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

[Ray]

Hey Trop.

I think that you may be missing the differences between some acids (bear with me if I am explaining this poorly...): when placed in environments with other chemicals, the bond strength makes a huge difference in whether it becomes a "bad player".

Citric acid and HF, at the same solution pH, do not react the same. The citric acid (H3C6H5O7) can be dissociated very easily, and actually has 3x the number of hydrogen atoms (which is how the pH is affected) than does the HF, rendering its acidity to be very weak and easily overcome by the other chemical species present. The HF, on the other hand, is very strongly bonded, so when it is added to a solution, it "takes over" and tends to influence other species, instead of itself being "pushed around".

Also, don't let that old chart of nutrient availability versus pH influence you too much. It was a plot of experimental data from a single fertilizer formula, poured through a single SOIL sample, and soils have a very large cation exchange capacity compared to orchid media. The pH affects the charge polarity and density of the exchange sites, and that's what determines whether the nutrient ions were bound to those exchange sites in the soil or not - with "not" meaning it's available to the plant.

As our media do not have nearly that level of CEC, the pH does not play a major role in determining if the ion gets trapped or not, so basically, if it is still in solution, it is available to our plants.

As to heating, I try to mix my solutions with hot water, then let them cool. The solubility is, as you mentioned, higher at higher temperatures, and they tend not to precipitate when they cool slowly. I see some sediment whenever I use cold water. It's probably undissolved soluble species, rather than an insoluble precipitate.

Let me add that I, too, would be concerned if I saw a very low solution pH, but I don't view it as a "life or death" situation.

[DavidCampen]

Originally Posted by 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.
...

I don't think the nutrient availability vs pH information that I have seen is very accurate for orchids in bark or even spaghnum. Perhaps those numbers were accuate for some (unspecified) type of soil.

I would be concerned that a 2-3 unit variance of pH from specification indicates that the material is not well mixed. To get a multicomponent mixture like these well mixed so that it will not separate during shipping you really need to make a solution of all of the components and then use spraying drying to get it back to solid.

A significant component of my decision to use the Dynagro products was that they are supplied in liquid form. Liquid concentrates like Orchid-pro though have problems with solids forming over time so that when I make up a batch of Dynagro Orchid-Pro I use the entire bottle and start a week ahead of time trying to dissolve the stuff that has come out of solution (and not all of it will dissolve).

Soon I will start preparing my own formulations from pure chemicals. I plan to use a two component liquid concentrate formulation with phosphate, sulfate, borate and molybdate in one component and calcium, magnesium and the other minor elements in the other component. The cationic minor elements - iron, zinc, manganese, copper and cobalt I will use as amino acid chelates. At the moment I have prepared zinc and copper glycinate and have iron as ferric ammonium citrate but I have aspartic and glutamic acids on order and will probably switch over to using those to prepare the chelates.

[tropterrarium]

Hi Ray,

thanks again for the detailed info. I fully understand difference between single, dual and triple etc. protons on a particular compound, and how it affects pH (molarity vs. normality: 1M citric acid is 3N).

What I was getting is, whether you add x amount of citric acid to reach pH n, or y amount of HF to reach pH n, it is the same pH, regardless of how strongly the respective acids/bases dissociate [let's ignore any other potential interactions of F- with whatever is floating around in the fertilizer; one of those "everything else being equal" exercises.]

Citric acid is quite reasonable, as it is a basic metabolite in plant physiology: citric acid cycle. Long forgotten any details from plant physiology classes I had to take way back ... Also is solid at room temp & pressure.

Interesting point re cation exchange sites and binding of nutrients. After a few fertilizations, those sites should be saturated and proper pH water in non-fertilizing waterings should get those also more available. Once the medium breaks down, it will be a different story. I mainly grow on mounts, either bare roots on bark/wood, or with some moss.

Will want to run that thought by some local growers and see what they think. We have Wed Society meeting and on Saturday a show.

Will kick myself, if it turns out I tried to fix a non-existent problem! :-o Good to think it through, anyway.

[Ray]

Originally Posted by tropterrarium

What I was getting is, whether you add x amount of citric acid to reach pH n, or y amount of HF to reach pH n, it is the same pH, regardless of how strongly the respective acids/bases dissociate [let's ignore any other potential interactions of F- with whatever is floating around in the fertilizer; one of those "everything else being equal" exercises.]

I totally agree, but was focusing MY thoughts on what happens once it's in the pot.

Will want to run that thought by some local growers and see what they think. We have Wed Society meeting and on Saturday a show.

Great idea - although I am always wary of the thoughts of growers without a scientific background, as they often respond with some of the same malarkey we've been hearing for years.

Will kick myself, if it turns out I tried to fix a non-existent problem! :-o Good to think it through, anyway.

Nonexistent, no. Serious, ????