Can I use my tap water for daily watering and fertilizing?

[itilien]

Hello,
First I want to mention that I don't grow my orchids in what you consider Semi-Hidroponic, but I don't see any better place to ask my question. I grow my plants, mostly pleurothallids, mounted bare root or in live moss in a closed tank that is misted heavily once per day. I use RO water and fertilize from time to time.
I live in a place with a soft water, and I found a document with its parameters, so I was wandering if I can use this water with added fertilizer instead

I measure the water with a TDS meter and it shows me about 55ppm, EC 116 uS/cm

But looking at this table, I cannot see where those 55 are coming from. There are about 10-12 from calcium, 2 from magnesium and 10 from sulfates. This is about half of 50, the rest is mystery.

This TDS is looking between 2-4 times higher then the numbers that Ray states in one of his articles and I don't know if it is a problem:

"Looking at the nutrient solutions that literally rain down on epiphytes in the forest, we find that they typically contain no more than 10-20 ppm total dissolved solids (TDS) at the onset of a rain storm (it’s almost pure water after that), and that the analysis shows it’s nutritionally almost all nitrogen."

But in another one he states that:
"We recommend feeding at 25 ppm N at every watering, using pure water, two or three times per week or more, flooding the plants each time, mimicking what they receive in nature"

He uses his K-Light fertilizer with formula 12-1-1-10Ca-3Mg at 25ppm N, so he has rougly 2 x (12+1+1+10+3) = 54ppm, from which 20 are comming from Ca, and 6 from Mg, so this contradicts a little bit the first statement, especially in the part "almost all nitrogen"

So what do you think, can I use this water daily, and what do I have to add?

[Clawhammer]

55 ppm TDS is great for just about any orchid.

[camille1585]

What is the column with the numbers in blue? I'm guessing that the one in gray in the last column on the right is the average, and the ones in white are a series of measurements used to calculate that average?

If so, you have very good water. The EC (97 uS/cm) and hardness are very low, and even lower than my tapwater which is so pure that I was told by Akerne that I can use their RainMix fertilizer (which is for pure water).

I don't have any salt sensitive orchids so can't advise it's use on pleuros with any certainty, but my hunch is that you could use it on them, and add some fertilizer (Rainmix or other)

[estación seca]

The TDS meter measures conductivity and extrapolates TDS. In other words, it is not a TDS meter. Your water TDS may be lower than 55. If you or a friend have access to a good laboratory scale you can measure your TDS by weighing a vessel with 1 liter of water, letting it evaporate, and weighing the vessel again. The number of milligrams of solids remaining is the TDS in parts per million.

Ray was describing water in situ. Horticulture does not need to duplicate nature. Fertilization does make orchids grow faster in cultivation than in nature if growing conditions are good. It becomes a question of how much fertilizer can be tolerated without damage.

[Ray]

In a 25 ppm N solution, those 5 cations may only account for 54 ppm dissolved solids, but the true dissolved solids contribution is 193 ppm, yet that is perfectly fine for constant contact, as is the case for semi-hydroponics.

My "almost all nitrogen" comment is not contradictory, as that's what's in the solution the plants are exposed to in the wild, but we must consider that that is very close to a "subsistence diet" and we want to do better.

EDIT That was a “25 ppm N solution” of K-Lite. Other formulas will have different numbers.

[itilien]

Originally Posted by camille1585

What is the column with the numbers in blue?

Those are the maximum allowed values by the standards for drinking water.


"but the true dissolved solids contribution is 193 ppm" - why is that? And is it OK for pleurothallids too? And is it OK if the roots/foliage dry out at some point before the next watering occurs? Is it going to make deposits at such concentration?

So I suppose that I need to add those 25ppm nitrate + 2 potasium + 2 phosphorus, I allready have 10 calcium, and some magnesium, so I need to recalculate them in order to get the final values.

Edit:
Actually there is a problem here. If I already have 10ppm calcium, I cannot use CaNO3 to bump the nitrate in order to lower the K in the mix. Even the present calcium will be more than needed.

[camille1585]

IMO, you are overthinking it a bit. Yes, fertilizer is important, but it is but a small part of the entire equation involved in growing orchids, and most plants will do quite well with a wide range of fertilizers as long as their other needs are met as well.

With my fairly pure water I alternate between a fertilizer meant for pure water (and so has added Ca and Mg) and a plain old 20-20-20, which served me well for many years.

If precision is important for you, you could simply continue with your current regime of RO water + fert. Otherwise with tapwater as a base you're going to have to handmake your nutrient solution with individual compounds to reach the same results.

[itilien]

Probably you are right about overthinking, but I'm an engineer, so this is how I think It is part of the fun too
I buy RO water, which is not very expensive, but it is a hassle, so it will be much simpler if I can use the tap water. I want to automate the routine work as much as possible.

[Ray]

Originally Posted by itilien

Probably you are right about overthinking, but I'm an engineer, so this is how I think It is part of the fun too
I buy RO water, which is not very expensive, but it is a hassle, so it will be much simpler if I can use the tap water.

Hah!!! I am an engineer, as well, but after 50 years of growing (and agreeing with Roberta’s comment), I still like to do such calculations.

However, think of an orchid as a Ferrari and fertilizer as a key: a key is a critical component, as the car won’t run without it, but the design of the key is relatively unimportant.

I’ll bet your thinking (or overthinking) is disregarding uptake dynamics. I know mine did for a long time. There are some mineral ions for which the uptake is controlled by the concentration surrounding the roots. There are others that are taken up as controlled by the plant needs, and others - notably phosphorus- that are pumped into the plant whenever it is available, whether the plant needs it or not.

Then there is internal translocation to be considered. Almost all mineral, once absorbed, are easily moved from old tissue into newly-growing cells, which means deficiencies are rare, especially in such slow-growing plants as orchids. Calcium is a significant exception: once it is absorbed and bound into the plants’ chemical structures, it is quite immobile, which is why it is important to supply calcium to all actively growing plants.

Even with all that rattling around in your brain, there is another set of facts that put fertilizer in perspective:

In the plant world, the fixation of carbon is the controlling factor for growth, and the nutrient minerals support that.

In order for a plant to add about a kilogram of mass (a few weeks for maize in the summer, 5 years or so for a phalaenopsis, or a lifetime for a tiny pleurothallis colony), it must absorb and process only about 10 grams of NPK fertilizer, but requires about 2000 kg of water.

Plus, if you take into account that the average plant transpires 95%-99% of the water it absorbs, that means that, in order to grow 1 kg, the plant must take in 10g of fertilizer and as much as 40000 kg of water.

To me, that means focusing on maximizing water availability without root suffocation, and throwing a tiny bit of fertilizer.

Since we have taken this way out on a tangent, I’ll address your original question and say your tap water is fine.

[MateoinLosAngeles]

Using RO or not is really location dependent. I'm not an expert so I can only talk from personal experience: I always used tap water without an issue when I lived in NYC. When I moved to Los Angeles I also expanded my interest in plant growing (perhaps had something to do with having more space) and for a long time I struggled to keep a plant alive. Using tap water here seemed to "burn" orchid roots, when my partner and I united households his plants arrived with brown tips, experimenting with using RO water across the board improved our plants health dramatically.

I think RO water is an extremely helpful tool in many parts of the world and it allows you to have total control over what your plants get. Having said that, your tap water analysis looks okay. You can and probably should use a fertilizer to boost N levels but I would feed based on the ppm N of your fertilizer as for what I can see the hardiness of your tap water won't make a significant difference.

You can always experiment with using RO for flushing if you notice mineral deposits to develop faster.

Regarding your question about Ray's website, the 25 ppm N recommendation assumes that the plant will absorb nutrients washed down the canopy by the rain water and that weren't initially contained in the rain water. These nutrients are exhumed by the host trees and the multitude microorganisms, insect castings, neighboring plants, and the sorts. However, it would make sense that in an environment where the plants get drenched several times a day but for a short period of time, that the amount of nutrients that an epiphytic can capture will be limited. More on this logic is found in the AOS article discussing the development of K-Lite which is also available on Ray's website.

Keep us updated on your research.