# Salts from fertilisers



## kiwi (Jul 15, 2016)

Hi all,
I have just received an email from a local orchid grower regarding salt build up in potting mixes. Could The chemists out there help with an answer please? The email was as follows:

Diatomite absorbs and accumulates salts. Do organic fertiliser, like Nitrosol, contain salts of the sort that build up and become harmful? I understand "chemical" fertilisers do contain salts, and these can build up or exist in mix like coir, and can build up in any mix, but if I use only organic fert will I avoid the buildup of "salts". I may not understand what "salts" are, this may be obvious by this question.


My problem is, I have diatomaceous granules that I am using to pot Paphs, I understand this material absorbs "salts" and these salts can't be flushed out like "normal" the little crustacean shells hold on to it like crazy, apparently. 

So if i use organic fertiliser only, will I avoid this accumulation of "salts"? Could this be the same for perlite?

I am assuming my water, being rain water is "very low salt".

I knew I should have listened in chemistry class!


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## Ozpaph (Jul 15, 2016)

All I can add is that diatomite was all the rage but my experience and that of many others was less than favourable over time. It was said that 'salt build up' was the problem. I would not use it again.
I have not seen or heard of the same issue with perlite.


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## JAB (Jul 15, 2016)

I am certainly no Chemist, and this is the first I have heard of folks using Dio as a substrate (usually used for top dress to kill fungus gnats, no?). But I will say this... ANY nutrient or additive can leave behind residual traces of whatever it is. This is why it is SO important to flush with pure water at least every other feed. 

Cheers
JAB


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## Ray (Jul 15, 2016)

Many, many fertilizer components are "salts", even organic ones, but focusing only on salts is too narrow thinking.

ANYTHING that is in the solution, and that includes all fertilizer and additive components, as well as plant wastes, will be absorbed by the potting media components, and they will build up over time. It cannot be prevented, but it can be slowed by never letting the potting medium dry out (the drier they get, the more deeply and permanently will be the deposits), and flushing heavily, although the latter will not extract enough previously-precipitated chemicals to matter.

Unfortunately, diatomite is one of the worst materials for building up wastes, but all that means is that you need to be diligent about regular repotting.


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## Linus_Cello (Jul 15, 2016)

Ray said:


> Unfortunately, diatomite is one of the worst materials for building up wastes, but all that means is that you need to be diligent about regular repotting.



What about watering/flushing with RO/rainwater?


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## gonewild (Jul 15, 2016)

Linus_Cello said:


> What about watering/flushing with RO/rainwater?



If salts build up over time then you are not watering enough each time you water. Best to use enough water when you water to flush out any accumulating salts constantly. That keeps the root zone it a healthy condition.


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## Ray (Jul 15, 2016)

Linus_Cello said:


> What about watering/flushing with RO/rainwater?





Deposits occur because the solvent (water) has evaporated, and as that happens from the surface, in, the solution becomes more and more concentrated toward the center of the particles. With each wet-dry cycle, they get deeper and more concentrated. Surface rinse with any water isn't going to do any appreciable good.


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## Stone (Jul 15, 2016)

Ray said:


> > It cannot be prevented, but it can be slowed by never letting the potting medium dry out (the drier they get, the more deeply and permanently will be the deposits), and flushing heavily, although the latter will not extract enough previously-precipitated chemicals to matter.
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## Ray (Jul 16, 2016)

I don't thing "exchange" plays much of a role in this situation; it's more a case of physical access and time that are the limiting factors.

Imagine having dipped the end of a sponge in water containing food coloring - the end simulates the outside of our potting medium particle. If we dip it and wait, some of the coloring will move toward the dry end, the "interior of the particle", if you will. Repeat that enough, and the sponge will get pretty well loaded up with the dye. If drying occurs between dips, the dye concentration will end up being greater than it was in the solution.

Now dip the end into plan water for a few seconds to simulate flushing with plain water. Some of the dye will certainly be extracted, but not all of it. Moreover, now being wet again, water will move up the sponge ( to the particle interior), and pick up more dye, allowing it to migrate to the surface.

With sufficient concentration or improper chemistry (specific combination of minerals or wastes), it could easily become toxic to the plants.

That is exacerbated by letting the potting medium dry. If the potting medium never, ever dries out, that will be lessened, but never eliminated.


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## kiwi (Jul 16, 2016)

Thanks everyone,
So are you saying that regardless of the fertiliser being inorganic or organic there will be a build up of salts and other constituents but so long as there is sufficient flushing this should not be an issue with regards to plant/root growth? Will the buildup be slower or less with organic fertiliser say kelp?
What about perlite versus diatomite versus coir? Are any of these significantly worse than the other with regards to build up of salt and other compounds? 
Thanks


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## gonewild (Jul 16, 2016)

kiwi said:


> Thanks everyone,
> So are you saying that regardless of the fertiliser being inorganic or organic there will be a build up of salts and other constituents but so long as there is sufficient flushing this should not be an issue with regards to plant/root growth?



Yes



> Will the buildup be slower or less with organic fertiliser say kelp?



Probably slower but only because there are less nutrients in kelp.
Organic fertilizers have nutrients in the form of salts too so don't let the term organic lead you astray.

What about perlite versus diatomite versus coir? Are any of these significantly worse than the other with regards to build up of salt and other compounds?[/QUOTE]

Perlite is probably the least affected because it has lower CEC. But as long as you are applying the correct nutrients in the correct amounts along with plenty of water you wont have a salt buildup problem.
The ability of the media components to absorb salts (CEC) is not an issue of concern when thinking about salt buildup. The different media components absorb nutrients by +/- energy charges. They absorb nutrients not salts so again not what your concern is.
As Ray has said the salt buildup is from when water evaporates and deposits solid salt crystals on the media surface. These salt deposits cause a problem for plants when roots come into contact with the crystals. The biggest problem is when small amounts of water are applied and allowed to dry out before more is applied and that leaves salt behind then the next time you water some of the salts are dissolved and add the the salt content of the water you are applying. Continue this enough times and the water can become too salty for the roots. It makes not difference what media is used the problem is the same. The solution is to always use enough water to flush through the pot so that excess salts flow out the bottom and out of the root zone.

Adding to the problem is that most of the salt deposits that form are not good nutrients for the plants, worst being sodium.


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## Stone (Jul 16, 2016)

Ray said:


> > Imagine having dipped the end of a sponge in water containing food coloring - the end simulates the outside of our potting medium particle. If we dip it and wait, some of the coloring will move toward the dry end, the "interior of the particle", if you will. Repeat that enough, and the sponge will get pretty well loaded up with the dye. If drying occurs between dips, the dye concentration will end up being greater than it was in the solution.
> >
> > Now dip the end into plan water for a few seconds to simulate flushing with plain water. Some of the dye will certainly be extracted, but not all of it.
> 
> ...


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## Stone (Jul 16, 2016)

gonewild said:


> > The ability of the media components to absorb salts (CEC) is not an issue of concern when thinking about salt buildup. The different media components absorb nutrients by +/- energy charges. They absorb nutrients not salts so again not what your concern is.
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## Stone (Jul 16, 2016)

kiwi said:


> > Will the buildup be slower or less with organic fertiliser say kelp?
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## Ray (Jul 17, 2016)

Back to the sponge for a moment, Mike, I think you're incorrect about that.

Starting with the sponge saturated with dye, if we "flush" the end with fresh water, that will extract some of the dye, but will also leave it wet, allowing more dye from the sponge to dissolve and relocate to the end.

I absolutely agree that keeping the medium wet is the only way to avoid that, although even that's not 100%. That's one of the benefits of semi-hydroponics, if you flood it regularly.

By contrast, one esteemed member here, who will remain unnamed, tried S/H culture, but only "topped up" the reservoir, so the LECA above it regularly dried out, concentrating the salts and wastes faster, causing those plants to decline drastically in only a couple of years.


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## Linus_Cello (Jul 17, 2016)

Ray said:


> Back to the sponge for a moment, Mike, I think you're incorrect about that.
> 
> Starting with the sponge saturated with dye, if we "flush" the end with fresh water, that will extract some of the dye, but will also leave it wet, allowing more dye from the sponge to dissolve and relocate to the end.
> 
> ...



So s/h plants in Leca have to be periodically repotted in new Leca?


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## gonewild (Jul 17, 2016)

Mike, You are confusing precipitation with evaporation.
The salt build up growers are concerned about is caused by evaporation not precipitation.

Precipitation is when a chemical reaction between two dissolved chemicals from a new compound and that compound falls out of solution as a solid. That does not easily happen in wet soil media.
Evaporation on the other hand is what causes salt build up in growing media. Built up salts left behind by evaporation (crystals) are easily redissolved when they come back into contact with moisture. This is what caused water in the potting media to become too salty.


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## gonewild (Jul 17, 2016)

Linus_Cello said:


> So s/h plants in Leca have to be periodically repotted in new Leca?



Not if you flood it regularly from the surface.


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## gonewild (Jul 17, 2016)

Stone said:


> Salts are nutrients



Not when they are at toxic levels.



> Only in theory. If we are talking about potted plants, the mo isture content of the mix would need to be so low that it would kill the plant roots. salts or no salts.



It takes some time for plant roots to die when the mix becomes to dry. But it only takes a moment for salts to damage or kill a root..



> Had how could you possibly know that?



Lucky guess!



> As for sodium ''deposits'' Unless you are using a high sodium content water, it will never be a problem



High sodium content water is a big problem in a lot of areas of the world.


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## Stone (Jul 17, 2016)

Ray said:


> > Back to the sponge for a moment, Mike, I think you're incorrect about that.
> >
> > Starting with the sponge saturated with dye, if we "flush" the end with fresh water, that will extract some of the dye, but will also leave it wet, allowing more dye from the sponge to dissolve and relocate to the end.
> 
> ...


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## Stone (Jul 17, 2016)

gonewild said:


> > Mike, You are confusing precipitation with evaporation.
> > The salt build up growers are concerned about is caused by evaporation not precipitation.
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## Bjorn (Jul 18, 2016)

Lots of interesting reading; and lots of misconceptions.
Why not just reduce fertilisation level and increase watering? Then salts buildup will be much less of a problem. 
There seems to be a conception amongst many ochidists that water is necessary but dangerous; so they try to water as little as possible - and never in the crown!.
As if it is not raining where the plants grow.......in my opinion many problems are related to cultural factors other than getting wet or not.
reverting to the issue about so-called salts deposits, these are normally insoluble and consist of not only carbonates like Calcium carbonate, but also silicates. Silica is, opposite to common belief, fairly soluble, up to 50-100 ppm as monosilicic acid. This Depends on the sopurce however silica sand is not very soluble, while DE is highly soluble (ok its not very soluble more the 50ppm range). Upon drying the monosilicic acid polymerises to polysilicic acid , and this is not soluble. If watering is withheld until the deposit is formed, it will become significant and may possibly create problems.


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## Ozpaph (Jul 18, 2016)

To address kiwi's question.
I use and have not seen problems with perlite (with bark and charcoal). I have had, as have many, problems with diatomite. Coir is too unpredictable. I have had bad experiences with it - so never use it. Im told the 'pre-treatment', at its source, is highly variable and determines its usability (too much sodium, I understand). many have moved away from it here.

You have the best bark in the world, I'd use that!


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## Ray (Jul 18, 2016)

Sorry Lance. "Precipitation" is any formation of a solid species from a liquid solution, whether it involves the formation of a new compound or not. "Evaporation" is the conversion of a liquid to a gas, whether anything drops out of solution or not.

In our context, evaporation of the solvent (water) causes the concentration of the dissolved solutes (nutrient chemicals and plant wastes) to exceed their maximum solubilities, therefore precipitating out of solution.

Bjorn, you are absolutely correct that decreased concentrations and increased watering - both volumes and frequency - will reduce the buildup rate.

Mike, all media will build up minerals eventually. LECA in S/H culture that is flooded frequently may take a very long time, but it's not never.


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## consettbay2003 (Jul 18, 2016)

*Gonewild*

How long does it take for plant roots to die when the mix becomes dry?


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## Stone (Jul 18, 2016)

Ray said:


> > Mike, all media will build up minerals eventually. LECA in S/H culture that is flooded frequently may take a very long time, but it's not never.[/
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## gego (Jul 19, 2016)

I agree with Mike. Eventually the system will be in equilibrium and there will only be so much build up as the system can sustain. The variable here is the amount of water/rain, the amount of mineral and the holding capacity of the material. A balance will be achieved for our dear plant to grow. 

Example of a good deposit of mineral is the formation of stalagmites and stalactites. Water with lime (Ca) drips at a rate to sustain build up. Too slow or too fast will have a different result.


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## Ray (Jul 19, 2016)

Aw, c'mon.

I hardly think one can compare a natural ecosystem containing many, many species of flora and fauna to that of a plant in a flower pot...



> Example of a good deposit of mineral is the formation of stalagmites and stalactites. Water with lime (Ca) drips at a rate to sustain build up. Too slow or too fast will have a different result.


 Different, but still a buildup, as is also the case for differences in Ca concentration. That is a case of constant flow, never drying, and yet still precipitation....in a natural environment.


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## C. Rothschild (Jul 29, 2016)

Vitamin B1 and liquid seaweed are both supposed to be good for orchids. Also pouring through a coffee filter may help. But yes I assume anything can build up in the pot. If you have some moss on the top you could just pick it off and replace it a couple times a year.


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## Ray (Jul 29, 2016)

A coffee filter is not going to affect dissolved minerals at all.


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## Rick (Jul 30, 2016)

http://www.slippertalk.com/forum/showthread.php?t=31056&highlight=potassium&page=3

I threw this old thread back in because it illustrates the importance of CEC of different mix components

In this experiment conducted by the Orchiata folk a ways back, it shows that the low CEC substrate (Orchiata) retained the least amount of K from fertilizer (under supposedly identical water and feed rates) compared to moss and coconut coir. In fact Orchiata only retained about 1/2 to 1/3 the amount of K compared to coir. 

This also affected the amount of K going into the leaves and roots of the plants (in this case a hybrid Phalaenopsis), although for this (short) study, no significant difference in plant growth was noted.

I've seen plenty of other studies on the use of coconut products that demonstrate that once it gets loaded with monovalent cations (Na and K for most purposes) you can't get it out of the mix no matter how much you soak it in pure water. You need to soak it in a high concentration bath of calcium and magnesium salts to draw it out of the media. 

Using a low CEC media retains less monovalent cation (or ionic forms in general) and easier to "flush".

But as Bjorn stated if you don't add concentrated fertilizer salts in the first
place you can water to your hearts content and not have to worry about flushing to clean out the retained salts in the mix.


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## gonewild (Jul 30, 2016)

Rick said:


> You need to soak it in a high concentration bath of calcium and magnesium salts to draw it out of the media.



Which you cant do with a plant growing in the media.


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## Rick (Aug 21, 2016)

gonewild said:


> Which you cant do with a plant growing in the media.



Yup which is probably where the tradition of repotting came from.:wink:


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## Stone (Aug 21, 2016)

Rick said:


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## Rick (Aug 22, 2016)

Stone said:


> Rick said:
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## Rick (Aug 22, 2016)

Remember the original question of this thread was how to manage salts in potting mixes.

Given that his irrigation water (rain or RO) the only salt going in is going to be from the fertilizer (which depending on choice is mainly K as an accumulating ion).

So the management answer is "it depends" on:

1) choice of fert mix
2) frequency and concentration of fert application.
3) choice of potting mix.
4) frequency of non feed irrigation rate
5) frequency of repotting.
6) pot size and configuration.

The way I feed and water has made items 3,4,5, and 6 almost irrelevant in mine and others collections.

Back before 2011 when I used MSU pure water in RO water I was constantly in the strategy battles of potting mix / pot / repot wars. Now these are the least of the things I worry about.


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## Stone (Aug 22, 2016)

Rick said:


> Stone said:
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> > Lack of K doesn't seem to cause any problems either. So why do we insist on wasting so much?
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