pH

[dodidoki]

Few measurments.
Tap water here, EC 522 uS, pH 7,2, hardness 22 Nk

Contained rain water here, EC 46 uS, pH 7,06.

Fertilizer solution, Peters Excel CalMag + rain water, EC 600 uS, pH 3,66 (!!!!)

Same solution 2/3 part + tap water 1/3 part, EC 535 uS, pH 6,5.

I will use this last recipe in the future.

I think everybody should check the fertilizing solution before using and if nessesery, should correct its pH!!! Unfortunately there is no any word mentioned about that by factory.

 

[Martin]

Good advice! Since i do so i have better results. For example my rain water from some days ago. After adding fertilizer i end up at a ph of 3!

 

[Ray]

From what I've read and been told by PhD's in the field, the pH of the solution has very little impact on the substrate pH, unless it has such high alkalinity (resistance to pH change upon the addition of an acid) that is results in the gradual increase of the substrate pH.

I remember when I was first experimenting with the development of S/H culture for orchids, using a neutral substrate, I thought the solution was the only thing that determined to pH. Boy, was that wrong. Using a carefully adjusted 6.0 solution, I saw the pH swing as low as 3.5 and as high as 7.5 from a single plant, depending upon the time of day (which I interpreted as which chemical processes were going on within the plant). When I used an unadjusted pH=4.0 solution , I saw the same thing.

 

[terryros]

From what I've read and been told by PhD's in the field, the pH of the solution has very little impact on the substrate pH, unless it has such high alkalinity (resistance to pH change upon the addition of an acid) that is results in the gradual increase of the substrate pH.

I remember when I was first experimenting with the development of S/H culture for orchids, using a neutral substrate, I thought the solution was the only thing that determined to pH. Boy, was that wrong. Using a carefully adjusted 6.0 solution, I saw the pH swing as low as 3.5 and as high as 7.5 from a single plant, depending upon the time of day (which I interpreted as which chemical processes were going on within the plant). When I used an unadjusted pH=4.0 solution , I saw the same thing.

Ray, I have read that most orchid roots take up the substrates that initially contact them and have a limit to what they can take up at a time. Much of what we pour into the pot probably runs out the bottom. In a completely hydroponic system (frequent solution flowing through from the top), the pH of the solution probably has some impact on the solubility and uptake of at least some ions. Semi-hydro with an upward-wicking reservoir at the bottom may be more complicated. An organic medium that stays wet is also different. Since I am using only LECA in pots and watering frequently from the top, I think I need to have a fertigation solution that is between 5.5 and 6.1 with 5.8 the sweet spot.

 

[Arne]

Ray is right, the pH of the water doesn’t mean much, especially not in pure water without any salts. So when everyone gets their coveted pure water they also allow the pH to plummet when they add fertilizer. Then they get worried and add salts back to restore the pH.

If you check the pH immediately after the water has been in contact with orchid medium you would have noticed that it is much closer to neutral.

 

[dodidoki]

If you check the pH immediately after the water has been in contact with orchid medium you would have noticed that it is much closer to neutral.

Yes, in the begining. Medium has some buffer capacity but if you flush it through regulary , this capacity will lost.

 

[Ray]

Ray, I have read that most orchid roots take up the substrates that initially contact them and have a limit to what they can take up at a time. Much of what we pour into the pot probably runs out the bottom. In a completely hydroponic system (frequent solution flowing through from the top), the pH of the solution probably has some impact on the solubility and uptake of at least some ions. Semi-hydro with an upward-wicking reservoir at the bottom may be more complicated. An organic medium that stays wet is also different. Since I am using only LECA in pots and watering frequently from the top, I think I need to have a fertigation solution that is between 5.5 and 6.1 with 5.8 the sweet spot.

Terry, I have referred to that in the past as "exposure period" when considering the mass of uptake.

A bare-root vanda, for example, is only exposed to the solution while it is being irrigated, and I suppose with a lot of measurement assumptions, once could calculate the mass of nutrients absorbed by calculating the total volume of the plant's velamen. In potted plants, there is bound to be a period of continued uptake from the solution in contact with the roots, whether immediate or wicked to it. I am not aware of any studies to measure that velamen-to-vasculum transfer rate.

In both cases, however, the plant has processes that allow it to maintain ionic neutrality. When they take up a negatively-charged nitrate ion, they emit a negatively-charged OH or HCO3 ion. When they take up a positively charged ammonium ion, they emit a positively-charged H ion, and those emitted ions are what affects the pH.

Unless there is a part of plant physiology/chemistry that I don't know about (and there is an extensive list), I cannot see how a change in the solution pH - within a reasonable range, that is - can affect uptake. The chemicals ions in solution are the same at pH=4 as they are at pH=9, orchid media have extremely low cation exchange capacities, so that's not affecting it like soils do, so unless the pH has a direct effect on the plant's actions (the unknown part), the uptake is unaffected.

Plus there's both the alkalinity of the solution and the time factor to consider. "Alkalinity" referring to the resistance of the solution to pH change upon the addition of an acid - buffering capacity, if you will. Pure water has none, so adding a fertilizer causes the pH to swing drastically, and the fertilizer additions we are making are so small, they really don't affect the alkalinity much, allowing the plant to continue absorbing and expelling ions at will. If the water alkalinity is high, however, it resists changes, causes the overall rhizosphere pH to gradually increase, which stifles those ionic transfers, negatively affecting nutrient uptake.

 

[terryros]

Many thanks, Ray. Are you skeptical about pH affecting availability/solubility of ions for the whole hydroponic situation? Here is my cropped screen shot of a chart I found online some time ago, but I am sorry I can't cite where.


This is what gets everyone fixated on pH. I suppose if you have a pH at which an ion is poorly soluble it might not be able to enter the velamen and then the ion exchange wouldn't matter. When I have tested K-Lite at standard concentration in RO water , I get approximately the EC that you state and I get a pH that is within the "safe" range on the picture. I think the same is true for MSU.

 

[Tom-DE]

Thank god I have good water with ~6.0pH, and I don't feed them often.......

 

[Ray]

Terry, the problem I have with charts such as that is that they imply that the ions are soluble in a specific range and insoluble outside of that, and that solubilities are independent of each other, and that's simply not how chemical solubility works.

For example, that chart suggests that calcium is soluble between a pH of roughly 4.5 to 5.8, but the solubility of CaNO3 in 20°C water at pH 7 is roughly 1200g/L.

 

[dodidoki]

.

For example, that chart suggests that calcium is soluble between a pH of roughly 4.5 to 5.8, but the solubility of CaNO3 in 20°C water at pH 7 is roughly 1200g/L.

Ray, the problem is with this example, that Ca(NO3)2 dissolves in water in 3 parts, 2NO3- and Ca2+. No3- is a strong acid , Ca2+ is a medium strong basis, so this solution will be strongly acidic. If you increase pH, with other words, the cc of OH-, Ca(OH)2 will precipitate, it is weakly soluable, so free ca2+ cc in the solution will decrease.

 

[terryros]

Ray, the problem is with this example, that Ca(NO3)2 dissolves in water in 3 parts, 2NO3- and Ca2+. No3- is a strong acid , Ca2+ is a medium strong basis, so this solution will be strongly acidic. If you increase pH, with other words, the cc of OH-, Ca(OH)2 will precipitate, it is weakly soluable, so free ca2+ cc in the solution will decrease.

OK, Istvan is getting serious. There must be some experimental evidence behind images like I showed. What were they measuring at the different pH? Is it free ions in solution? I know this is talking about a pure hydroponic situation and the media in a pot will alter these things, but the closer your situation is to hydroponic the more the pH of the fertigation solution might matter.

 

[Ray]

Ray, the problem is with this example, that Ca(NO3)2 dissolves in water in 3 parts, 2NO3- and Ca2+. No3- is a strong acid , Ca2+ is a medium strong basis, so this solution will be strongly acidic. If you increase pH, with other words, the cc of OH-, Ca(OH)2 will precipitate, it is weakly soluable, so free ca2+ cc in the solution will decrease.

I'm in 100% agreement, but in the concentrations that we are dealing with and the pH ranges we're dealing with, I question whether these are issues from a practical perspective.

 

[Roth]

In short as it would need hours of explanations:
- pH does affect heavily the ion uptake by the plants...
- Soluble vs. insoluble elements is not really relevant, as plant roots are adapted, depending on the species, to extract insoluble ions if needed. That's what can pose a problem indeed sometimes, as some are adapted for minimal uptake of boron, in an environment that is loaded with it. With a proper fertilizer, the plant will be deficient, where a plant adapted to take up a lot of boron will be heavily intoxicated by the same concentration.

= The pH importance is dependent on the organic content of the substrate, and the EC. That's what people call 'fertilizer burns' all over the place on internet. A pH of 3 with an EC of 10 microsiemens will be compensated ( except organics) within seconds by the plant. A pH of 3 with an EC of 2 will eventually make the plant deficient in some key nutrients, such as potassium. Leaf tip burns, roots stop growing, and people wrongly assume the ' fertilizer was too strong' as one of many examples.

A ' fertilizer burn' at an EC of 2 is borderline impossible for orchids, even Disa ( been there done that). That the leaves become yellow, tips crinkled, roots stop growing is very likely if the pH is 7.5 or 4 with such a fertilizer ( or if there is no ammonium/urea nitrogen, but that's another story.

Feeding is very complex and most articles published are either voodoo or hearsay. The only way to really discuss about feeding orchids is tests of several parameters with control, potting mix analysis, drain water analysis, and leaf mineral analysis with comparisons...

Roots that stop growing or leaf tips that become yellow then become brown are very rarely the result of a 'fertilizer burn'. Until now, I have seen only 2-3 times a real fertilizer burn in orchids with symptoms ( and leaf mineral analysis...), and those were for EC of 4-5, way above anything any hobbyist will ever use, or any commercial hobby plant grower.

100% of the Ludisia jewel orchids sold as pot plant in Europe are raised by 1 company ( and 1 contract grower of it...) with an EC of 2.8-3... but the formulation is perfectly adjusted, in terms of micronutrients, ammonium/etc... contents and much more.

 

[Ray]

Well-stated, Roth.

I will say that I have seen fertilizer burn (or maybe poisoning is a better description).

When I volunteered at what is not the Atlanta Botanical Gardens, back in the 70s, they fed their cattleyas monthly. Their technique was to mix up a very high concentration of 30-10-10 - several tablespoons per gallon - and meter it through a siphon-type device AFTER heavily watering with plain water.

Guess what? A new employee didn’t water first…

 

[dodidoki]

In short as it would need hours of explanations:
- pH does affect heavily the ion uptake by the plants...
- Soluble vs. insoluble elements is not really relevant, as plant roots are adapted, depending on the species, to extract insoluble ions if needed. That's what can pose a problem indeed sometimes, as some are adapted for minimal uptake of boron, in an environment that is loaded with it. With a proper fertilizer, the plant will be deficient, where a plant adapted to take up a lot of boron will be heavily intoxicated by the same concentration.

= The pH importance is dependent on the organic content of the substrate, and the EC. That's what people call 'fertilizer burns' all over the place on internet. A pH of 3 with an EC of 10 microsiemens will be compensated ( except organics) within seconds by the plant. A pH of 3 with an EC of 2 will eventually make the plant deficient in some key nutrients, such as potassium. Leaf tip burns, roots stop growing, and people wrongly assume the ' fertilizer was too strong' as one of many examples.

A ' fertilizer burn' at an EC of 2 is borderline impossible for orchids, even Disa ( been there done that). That the leaves become yellow, tips crinkled, roots stop growing is very likely if the pH is 7.5 or 4 with such a fertilizer ( or if there is no ammonium/urea nitrogen, but that's another story.

Feeding is very complex and most articles published are either voodoo or hearsay. The only way to really discuss about feeding orchids is tests of several parameters with control, potting mix analysis, drain water analysis, and leaf mineral analysis with comparisons...

Roots that stop growing or leaf tips that become yellow then become brown are very rarely the result of a 'fertilizer burn'. Until now, I have seen only 2-3 times a real fertilizer burn in orchids with symptoms ( and leaf mineral analysis...), and those were for EC of 4-5, way above anything any hobbyist will ever use, or any commercial hobby plant grower.

100% of the Ludisia jewel orchids sold as pot plant in Europe are raised by 1 company ( and 1 contract grower of it...) with an EC of 2.8-3... but the formulation is perfectly adjusted, in terms of micronutrients, ammonium/etc... contents and much more.

Dear Xavier, how do you mean this , eg. EC 2 ??? 200 uS/cm?

 

[Roth]

Dear Xavier, how do you mean this , eg. EC 2 ??? 200 uS/cm?

No no, 2000 microsiemens... It works great, but only if the pH is carefully controlled at the roots, including when the plants takes up nutrients... and with an optimization of the micronutrients.

Most Phalaenopsis for pot plant are grown at an ec of 2000-2400 microsiements so far....

 

[dodidoki]

i wrote 200...no 2000....2000 is horrible high, i know...My equipment measures uS/cm.

 

[terryros]

I was assuming Roth's 2 meant 2,000 on my meter. Those of us who have been using under 1,000 for a while can forget that values between 1,000 and 2,500 have long been considered safe and are still in use. If I am reading Roth correctly, the lower the fertilizer concentration I use the more important the pH can be for at least certain ions. Please correct me, Roth!

 

[dodidoki]

I was assuming Roth's 2 meant 2,000 on my meter. Those of us who have been using under 1,000 for a while can forget that values between 1,000 and 2,500 have long been considered safe and are still in use. If I am reading Roth correctly, the lower the fertilizer concentration I use the more important the pH can be for at least certain ions. Please correct me, Roth!

Thank you for correction.