Questions on Avoiding Fluoride

snowflake-rosaSeptember 13, 2008

Hello! I recently purchased a slightly bedraggled Dracaena of the Warnekki variety. The plant was on clearance because of brown spots on the leaves but other than that it seems very healthy. I believe the brown spots meet the descriptions I have read from fluoride toxicity. I have been leaving water out over night before watering. (I have a prayer plant and calathea which both require this anyways) I was just wondering how to avoid fluoride problems with fertilizers and soils. I have the standard Miracle-Gro Houseplant Fertilizer and the other plants have responded well to it. Does anyone know if this would be okay for the dracaena?

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tapla (mid-MI z5b-6a)

"I was just wondering how to avoid fluoride problems with fertilizers and soils."

First - it's more likely that the source of the lesions is something other than fluorine toxicity. The best way to avoid fluorine toxicity issues is to insure that the soil and soil solution pH is between 6.0 - 6.5, and use fertilizers w/o superphosphates at reduced rates and more frequent intervals. This will prevent F toxicity in all but the most extreme cases.

Occasionally, symptoms of fluorine toxicity begin as wet lesions (spots) near leaf margins and tips that eventually involve large portions of the leaf, but in most cases symptoms are limited to leaf tips and margins. In a large % of cases, symptoms believed to have been caused by fluorine toxicity can be traced to other issues - too much light, over-fertilization, high soluble salts levels in the soil, ....


    Bookmark   September 13, 2008 at 2:38PM
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Ah, the perils of houseplants. I agree that its difficult to nail down one particular ailment on a leaf as being from one particular cause.
One of the houseplant guides I go by suggests leaf tips and leaf margins having browned can be from
over, or under watering
too much sun through a window, too much fertilizer, water with a too high degree of salt concentration, not enough humidity, locating the plant in drafts.....

or.....a combination of any and all of them.
I don't wish to go into why yellow leaves sometimes show up because it'd take maybe too much space.

    Bookmark   September 13, 2008 at 5:59PM
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rhizo_1 (North AL) zone 7

snowflake, fluorides won't evaporate from your drinking water....sorry. Chlorine does, and that's why people get confused.

Yes, MG fertilizer would be perfectly fine for any of your plants, as long as you understand the problems of using too much, too often.

    Bookmark   September 13, 2008 at 9:19PM
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john_z(Z 7b Ga.)

Hi Snowflake-rosa!

There is naturally occuring flouride in perlite, and even in peat moss. But the toxic levels come from what is added to tap water, as well as fertilisers whos source of phosphorous is superphosphate or triple-superphosphate. I don't know offhand if Miracle-Gro uses phosphorous from these sources. (Personally, I boycott the Scotts company's products).

Dracaena 'Warneckii' is at the top of the list for fluoride sensitivity, along with D. 'Janet Craig'. Maranta species ("Prayer Plant" included) are also sensitive to fluoride, but Maranta is also sensitive to accumulating fertiliser salts. Fertilise this one sparingly, as well as Calathea! And although chlorine is the most used micronutrient for most plants, most of the plants in the family Marantaceae are damaged by the amounts of chlorine added to tap water. Calathea is one genus in this family.

The brown spots on the Warneckii are most probably fluoride damage. If wet lesions appear - there is a bacterial problem. If raised bumps occur on the epidermal surface of the leaves, this is oedema. Bacterial infections and oedema are not directly related to fluoride content in water.

To avoid fluoride toxicity, don't give fluoridated water to fluoride-sensitive plants. Either filtre it out with an activated charcoal filtre, use bottled spring water (insignificant amount of naturally occuring fluoride), and don't use fertilisers whos phosphorous is derived from superphosphate or triple-superphosphate.

There is a huge problem with attempting to get the pH up to 6.0 to diminish fluoride uptake. (And by the way, many sources of dolomite have enormous amounts of naturally occuring fluoride). Without a very pricey pH meter, there is no way to assess what the actual pH is in the potting medium, and therefore no way to assess how much dolomite to INCORPORATE into the potting soil. If that pH is too high (especially at 6.5 +) you will indeed begin micronutrient deficiencies. The pH controls or regulate nutrient uptake.

    Bookmark   September 16, 2008 at 8:51PM
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tapla (mid-MI z5b-6a)

Container soils are generally adjusted to pH levels of 6.0-6.5 to allow for upward creep of pH as soils age. Of course pH affects nutrient uptake, but pH of 6.5 presents no problem (and certainly not a huge problem, in either container or in situ soils, unless the plant is possibly inefficient at assimilating Fe or Mn). We shouldn't expect pH related deficiencies of either macro OR micronitrients until soil solution pH is approaching a pH of 8.0, and then the deficiencies will likely only be the Fe/Mn mentioned earlier.

BTW - activated charcoal filters (especially like those sold to treat tap water) do not remove fluoride. Fluoride is a negatively charged ion and does not react with charcoal or the ion exchange resins in the filters.


    Bookmark   September 16, 2008 at 10:09PM
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Thank you John for your response. I know that flouride is my problem with some plants along with other things in our very hard water.

    Bookmark   September 16, 2008 at 10:10PM
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john_z(Z 7b Ga.)

Hi Al!

Thanks for your response. It appears that I have received erroneous information regarding fluoride removal from what I thought were the most reliable sources: I had two horticulture instructors (both with a Ph.d)tell me that charcoal filtration will remove most of the flouride unsafe for plants after I expressed my concerns about tap water for the flouride-sensitive ones. Then, when I considered getting filtres for fluoride removal at home, two sales reps (roll eyes)said essentially, "yep - charcoal filtration will do the trick". Who was I to think otherwise? Good thing I never bought those filtres. For years I've been relying on bottled spring water without added fluoride, and there have been no signs of toxicity on my Chamaedorea, Dracaenas, Maranta, etc. (Of course, there are other factors to take into consideration).

I also googled what you said regarding charcoal and fluoride removal, and it appears you have others to support your statement. I see now that there are filtres especially designed for fluoride removal (many with activated alumina and/or a SIR-99 synthectic alumium oxide). Some sites mention that charcoal filtration will remove some fluoride (what percent?), but that it should not be relied upon for sufficient fluoride removal for humans. Fluoride removal for plants is not mentioned, but reverse-osmosis is generally used by professional growers.

Regarding raising the pH above 6.0 to 6.5, the graphs that I have (for various soilless potting media)show that elevating the pH above these levels spells danger for nutrient uptake. But the larger question is how the average plant lover will be able to assess and monitor the pH without pricey equipment. The metre I used in a lab when I was a student cost over $2,000 USD - and that was just for pH assessment.

I'm using a Hanna "Checker" metre at home that requires calibration, has an electrode cleaning solution, ph buffering solutions, etc. Because it is not laboratory quality, yet not the utterly useless metre you'll get at the garden centres - I apply dolomite modestly, and strictly for the nutrient value. I do not rely on elevating the pH for reduction of NaF uptake.


    Bookmark   September 17, 2008 at 8:05PM
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Hey Al,
Just regarding your comment that pH of 6.5 offers no problems for container plants unless they are inefficient at processing Fe. I have previously seen you mention that 'Growing Media for Ornamental Plants and Turf' by Kevin Handreck & Neil Black as a good source. I am currently studying this book for a Tafe course, if you see on p. 91 - Figure 11.2 it shows the relative nutrient availability of container plants, and at 6.5 the availability of Iron decreases to nothing. I agree that in soils there is no issue with pH until it reaches 8 but in container situations the book suggests the pH cannnot increase beyond 6.5 without significant plant impact. Beyond 6.5 there is no Iron available for plant uptake.

Are you basing this statement on an organic based mix, or the inorganic based mixes that you use? Just curious if it makes any difference!

    Bookmark   September 17, 2008 at 10:15PM
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tapla (mid-MI z5b-6a)

John/Brent. I just returned from an evening engagement and I'm kind of tired, not fully possessed of my wits. I have another commitment tomorrow night, but I'll be back to outline some thoughts about the effects of pH on nutrient availability in container media as soon as I get a chance - i.e as long as Snowflake-rosa is ok with all the tech-talk on her thread.

Brent - To answer your question - 'on an organic or largely inorganic mix'. I'll comment more later.

John - I'm usually very consistent in pointing out that trying to chase/maintain any kind of "ideal" media pH is an exercise in futility for the average hobby grower because pH often changes by the hour and is affected by many cultural conditions. I should have mentioned that fact to S-r so she didn't get the idea I was suggesting that she should try to maintain an "ideal" pH, even though it is a primary strategy employed for F-sensitive plants by commercial growers.

"I do not rely on elevating the pH for reduction of NaF uptake."

... nor would you need to, if you're taking steps to insure a minimal presence of F in media and nutrient solution. In the cases where it IS present, it is similar in effectiveness to raising pH to prevent Al toxicities.

I did some nosing around to check the accuracy of what I said - following is a quote from the first meaningful site I found.

From a U of FL production guide:


1) Fluoride toxicity

Symptoms -
Elongated tan to dark brown areas form first in the white tissue of `Warneckii', then progress to marginal necrosis. `Janet Craig' tips and margins have chlorotic and necrotic areas.

Control -
Maintain pH between 6.0 and 6.5. Avoid water, medium components, and fertilizers containing fluoride. Dracaenas are also very susceptible to aerial fluoride pollution.


PS - The P in MG comes from monoammonium phosphate.

    Bookmark   September 17, 2008 at 11:56PM
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tapla (mid-MI z5b-6a)

We know that in most cases micronutrients are already in mineral soils, and lowering pH generally makes micronutrients more available as H+ ions combine with elements to form acids, which have the effect of dissolving micronutrient-containing compounds and making them available for plant uptake. As pH increases, OH- ions combine with the same elements to make hydroxides that are less soluble or insoluble. A pH change has no effect on the presence of the elements, only the state of their solubility/availability.

Container media are different in that the quantity of micronutrients present will be extremely small and held tightly within the organic structure of the ingredients. We either wait for the organic component to decompose and release the elements, or supplement with a fertilizer containing micronutrients, so micronutrient availability in container media is no where near as pH dependent as in mineral soils.

I mentioned Fe and Mn upthread, and they are the first listed as being "unavailable because of elevated pH status", so letÂs look at that claim. It is only when we have high levels of mainly Ca, but Mg, Na, or HCO3- anions (bicarbonates) too, that pH begins to come into play and Fe and Mn become unavailable. As pH rises from the addition/accumulation of these base salts, you can certainly expect that Fe and Mn will become increasingly unavailable, but you cannot blame the unavailability on pH. Media pH rise is simply a symptom of of the presence of the base salts and it is the base salts, not pH that makes the Fe and Mn unavailable.

Let me shore my position by relating an experiment conducted by Drs. Whitcomb/Ward/Hathaway in which they grew azaleas, a plant known to be very inefficient at absorbing Fe, in media that ranged from a pH of 3.0 to 8.2. In the experiment, they used nonessential elements to raise/lower the pH. They discovered that as long as a slow release form of Ca/Mg was available (they used dolomite at 2:1, Ca:Mg), the plants grew equally well throughout the range. I've read similar studies with the same results, one was recent - blueberries the subject.

Micronutrient nutrition in containers revolves around our ability to provide the essential nutrients in a favorable ratio. If you donÂt provide them, deficiencies are pretty much assured, when you do provide them in the proper proportions, they are there for the plant to use and pH has little effect on availability.


    Bookmark   September 21, 2008 at 10:13PM
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john_z(Z 7b Ga.)

Thanks for sharing that, Al! I had Whitcomb's books as required text for courses in college. I also spoke with him indepth at a conference, and had even further admiration of him because of it.

While he, as well as other sources, point out that an excess of one fertiliser element can create a deficiency in others, I have never read statements that nutrient availability in potting media is irrelevant - including with Whitcomb's research.

Sodium fluoride (NaF) is, of course, not a nutrient; it is a toxin to many plant species. But pH will affect its uptake, as well as its unavailability. Why should this be different for other elements?

    Bookmark   September 26, 2008 at 8:00PM
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tapla (mid-MI z5b-6a)

My reply was in answer to Brent's question about pH and nutrient availability. A careful reread of what I wrote will show I never said or implied, nor did I say that anyone else stated that nutrient availability in container media is irrelevant. I also stated that pH does influence fluoride uptake and stated the favorable range to help minimize it. After disagreement from you, I offered copy from a U of FL production guide to support what I'd said. The info is fairly common knowledge & found in a variety of places.

I took the time to check - if you need hard proof of Dr Whitcomb's statements about (the lack of) pH's effect on micronutrient uptake, I'll direct you to chapter 8 in Plant Production in Containers II.

BTW: Cl is far from "... the most used micronutrient for most plants". It's actually one of the least used. Plants use many times more Fe, Mn, B, and Zn than Cl.

Since the conversation has moved well beyond what's usable to the OP and most readers, I'll move along & offer you the last word if you'd like to take it.

Take care.


    Bookmark   September 26, 2008 at 10:12PM
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paul_(z5 MI)

Out of curiosity.....

Why would "superphosphate or triple-superphosphate" fertilizers contribute to fluoride toxicity? Is it simply that these compounds have a greater tendancy to bond to fluorine atoms thus allowing a build of fluorine in the soil?

    Bookmark   September 27, 2008 at 2:12PM
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tapla (mid-MI z5b-6a)

Like phosphorous, fluorine is so reactive it is never found naturally in elemental form, but it's the fact that fluoride can comprise up to 5% of the phosphate rock the super phosphates are made from, not the fact that it's so reactive, that accounts for the increased likelihood of fluoride toxicity arising from the use of the superphosphates.


    Bookmark   September 27, 2008 at 6:20PM
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paul_(z5 MI)

Ah! Thanks, Al.


    Bookmark   September 27, 2008 at 7:58PM
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tapla (mid-MI z5b-6a)

You're welcome. :9)


    Bookmark   September 27, 2008 at 8:40PM
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tapla (mid-MI z5b-6a)

You're welcome. :9)size>


    Bookmark   September 27, 2008 at 8:50PM
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