Salts from fertilisers

[kiwi]

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!

 

[Ozpaph]

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.

 

[JAB]

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

 

[Ray]

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.

 

[Linus_Cello]

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?

 

[gonewild]

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.

 

[Ray]

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.

 

[Stone]

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.

So how do precipitated (therefore insoluble) minerals affect plants?
Obviously soluble dissociated compounds which have recombined (crystalized due to high enough concentration - complete drying out, which never really happens), are easily redissolved and flushed with enough water. Any which are held as cations on negatively charged surfaces on diatomite or any other material are of no concern and in fact are of benefit to the plant as they are exchangeable. (as long the potential for accumulation without flushing is recognized - just as it is with sphagnum or CHC)
I fail to see a problem with salt build up which cannot be managed with correct irrigation.

 

[Ray]

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.

 

[kiwi]

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

 

[gonewild]

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.

 

[Stone]

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.

Yes but the dye which is not removed is insoluble therefore inactive. It is only in a soluble form where bio-damage can occur to a plant. Plants cannot swallow and break down solid minerals as we can. A solid cannot alter the salt concentration of water.
Consider limestone. It's solubility is very low (the dye) Only what dissolves becomes potentially active and able to affect the plant. The rest (the vast majority) remains static, inert, no problem. It just becomes part of the potting mix furniture. If it where completely soluble in water (just as ammonium sulphate or calcium nitrate or sodium chloride are) then it could cause serious harm by raising the salt concentration of the media solution. But it isn't therefore we can add relatively large amounts.
If you are concerned about crystallization rather than precipitation, then you need to 1, lower the rate of added salts, and/or 2, increase the water content of the media.
About ''wastes'' from the plant and microbes, flushing should also take care of those as they are mostly organic in nature.

 

[Stone]

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.

Salts are nutrients

As Ray has said the salt buildup is from when water evaporates and deposits solid salt crystals on the media surface.

Like I mentioned above that only happens if you add too many salts when allowing your mix to dry out. I cannot happen in a wet media unless you pour on your ferts straight from the packet.

These salt deposits cause a problem for plants when roots come into contact with the crystals.

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

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

.

Yes but this has nothing to do with precipitation. It is a problem of high electrical conductivity of the solution. (Again, too many salts being added(now or previously) or not enough water to dilute them).

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.

Correct..

Adding to the problem is that most of the salt deposits that form are not good nutrients for the plants

,

Had how could you possibly know that?

worst being sodium

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

 

[Stone]

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

No ''build up'' at all unless you don't water enough or your water is not very good. (and I'm sure it's pretty good in NZ)

Even if you fertilize strongly there will be no ''build up'' keeping the above caveats in mind.

 

[Ray]

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.

 

[Linus_Cello]

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.

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

 

[gonewild]

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.

 

[gonewild]

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

Not if you flood it regularly from the surface.

 

[gonewild]

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.

 

[Stone]

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 that would make it ''slightly soluble'' and only ''slightly'' active