Help with quality of tap water + high Sodium content?

[ChrisMalaga]

I was able to find a report of the quality of my tap water (wasn't so easy, as my city has a few water treatment plants). I use tap water for my plants, but in winter I cut it with water that comes from my dehumidifier (about 60-70% tap water).

I think the quality of my tap water is generally "good" - I thought it was much harder, but I was misinterpreting readings from a TDS meter which reads between 200 and 300, but has nothing to do with hardness.

I'm concerned about the quantity of salt (Sodium), which is currently around 80 mg/L (ppm) and how this can harm plants (I've never seen any damage I could directly associate to salt so far, but it's really hard to tell...).

Having a quick read here and on the internet, it seems the biggest problem is that Sodium is an antagonist of Potassium (means that the plant will take Sodium instead of Potassium and could have a Potassium deficiency). I have not found any study about the effect of salt on orchids though. I've also read someone was suggesting adding humic acid - something I also started doing recently at a very low concentration (water turns deep black with just 1 teaspon every 10 liters, so I use 1/4 of teaspoon every 10 liters). This also contains 10% Potassium . which I don't think I'm supplementing enough though - considering the amount of Sodium, and I use a 3-4-5 fertilizer at very low dosage (1 teaspoon every 10 liters with every watering + some are in self watering with leca).

Has anyone had any clear issues with salt, or are you aware of any study about the subject and possible solutions? Unfortunately, osmosis water it not a solution right now for me for different reasons.

This is the analysis of my water today (translated from Spanish, sorry if you see some mistakes, I'm not sure if "Cloruros" is the same as "Chloride"):

Conductivity µS/cm (20ºC) 544
pH 8,2 -> I tweak this to 5.8 with Phosphoric Acid
Turbidity NTU 0,42
Chloride mg/l 123
Hardness mg/l - 90
Sodium mg/l 80
Free residual chlorine mg/l 0,88

[Ray]

A couple of comments.

I think the quality of my tap water is generally "good" - I thought it was much harder, but I was misinterpreting readings from a TDS meter which reads between 200 and 300, but has nothing to do with hardness.

First, don’t trust any TDS meter to give you reliable, accurate readings. Read this.

Second, TDS and hardness are definitely related. The “hardness” minerals are a part of it.

Does your info include alkalinity data? That maybe more important than the pH itself.

I seriously doubt that there will be an issue with P uptake being blocked by the Na. Plants don’t need a lot of phosphorus to start with, and they store excess in cell vacuoles.

Personally, I’d be more concerned about the chlorides.

Several years ago, Bill Argo, the guy who formulated the MSU fertilizers, published a series of articles in the Journal of the IPA. They are really not focused specifically on orchids, but definitely worth a read.

[ChrisMalaga]

Thanks Ray. Yes, I understood from your article some days ago that the result of a TDS meter does not correspond to hardness. I didn't express myself well, I didn't mean to say that "they have nothing to do" with each other, but I meant to say that they're not the same thing.

I didn't know about alcalinity, I don't have this data unfortunately. I can say that I use a syringe to add Phosphoric Acid to water, and the amount is 0.2 ml of Phosphoric Acid - 2-3 drops more or less, every 10 liters of water to get from 8-8.7 to 5.8, do you think I'm able to find out alcalinity from this?

I'm "a bit" noob about Chlorides, ahem... aren't these volatile? I always let water sit for at least one day before using it, or this works only with chlorine?

[Ray]

Originally Posted by ChrisMalaga

I didn't know about alcalinity, I don't have this data unfortunately. I can say that I use a syringe to add Phosphoric Acid to water, and the amount is 0.2 ml of Phosphoric Acid - 2-3 drops more or less, every 10 liters of water to get from 8-8.7 to 5.8, do you think I'm able to find out alcalinity from this?

I'm sure there is, but I'm not familiar with the calculation, but you'd need to know the acid concentration to start.

I'm "a bit" noob about Chlorides, ahem... aren't these volatile? I always let water sit for at least one day before using it, or this works only with chlorine?

Yes, that's only chlorine. If chlorides were volatile, you'd put table salt in water, the chlorides would be lost, leaving the solution no longer "salty".

[ChrisMalaga]

Originally Posted by Ray

I'm sure there is, but I'm not familiar with the calculation, but you'd need to know the acid concentration to start.

It's 81%, but I will check that!

Originally Posted by Ray

Yes, that's only chlorine. If chlorides were volatile, you'd put table salt in water, the chlorides would be lost, leaving the solution no longer "salty".

Makes sense
I was searching for information about the effect of excess chlorides on orchids but so far, I haven't found anything clear. On some websites it talks about how Sodium + Chloride can cause a stress similar to drought, but I'm not sure at what levels of chloride or if this information is correct. I haven't seen my plants suffer from such symptoms so far (except those I underwatered).

[Ray]

I have seen some discussion of excessive chlorides causing decreased chlorophyll production, smaller leaves if not leaf loss, and reduced CO2 assimilation, but the data was only for woody plants, fruits and grasses and varies all over the place.

In nature, most orchids see almost none, so that's what base my assessment on.

[ChrisMalaga]

I haven't seen this on orchids, but I use the same water for other plants and I've seen "strange" things happening with Peperomias - which are not orchids though, but grow in quite similar conditions. I have a P. dahlstedtii and a P. Hope that drop healthy leaves all of a sudden, with no apparent reason, and a P. polybotrya that instead of making heart-shaped leaves is making M-shaped leaves and has sometimes rolled leaves
All plants grow vigorously, it's more something esthetic.

At the same time my Stromanthe and my Calatheas seem to do great, and they are known to be sensitive plants... quite a mystery for me.

[camille1585]

About sodium, I read an article about it a few days ago, and managed to find it again. In 2027 dutch nurseries must fully recirculate their water, and with orchids they are concerned about the sodium levels that will rise over time. For standard pot plants it isn’t much of an issue, but they explain that as epiphytes, orchids get their water from rain and as a result have a very low tolerance for sodium. In practice, this means that with too much sodium in irrigation water the plants have to work harder to take up water, which reduces growth. They also bring up the potential problem of sodium hindering uptake of elements such as calcium and potassium.

The bad new for you is that the research is now just starting and there is no available data yet. They are testing sodium concentrations up to 10mmol, which works out to roughly 207mg/L.

EDIT : I forgot to mention that they did study this a few years ago with Phals, but they only looked at sodium values up to 1,2mmol, which is still far under your water (3,5mmol). No negative effect on growth, spiking or blooming was seen.

[ChrisMalaga]

Seems really interesting. I also read that Dutch nurseries will have to recirculate water and, living in a semi-arid climate, I think Spain should do the same as soon as possible, water is very precious and we cannot throw water full of nitrates like this, especially considering that in the next years we will probably face some water emergency. Also makes me really sad to know how many ecosystems were destroyed by agriculture here.

It is possible that it can affect orchids but it will be interesting to see at what levels. We already dope our orchids with lots of things they don't receive in nature, I wonder what level of Calcium and Magnesium epiphyte orchids growing on trees receive, maybe they are more resistant than we think... or maybe not?

[camille1585]

Recirculation is already mandatory in Dutch nurseries, but commercial orchid nurseries were exempted from this since orchids are extremely salt sensitive if you compare them to nearly all other pot plants. With the new 2027 zero environmental targets the exemption end that year.

Yes, we already give our plants nutrient levels that are far above what they see in the wild, but that is true for nearly all crops, and what we give orchids is still rather minimal compared to other plant types. In commercial environments growers are feeding with an EC of 0,8 to 1,2 mS/cm. For other crops this is usually an EC of 2, 3, or even higher. The tolerance margins of orchids are thus far smaller.

One solution they've found (in experimental conditions) so far to recirculate in orchid greenhouses without problems is to use controlled release fertilizers in the bark mix, and use only half of the usual fertilizer strength while irrigating. They can recirculate water far longer without nutrients building up to toxic levels. Of course, this dosn't help you for your specific problem, but I found in interesting to mention.

As to the situation in Spain, if I'm not mistaken most things are grown directly in the ground? I have no idea how much above ground cultivation they have.