pH swing in S/H

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ssknapp777

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Hello All!

I'm growing many of my orchids in S/H using PrimeAgra. For fertilizer I'm using MSU RO formula. For the last several months I've been adjusting the pH of my fertilizer water solution upwards for Paphs that like a more alkaline environment. I have been using a pH up with Potassium Hydroxide to get the pH up to 7.5-8 range.

I decided to test the pH of the water in the reservoir after a few days. I was surprised by what I found. It seems that as the water level drops in the reservoir, the more acidic the water becomes. I found that even though it goes in at above a ph of 7 it sometimes drops to 6 or even 5.5.

My best guess is that as water evaporates it leaves a higher and higher concentration of minerals that raise the acidity level (root uptake might have some influence also).

What do you all think?

This is the pH of the water remaining in the reservoir. Do you think this pH reading is accurate/representative of the whole growing medium (the water being wicked up)?

Has anyone else noted this for S/H with other types of medium and fertilizers?

How could you correct this or don't you want to? I guess it could be beneficial in that the root system experiences a wide range of pH levels which would help the roots absorb minerals at their optimal pH level.

I've heard of top dressing of oyster shell, calcium chips, etc. but would this help balance out the pH if this is only on the top and doesn't fall throughout the medium, especially into the reservoir and the water being wicked up through the medium?

If anyone has any ideas on this please do post.

Shayne
 
Thanks Candace.

So am I understading you correctly that from what people have posted before that this problem is restricted to the new P.A.? That with the old P.A. and all other S/H LECA the pH going in equals the pH in the reservoir after 3 or 6 days of evaporation? That pH doesn't drop over time except in the new P.A.?

Shayne
 
If you assume the media is absolutely inert as the plants consume the mineral (salt) content of the reservoir water you will see the pH and other chemistry change over time. If the change is rapid it is likely from the media itself.

When you are growing using the s/h method you have to be concerned to what is happening with content of the reservoir water just as if you were growing in true hydroponic culture.
 
a pH of 5.5 is not that low, it is well within the range of what plant roots deal with. The resevoir wicks up to the orchid roots, the roots are mostly just exposed to a thin film of this solution. A healthy orchid can buffer the thin film of soil moisture near the roots to its preferred range. If your water is potable - it is unlikely to have a pH that would really hurt your orchids.

My advice is throw out the pH meter. You need to spend over $2000 for a decent meter and use the referance solutions to keep it calibrated in order to get a meaningful pH measurement. Most cheap pH meters are wildly inaccurate, and can not be trusted one measurement to the next. You are far more likely to kill your plants by getting too technical.

my 2 cents - for what it is worth
 
Thanks everyone,

I guess I wasn't necessarily seeing this as a problem with the LECA but was thinking it was a function of the S/H culture. My thinking was based on my observations.

The more MSU you add to a gallon of water the lower the pH value.

You then fill the reservoir and as time goes by, the water level drops due to wicking and evaporation.

I assumed that minus what minerals/nutrients the plant uses, are left behind, and you end up with a higher amount of minerals in the remaining water that could lower the pH of the water in the reservoir.

I thought this would be similar to if you added salt to water. Boiled half the water off, you'd have double the ratio of salt to water compared to before you boiled half the water away. Likewise, twice the ratio of lowering pH MSU minerals would lower your pH after half of your reservoir water is gone.

Leo might also have a point on accuracy of pH meters. I remember doing research on hygrometers and you have to go near the five thousand dollar range to get +/- 1 % accuracy. Most are only accurate to +/- 3 % ( and then only for certain humidity ranges) and analog ones can be +/- 10%.

I use a liquid pH solution to get a general idea. However, I do like to figure out how things work and what the optimal conditions for growing would be.

Hence my four primary questions in my first post. Again, this might be normal for S/H culture and not really accurately measurable or a big concern at all, as Leo has suggested.

Shayne
 
My guess is that respiration from the roots, along with decomposition of the organic matter that makes it into the reservoir, adds CO2, which lowers the pH. Also, I think the pH of your fertilizer could be lowered, to say, 7.2. When you fertilize, pour out the old water and refresh rather than just top off..but I'm no SH expert....Eric
 
Leo, once in a while we disagree- this is one of those times. I think a decent, well-maintained pH meter is valuable to any plant, fish, and homebrew hobbiest. Even a pen style job for $30 on eBay is better than nothing or silly pH paper. Yeah, I have the convenience of an awesome lab-grade meter to comare it against, but it is pretty close all the time with only *the maintenance the manual recemmends* (calibration with proper, fresh standard solutions and proper storage for the membrane). But certainly, grossly neglected pH meters, no matter how cheap or expensive will "lie" to you.

I wouldn't call that a HUGE swing at least from the plants' perspectives. And is certainly within a good range for our plants. So Leo and I agree there.

Why? Good question. Could be the metabolite buildup as the others suggest. Or could be that and/or a change in the CO2 concentration as the liquid volume decreases. CO2 DIRECTLY influences pH, and gas concentrations in fluid is closely knit to surface to volume ratio- deeper liquid means gas molecules have farther to go to leave the liquid compared to a shallow liquid in a wide container where gas moves pretty rapdily just because there's not that far to go. An oversimplification, but you get the idea.

-Ernie
 
Thanks everyone,

Leo might also have a point on accuracy of pH meters. I remember doing research on hygrometers and you have to go near the five thousand dollar range to get +/- 1 % accuracy. Most are only accurate to +/- 3 % ( and then only for certain humidity ranges) and analog ones can be +/- 10%.

The less expensive meters may not be accurate to the degree of the higher quality meters but they do give a good reference. If you want your mix to be pH7 and your cheap meter reads pH5 you can trust that you need to correct something. without at least a cheap test method you will simply be guessing. Guessing is fine most of the time but if you have a sensitive plant variety why guess when a $30 meter will quickly point you in the right direction.

I use a liquid pH solution to get a general idea. However, I do like to figure out how things work and what the optimal conditions for growing would be.

That is a fun aspect of growing.

Hence my four primary questions in my first post. Again, this might be normal for S/H culture and not really accurately measurable or a big concern at all, as Leo has suggested.
Shayne

It is a normal part of s/h culture and it should be a big concern.
If you follow the basic guidelines for s/h culture you should flush the reservoir water out each time you add water. This rule of thumb is to avoid the problems you are asking about. Evaporation and plant metabolism both change the water chemistry very quickly.

You can use an inaccurate method (cheap) to test the pH and it will at least tell you when something has changed. If you test your water weekly and the results are about the same and then suddenly the tests result in a drastic change you can assume there is something going on in the water and make corrections.
 
a pH of 5.5 is not that low, it is well within the range of what plant roots deal with. The resevoir wicks up to the orchid roots, the roots are mostly just exposed to a thin film of this solution. A healthy orchid can buffer the thin film of soil moisture near the roots to its preferred range. If your water is potable - it is unlikely to have a pH that would really hurt your orchids.

Leo, Many articles and many times on this forum it is said that there are some orchids that like a higher or lower ph, depending on their native habitat. Advice is given on how to change the ph to meet those supposed optimums. Are you saying don't worry about this just use drinking water and do not worry about the ph?
 
Leo, once in a while we disagree- this is one of those times. I think a decent, well-maintained pH meter is valuable to any plant, fish, and homebrew hobbiest. Even a pen style job for $30 on eBay is better than nothing or silly pH paper. Yeah, I have the convenience of an awesome lab-grade meter to comare it against, but it is pretty close all the time with only *the maintenance the manual recemmends* (calibration with proper, fresh standard solutions and proper storage for the membrane). But certainly, grossly neglected pH meters, no matter how cheap or expensive will "lie" to you.

I wouldn't call that a HUGE swing at least from the plants' perspectives. And is certainly within a good range for our plants. So Leo and I agree there.

Why? Good question. Could be the metabolite buildup as the others suggest. Or could be that and/or a change in the CO2 concentration as the liquid volume decreases. CO2 DIRECTLY influences pH, and gas concentrations in fluid is closely knit to surface to volume ratio- deeper liquid means gas molecules have farther to go to leave the liquid compared to a shallow liquid in a wide container where gas moves pretty rapdily just because there's not that far to go. An oversimplification, but you get the idea.

-Ernie

Hey Ernie, No offense taken, I know you and I can get pretty good test results out of a $30 pH Pen. But you and I have had years of wrangling pH with professional grade equiptment and really understand the nuances of conditioning a pH electrode and calibrating a slope for the electrode. A couple years ago at a bonsai club meeting I watched an abysmal demonstration on measuring soil pH by a retired M.D., a physician who you would have thought would be able to make the $90 pH pen she was using work, and she so jazzed it up that she did our little bonsai group more damage than help. She was well meaning, I said nothing, as so to not embarras her, but the point is, even people you would think by their education could do it, getting the technique down to get valid pHs out of inexpensive equiptment is really not easy to teach or learn. Good lab technique is almost one of those 'Oral Traditions'. In this I mean most of the best little tricks are not written down, and are passed on generation to generation by our secret cult of Lab Technicians. ;) Even the PhD and MD researcher bosses don't know our secret Lab Tech Code. I know you haven't showed your boss everything. :evil:

So with that thought in mind I tend to downplay the need for going the technical route. You are right - it can be done, but trying to explain all the technique needed around conditioning and calibrating a pH electrode is exhausting.

Leo, Many articles and many times on this forum it is said that there are some orchids that like a higher or lower ph, depending on their native habitat. Advice is given on how to change the ph to meet those supposed optimums. Are you saying don't worry about this just use drinking water and do not worry about the ph?

This goes also to the point where Ernire and I agree. James Asher published in Orchid Digest in the early 1980's an article where he went to Sumatra, and found Paph glaucophyllum growing on a limestone cliff. He brought along a pH meter with a micro-probe. He measured the pH of the water trickling down the cliff, and then he measured the pH of the water film surrounding the root tip of the Paph glaucophyllum. He took a significant sample (I forget numbers, but it was more than 2) the results were interesting, and the reason Ernie and I both think a pH of 5.5 is not a problem for the s/h solution. The water trickling down the cliff was mildly alkaline as expected (I think ? pH 7.8 ?). The pH of the water on the root tip was I believe 5.6. Memory fails me for the exact number - but Asher and the rest were surprised at first. A mildly acidic pH is ideal for solublizing the over-all greater number of nutrients (see Ray Barkalow's excellent references on his First Ray's website). It turns out Paphs specificly and most plants in general, excrete buffers into the water film around their root tips to bring the pH into their ideal range for optimal nutrient absorption. If your plant is healthy, and has lots of root tips, the plant will modify its own environtment to its preferred range. This is one reason not to over-pot an orchid, also the reason seedlings in community pots grow faster than the same size seedlings in individual pots. Less media per root tip, each plant has to work less hard to buffer the mix and the plant has more energy left over for growing. I would say pH's between 5.2 and 8.8 are mildly acidic to mildly alkaline. This is the range that generally I would not get excited about. Healthy plants will be able to take care of themselves in this range.

Now Corbin asked about pH Calciphyles. This is not well understood, to the potting mix of all my calciphylles I add crushed oyster shell or horticultural lime. Paphs like glaucophyllum are found ONLY on limestone cliffs. Asher's article proves it is NOT soil pH that determines habitat selection for glaucophyllum, I believe the speculation is that either the adult plant itself has a higher need for calcium & magnesium OR the fungal symbiont that the seedlings need to sprout requires a higher presence of Calcium & Magnesium. So the extended thought from this observation from Asher is that the Paphs that are listed as calciphyles, DO NEED the limestone in their mix, but the reason is not for pH adjustment, but rather mineral nutrition.

So I believe, especially when it comes to Paphs, when the dicussion revolves around soil pH, most of the time the real issue is a need for limestone, a source of calcium & magnesium. Sometimes, especially with certain Phrags, it does actually seem to be pH itself as the issue. Phrag extaminodia, really does seem to want a lower pH, less than 6.0 for sure. Phrag fischeri and kovachii do seem to do better at higher pH's, above 6.0 and better with limestone in the mix. But for Paphs I really think the issue is limestone, not pH specifically.

If your municipal water starts you off at less than 800 ppm dissolved solids, is pH neutral enough that you can drink it without burning your mouth. (By this I mean 5.5 to 8.5). It is probably good enough for most Paphs and Phrags. (Note: it is not good enough for Disa, or Nepenthes and other carnivorous) Most of the problems people claim are hard water problems are actually problems with drying the plants out too hard between waterings.

That's my 2 cents. I try to keep my orchid growing non-technical, by being careful about setting up the growing conditions. By this I mean - I use a standard fir bark based orchid mix - which I know will have a mildly acidic pH, I add oyster shell for the brachypetalum, cochlopetalum & other calciphyles. I use a good scientifically designed fertilizer like MSU Orchid Special or some of the DynaGro products, I avoid unbalanced fertilizers like the so called 'balanced' 20:20:20 or the true death in can 'Blossom Boosters' 0:18:0. And I really try hard not to dry my orchids out hard between waterings. I make sure I have good air movement. Cover these bases and as an issue pH will dissappear.
 
Allot of good points. Here's another way to look at the pH drop in terms of what's already been stated.

The pH increase from OH is not stable and easily reduced by acidic substances including CO2 from plant respiration. The buffering content of carbonates and bicarbonates (from limestone and oyster shell) is much more stable and offers more stable pH levels against similar acidic substances.

The root zone pH ranges mentioned are generally safe and offer decent nutrient transport to the plant, although phosphorus uptake gets more restrictive below pH 6.
 
sourse of acidity

Potasium hydroxide traps carbondioxide from the air(CO2+OH---> CO3 + H+)
When al hydroxide is removed the pH will drop below 7 due to the acidity of carbondioxde.
a pH of 5.5 is very normal for water at equlibrium with carbondioxide.
Tap water is less sensitive to this pH drop.

I think the pH meater is correct (you can check this with buffer sollutions at fixed ph)
 
Throwing my 2¢ in... I recall that a few of us did some pH investigation of S/H reservoirs several years ago (discussion lost, unfortunately, upon a server change for the domain) and found levels as low as the low 4's, and that it varied not only with the length of time since the last watering, but with the time of day tested, that is, just before sunrise or just at sunset.

All of that, by the way, was with "old PrimeAgra".

On that subject, I still am not seeing the "problems" with pH that others have observed with the current version of the medium. I chalk it up to the fact that I have probably leached the manufacturing residues from the clay body. To test that, I took a pot of PrimeAgra that had been in the GH for about 6 months with no plant (damned thing got knocked out of it, but was hidden, then I just left it that way, so it had been watered regularly for the duration), soaked it for 24 hours in an excess of RO water, then put half of it into a plastic container of RO, and half in my well water (pH 7.1 when I drew it) and left them in my basement for 3 months. The pH of the RO liquid was 5.7, which is in the range expected of RO exposed to CO2 in the air, while the well water was 7. It looked to me like there was little-to-no interaction with the clay.

Some will write my comments off because of my business stake in this, but I like to think my scientific curiosity outweighs that. Besides, my plants don't give a damn about that, and they're fine.
 
I chalk it up to the fact that I have probably leached the manufacturing residues from the clay body.

That is exactly right. As I recall you said you water heavily so you not only leach out any impurities in the leca but you also replace all media moisture on a daily basis.

The problem is because most people rely on the s/h method to reduce the frequency of watering the plants. People often say they "top off" the reservoirs and this is when the chemistry goes from good to bad.

Did you do this test with old or new PA?
 
Hi, Lance. It was new.

I always recommend that folks not only top-off the reservoir, as it really does not do any kind of flushing, to speak of.
 

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