. Waters from black water amazonian streams are SUPER acidic because the abundant plant life,  tannins from dead leaves and biological interactions have acidified the water and used up all the minerals making the water very soft, and nutrient poor. Conversely, “White Water” rivers of the Amazon have a higher pH and are high in minerals; these rivers are generally unobstructed by as much plant life and the minerals haven’t been stripped from the water that flows from the Peruvian mountains. It’s an important concept to understand because it explains why some plants (such as Phrag klotzscheanum) require very acidic or alkaline conditions while other plants like Phrag kovachii requires a high pH – even though they’re both from South America in the Amazon region.

Fun Alkaline Water Fact:

The ocean’s pH is 8.3 – and it’s constantly buffered up by the dead skeletons of corals and crustaceans. The beach sand you find near the ocean’s edge in tropical areas…is a natural pH buffer. Ocean creatures (such as starfish, snails and clams) literally disintegrate in acidic water.

ANOTHER fun Alkaline Water fact:

Lithophytic and terrestrial orchids that DO NOT grow on trees often grow in mountainous regions where limestone and calcium carbonate are constantly buffering acidic rain to become alkaline – with that change in pH also brings Calcium Carbonate and a slew of other hard water minerals. That’s why in cities near the mountains, the water has a high pH; it’s also why some species of phrags and paphs need a higher pH to do well (including in flask) – they come from the base of the Andes or limestone regions of Malaysia where there’s LOTS of calcium carbonate and limestone.

 

Now that we know the terms, let’s get on with how this all relates to orchids (and plants)

Why Does pH Even Matter?

The pH of water affects the solubility of minerals and nutrients in it. It has to do with hydronium ions interacting with minerals making them aqueous or binding them up as non-usable compounds. The gist of it is, at very low pH (under 5pH) lots of nutrients are not available (including nitrogen, calcium and magnesium); and at a high pH (over 7.5) different nutrients are not readily available (iron).

Theoretically at a pH 5.8-6.5 you kind of reach an ideal balance of all “nutrient’s being available.”

Yahtzee, right?! The perfect pH is 6! Well not exactly…many plants have adapted to live in an environment with a pH outside of that ideal 6. They’ve either adapted to having LOTS of iron Fe+ or not having much iron but having LOTS of Calcium. This means a plant that grows in nature at a pH of 7.9 will not be able to do as well at a pH of 5 because necessary minerals it requires for growth are simply not available. This is why most people suck at growing blueberries – they love acid (and need lots of iron).

The plant/orchid pH “nutrient availability” chart

This isn’t a chart that says, “this is what the pH needs to be for orchids to take up nutrients.” This is a chart that shows how the solubility of plant-required nutrients are available (or not available) at specific pH levels in water. An orchid may or may not have adaptations to deal with low pH or high pH – so this chart is not an “orchid nutrient availability chart”, so much as it’s a “mineral and nutrient solubility in water chart”

Nutrient Solubility Chart

Why should we consider adjusting pH of our tropical plants and orchids?

The short answer is: because you want to offer the pH your plant has spent millions of years adapting to. That doesn’t mean you always acidifiy your water…it means you need to understand the context of what the plant requires and offer that (or select orchids that grow best at the pH of your tap water).

Orchids have adapted to niche environments

Epiphyte Orchids (grow in trees): do best with a pH close to that of rain (5.5-6) – this accounts for likely 60-75% of the orchids we keep
Lithophyte Orchids (growing on limestone or basalt rock): do best with a pH that’s close to 7-7.9
*lithophytic orchids growing on marble or non-alkaline rock: 6-7
Terrestrial Orchids (grow on the ground or near river banks): will need a pH dependent largely on the soil, mineral composition, and the factors within their natural environment.
*Terrestrial orchids growing in sandstone /sandy soils or regions with thick vegetation may require a very low pH (sometimes down below 5.8 toward 3.5-4.5pH – see Phrag klotzschianum).
*Those growing on mossy forest floors will likely be used to a pH of around 5.8-7.
*And terrestrial orchids growing in limestone-rich regions will likely require a higher pH (7-7.9).

In nature a plant has evolved over millions of years to exist in the place it grows. If it’s growing on a tree, the pH of rain water is low. If it’s growing on limestone rocks at the base of a mountain in Borneo or the Amazon, the pH is high. If you’re one of those who strongly believes that “rocks don’t dissolve in rain” – sure, you’re right, in one rain the mountains don’t dissolve like sugar…but it rains what… 400+ times a year? Over time that limestone is broken down by acid rain and slowly travels down the mountain and concentrates in the soils around the base and along water ways. When the rain stops, Calcium Carbonate dries and becomes immobilized until the next rain, then it carries on down the water table.

Plants growing in these areas have gone through evolutionary changes to be able to best takes up available nutrients. It’s an important concept to understand and here’s why:

Acidity can be bad for alkaline-loving plants

A plant that is native to a habitat that has a high pH will have roots adapted to restrict it’s calcium and magnesium uptake (both minerals that are abundant at higher pH) while having open pathways for absorbing iron (which unlike calcium and magnesium is not as available at a higher pH). If you take that same plant and put it in a low pH, with LOTS of iron, and minimal calcium, you risk nutrient deficiency AND potentially iron toxicity because the plant doesn’t have the tools to fix the nutrients it has adapted to use in its native habitat.

Alkalinity can be bad for acid loving plants

In an inverse scenario, if you take a fruit tree (for example the blueberry mentioned above) that requires VERY acidic soil and LOTS of iron, and you put it into a high-pH soil, it will get iron deficiency almost immediately because at a high pH, iron is not freely available. Even at a pH of 5.8…they don’t do well. This is largely why most growers struggle with blueberries, they need a very LOW pH of 3.5-4.5.

 

Now that we’ve covered all the details of water and pH AND HARDNESS AND NATURE…

Should you pH adjust water for your orchids?

I’ll be the first one to say you don’t HAVE to. Plants are generally pH versatile and you can get away with just using water as long as you leach your pots and provide ideal conditions (regularly watering, providing good light and nutrients, not burning your plants from too much heat, keeping them in their preferred temperature range, etc). I did this for years and my plants did well enough.

pH adjustment is just the cherry on top of the cake, it’s leather seats in your new car, it’s a 4K DVD player instead of your old standard definition player, it’s simply a step above where you were before.

If you’re really struggling with orchid care, I urge you not to mess with pH and first focus on the 90% of care, before you tinker with the last 10% of refinement.


Need some tips on orchid care and how to achieve that 90% before dealing with pH?
Read Orchid Care Basics


If you feel mucking with water pH is a waste of time

No problem—then don’t muck with it. However, I’ll leave you with this consideration: orchid seedlings, grown in a flask, require optimal conditions to sustain their tiny plant bodies and grow well. This includes a very specific pH (which varies depending on the orchid species being flasked). Seeds sown and grown outside of their preferred pH range generally germinate at lower rates, grow slower, and yield less plants overall. To me if the little orchid babies need flawless conditions and that means growing phalaenopsis seedlings at a pH of 5.5 in flask…then maybe it would smart to do this outside of flask also.

With that tidbit of info in mind, if you’re considering pH adjustment for your plants, let’s move forward..

 

Common Questions and Answers About Orchids, Water, and pH

What is the perfect pH for Orchids?

Around 5.5-6.5pH is ideal – but it depends on many factors including what species of orchid you’re keeping, the time of year, and where it grows.

Photo of my Phal schilleriana after 8 months pH adjusting
– The big leaves are from the period of pH adjustment –

How do you know if you need to acidify your tap water?

If you fill a glass with water and leave it for a few days, do you get a white ring of calcium around the glass as the water evaporates? Then you probably have hard water AND ALSO a high pH and you should consider adjusting your tap water pH for your orchids.

If your pH is high – Leach your pots!

If you have a high pH, your water is likely alkaline  and the pH of your soil (or substrate) can increase each time you water. As the water evaporates, calcium carbonate and other “alkaline and hard water minerals” will stay behind and quickly start to build up in your media. This leads to compounded pH increase at the root zone. Think about salt in a glass of water – if you don’t remove the salt and the water evaporates, all the salts starts to precipitate out but it stays in the glass. That’s how hard water works, and that’s why you get calcium carbonate rings in your glass. This is also why holes in the bottom of your pots are SO important! You want to fill your pots, soak them, allow old salts to dissolve, and then flush the mineral buildup away.

What happens if you don’t leach your pots?

I once did a test where I watered a pot but never let it drain or flush. I watered the top of the pot whenever the soil approached dryness. After 3 months the soil pH was over 9! That’s not good; it’s really bad in fact because at a pH of 9 virtually all the main nutrients are completely not available to the plant. They starve.

Can you use Tap Water to Water Orchids?

Yes – as long as your ppms are under ~400. Frankly even if they’re over that, I would say try it

Isn’t Chlorine in tap water toxic to orchids?

Doesn’t seem to be. There’s a bunch of people online saying it is and a bunch more saying it isn’t. I water my plants with water directly from my tap, from the city’s water processing plant and my plants look great. I’ve been doing it for YEARS. Also, “captive bred” orchid seeds are often disinfected with chlorine from household bleach with a MUCH HIGHER concentration of chlorine than what’s in your tap.

CHLORAMINE is/not toxic

Chloramine is a chemical used in some city water supplies; it’s a stable form of chlorine that has been combined with ammonia, and it’s allegedly more harsh than chlorine. That said, I couldn’t find any concrete information that actually proved chloramine was toxic to plants.

If you’re a chlorine-o-phobe, you can get neutralizing drops at a pet store or on Amazon which will deactivate both chlorine and chloramines. I use tap water, I don’t use drops, but my city uses chlorine only so I can’t say much about chloramines from experience.

 

To Decrease the pH of your tap water – add an acid

There are lots of “plant safe” acids – but beware, some of these are very strong acids and can burn your skin or harm you (DO NOT GET IT IN YOUR EYES).

Acidifying solutions for orchids include:

– Citric Acid (least strong and most safe)
– Phosphoric Acid (strong acid and potentially harmful)
– Nitric Acid (very strong acid and harmful)
– Vinegar (I’ve hard of acetic acid being used – but I tried it and didn’t like the results)

To lower your pH you’ll need:

– A jug (I use a 1 Gallon jug)
– A large spoon (for string the water)
– Orchid fertilizer (that’s the whole point of this, right?)
– A pH meter (to determine what the pH is as you lower it)
– A pH meter calibration solution (exactly 7.0)
– An acid to lower pH
– A dropper bottle or something to drop drips of acid into your tap water

Steps to lower pH for Orchids

1. Calibrate/reset your meter (you don’t want an off reading right out of the gate)
2. Fill your water jug with tepid temperature tap water
3. Add 1/4-1/2 tsp of fertilizer
4. Test your tap water’s pH
5. Start by adding drops of your acidifier to the water and stir with the spoon to distribute the acid
6. Test the pH
7. Continue adding drops if it’s still to alkaline
8. Once the pH reaches 5.8, stop and wait 5 minutes
9. Test again to ensure it’s still 5.8
10. Water your plants
*If you go too acidic, you can just pour off some water, and add tap water back in.

 

To Buffer Acidic Water – Add Lime or Crushed Oyster Shells

There are ways to increase water pH, but most gardeners, growers, and orchid hobbyists turn to natural methods. Most add limestone chips, limestone rocks, crushed (and baked) egg shells, or crushed coral to their potting mix. You can read more about natural methods for increasing pH here which although it’s geared toward aquariums, is equally applicable to plants.

 

That pretty much covers all the things I know about pH in relationship to orchids. I hope you found it helpful. If you have any additional info to add (especially about the relationship between Calcium Carbonate in the Calcium Cycle), hit me up on facebook and lay it on me.