pH drop 6.5->4 with MSU RO/DI fertilizer?!?

[tropterrarium]

I just got some MSU "pure water" fertilizer form First Ray. I use a DI system with TDS of ~0-5 after pH adjustment with KOH from dosing pump. After adding a good teaspoon of the fertilizer to the 2.25 gals of water, pH dropped to 4! Not good. I was under the impression that the pure water formula was pH balanced for pure water in the pH6.3-6.5 range [certainly not 4].
I could get pH back up with adding 1 teaspoon of Protect, but got the pure water MSU fertilizer to avoid that adjustment in the first place. It's no improvement over the Dynagrow+Protect I have been using before.

Questions:
- Is my assumption that pure water formulation is pH balanced around 6.3-6.5 wrong?
- If so, what is the point of pure water formula?
- Or is First Ray's no good?
- Or do you make a stock solution, adjust stock to pH 6.3-6.5, then add that to DI water? Does that work, or will pH change cause settlement of certain compounds (good old solubility products from chemistry)?

[tucker85]

Contact Ray. I'm sure he will be happy to help you with this issue.

[tropterrarium]

Hi Tucker,

thanks for the suggestion. I know that Ray is often on OB. I am looking to get a more diverse opinion.

[DavidCampen]

How are you measuring the pH?

Which Dynagro formulation are you using? Using a narrow range pH paper I get a pH of 5.5 for Orchid-Pro diluted to 275 ppm (using a Hanna Instruments EC/TDS probe) in RO water.

Also, I would suggest to you that it would be better to adjust the pH of your DI water with something like potassium acetate instead of potassium hydroxide. With KOH a slight overdose can make the water extremely alkaline, like pH 13. Also KOH has no buffering effect thus only a minute amount of acid would bring your pH very low.

Another point, after you have made a fertilizer solution, if you want to raise the pH I would suggest using potassium hydroxide instead of Protekt (potassium silicate) especially if you are making a concentrated stock solution.

[keithrs]

DI, RO, distilled water are all unbuffered.... This means at there pH can change very fast when you add chemicals. You can try adding back 10% tap water to buffer the water.

My notes show a pH of 5.8 with my RO MSU and 5.9 with K-Lite in RO water. And that is when I was feeding @ 125 ppm N. I have not tested it since extremely reducing my feed. Just for reference.... My RO water has a pH of 6.7-6.9.

I tested my stock and it has a pH of 1.9-2.

[tropterrarium]

Thanks for the pointers. I am well aware that DI water has close to zero buffering capacity. Blending in tap water is not appealing, as it will raise TDS. I try to keep TDS low before adding fertilizer.

I have not checked TDS after adding fertilizer.

I use the Dynagrow green label.

I measure pH with a glass bulb electronic pH probe, part of my HerpKeeper control system. And yes, I do a two point calibration on a regular basis.

Re overshooting with KOH, that I have under control with lower concentration stock KOH (~0.5M), and using a dosing pump, having water circulation pump in storage 30 gal tank, and automated feedback loop via pH probe:
-Check pH
- if pH<6.1, run dosing pump for 3 seconds.
- if pH>6.4, do NOT run dosing pump.
- wait 15 minutes
[Loop back]

Maybe once I run out of KOH, I may switch to Kacc.

Adjusting pH of a stock solution rather with KOH than Protekt is certainly the way to go.

So, what I gather is, that the pure water formula is not pH balanced for optimum 6.3-6.5. Interesting.

Will try to do a test batch with stock solution with KOH adjusted pH, see whether I get any precipitation.

[Ray]

The MSU RO formula is designed to give a pH between 5.5 and 6.5. Some of the adjustment is done via the addition of citric acid.

The product, however, is manufactured at a plant that produces fertilizers for a large–scale growers that are likely to use full, 25 pound bags at a time, if not several, so the homogeneity within a single bag is not of paramount importance to them.

When I break down a bag into smaller containers, I try to mix the bag contents manually to improve upon that, but cannot guarantee that the contents of the jar is homogeneous, which is why I recommend right on the label that folks make a concentrated stock solution out of at least half the jar – the whole jar is better – and then use that to mix up their final irrigation solution.

I suspect that you simply grabbed a portion of the contents that was somewhat higher in the citric acid, or that your particular jar was.

[tropterrarium]

Thanks Ray. I did not get that you recommend making a stock at all. Neither on website, nor on label. It just looks like an option, nothing more. Possibly consider revising description on website as well as on label.

The main table is for teaspoon of powder per gallon for different ppm fertilizer concentration, so indicates (wrongly) that this is the suggested application.

Tried to do a somewhat scaled-down stock (1/5 gal). Took about 5 g of pellet KOH to get to pH 6.3-6.5. The solution turned from transparent azure blue, to milky light blue, so there seems to be a lot of precipitation going on. Will have to wait for a day to see what is going to settle out.

After that, I will have to find some 2 gal container to keep the stock of the remainder of the 2 lb powder container.

I guess the precipitation problem will remain, and will have to stir the stock before taking out the 1 ounce/gal (or so).

[Ray]

The recommendations on concentration are from the inventor of the formulations. Those numbers are not "wrong" at all, they do give the approximate amount needed for the stated ppm levels. Approximated because ppm is based upon mass, not volume, and the bulk density of the solid matter may vary a little bit.

I suggest that you need not be so concerned about the pH. Citric acid is a very weak acid, added to enhance the solubility of some of the other minerals, and once the solutions come in contact with potting media and residues in the pot, its impact is minimized.

Several years ago, a number of us did some testing of the solution in the reservoir of S/H pots, and depending upon how recently one watered, and when in the day the measurement was taken, we saw pH's as low as 3.5 in healthy plants.

I have used the Greencare MSU formulas for several years, and have never had any plant health issues, even early on, when the citric acid addition was even greater.

Try warming the solutions before mixing to avoid the precipitation.

[tropterrarium]

Hi Ray,

Do I understand you correctly, that you think that pH deviation 2-3 steps of "recommended" do not matter? This goes very much against anything I've ever heard. because of pH dependency of nutrient availability. I don't believe in the singular perfect value either, but would think that variation of a total of 0.5-1 pH value (+/- 0.25-0.5) would be more like an acceptable range.

Re healthy plant at pH 3.5, that may be the case for some bog orchids, or some hardy species/hybrids. It may also be ok for a one-time aberration, but doubt it is sustainable over long time for many species. I grow mostly odd balls (pleurothallids, Oberonia, miniatures) so try to be rather careful.

Re citric acid being "only" a weak acid does not make sense. Whether you add a little of HCl (or even HF) or more of citric acid, if it reaches the same pH, it's all the same, particularly in a more concentrated solution (such as a stock) acting as a buffer system with buffering capacity.

Heating certainly improves solubility. However, heating 2 gals of stock every week is not terribly practical. Stirring the pot may be more reasonable, and when adding the stock to the water, the precipitates should dissolve again, assuming they are of similar compounds as the original mix, not new compounds with lower water solubility. I will know soon enough how it looks and which approach works.

Good to hear that you've never run into problems. I guess one is more paranoid when changing any parameter, so I am extra careful.