Where do phosphates, potassium, calcium go? - The Planted Tank Forum
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post #1 of 16 (permalink) Old 11-04-2011, 11:05 PM Thread Starter
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Where do phosphates, potassium, calcium go?

I can't seem to answer what eventually happens to these nutrients after they've been absorbed by plants. We mostly all dose these nutrients into our tanks, but it's bothering me that I really don't have a clue where they disappear to thereafter!

Clearly they must undergo some chemical change where they're converted into some other compounds, but what chemical compounds could these possibly be in once those elements leave the plants?

And either these compounds are expelled continuously by the plants, or the plants retain them all in their tissue until they die at which point they're released once more? Either way all these elements our plants use remain in the tank (or nature) in some form or another. And somehow nature recycles them back into forms usable once more by the plants, but what processes could those possibly be?

I'm not even sure what happens to the nitrogen in Nitrates once the plants have broken the nitrates down, is it off-gassed simply as nitrogen where it escapes into the air, in a similar way to what denitrifying bacteria do? But these elements of course cannot just continue to build up infinitely in the plants, so can anyone answer where they go please?
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post #2 of 16 (permalink) Old 11-05-2011, 12:35 AM
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Interesting question.

All I've heard mentioned as waste products are oxygen during the day, CO2 during the night, and little else.

The rest may just accumulate. I've heard it said that trees that lose their leaves each year pump their accumulated waste into the leaves right before they drop. Haven't looked at rolling in a pile of autumn leaves quite the same way, since hearing it described as "plant poop".

Would love to hear if there's some other waste disposal method.
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post #3 of 16 (permalink) Old 11-05-2011, 12:44 AM
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They build the plants structure and then are later sucked up as brown goo into your filter and you wash it down the drain.

What happens after you eat, just a little different a plant loses leaves, or leaves melt, plant poo.

I don't know for sure but I don't think tree haves any waste like this at all, but the do eat make leaves and flowers then they shed them to be replaced with new leaves, I would guess the nutrients are transformed in some manner and later can be converted back to there original state in some type of cycle, other wise we/plants would run out of these nutrients and life would cease to exist. We do have some very smart people here and if you ask this question in the plant section a Botanist that works for the Smithsonian is likely to answer your question, no joke.

I'm just not one of the smart people, but I have met some of them.


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post #4 of 16 (permalink) Old 11-05-2011, 01:11 AM
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Phosphorous is bound up in the plant DNA and enzymes. Potassium is used for a zillion different things, including osmoregulation and maintaining electrical potential, and calcium is primarily used structurally. Basically every nutrient has some enzymatic use. In an aquatic environment small amounts of nutrients can pass membranes back into the water, but for all practical purposes, the nutrients are retained by the plant. When a plant cell dies, bacteria and fungi break it down into it's mineral components, which are mostly used in building the bacteria, or excreted as waste. Bacteria primarily produce their energy by reducing compounds. Photosynthetic bacteria are a different story.

Plants don't produce much waste simply because their ingestion is so small compared to an animal, which has very high energy requirements.
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post #5 of 16 (permalink) Old 11-05-2011, 01:14 AM
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Originally Posted by Sharkfood View Post
Phosphorous is bound up in the plant DNA and enzymes. Potassium is used for a zillion different things, including osmoregulation and maintaining electrical potential, and calcium is primarily used structurally. Basically every nutrient has some enzymatic use. In an aquatic environment small amounts of nutrients can pass membranes back into the water, but for all practical purposes, the nutrients are retained by the plant. When a plant cell dies, bacteria and fungi break it down into it's mineral components, which are mostly used in building the bacteria, or excreted as waste. Bacteria primarily produce their energy by reducing compounds. Photosynthetic bacteria are a different story.

Plants don't produce much waste simply because their ingestion is so small compared to an animal, which has very high energy requirements.
and then you trim and it removes them =)

-VeeSe

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post #6 of 16 (permalink) Old 11-05-2011, 01:58 AM Thread Starter
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Interesting (and worrisome), that we don't have a complete answer for this!... I spent a little more time Googling it and there were differing notions. Some suggested that plants do excrete certain substances, such as organic acids, either through their roots, through lost leaves, etc. And others suggest the plants store their, um, excrement internally throughout their lives.

But whether the plants are 'pooping' these things constantly through roots or leaves, or releasing them only when they themselves dissolve, those nutrients will remain in the tank and eventually be in some dissolved form.

Pure nitrogen, being a gas at room temperature can of course off-gas, while the other elements cannot.

As a denizen of Earth, I have to suggest that it does work in a closed system, that these elements do get recycled back to their original forms. I just wonder why it doesn't happen to work quite the same way in our aquariums, and why we are having to add additional phosphates and potassium constantly, (trimming and water changes aside).

With the Natural Planted Tank/low-tech approach you're typically adding those nutrients only in the form of fishfood, and often not doing water changes, but you're adding them nevertheless. So perhaps in such a system as this the nutrients are being recycled in some manner. Those systems often do have a rich soil substrate though, which is a significant store of many nutrients to last for some time in any case.
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post #7 of 16 (permalink) Old 11-05-2011, 05:52 PM
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Nutrient import: Fish food and ferts.

Nutrient export: Trimming and water changes.

Nutrient recycling: Limited almost exclusively to plants recycling fish waste. The rest of the cycle is broken, with nutrients stopping at plants. We do not typically keep critters that eat our plants, allow plants to choke the tank until some partially or fully die off from lack of light, allow rotting plant material to remain, or have massive seasonal or weather-related parameter shifts that periodically kill off many plants. If we did, it wouldn't be considered a very pretty tank; and given the small volume and diversity, it would be quite easy to reach points where species simply become extinct over short time periods.
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post #8 of 16 (permalink) Old 11-05-2011, 06:24 PM
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You can, if you have the equipment and training, dry aquatic plants and measure the elemental composition of the dry plant matter. That would show you where the nutrients went - they went into building the plant matter. All of those leaves are made of carbon, nitrogen, potassium, etc. along with hydrogen and oxygen.

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post #9 of 16 (permalink) Old 11-05-2011, 08:31 PM
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As a denizen of Earth, I have to suggest that it does work in a closed system, that these elements do get recycled back to their original forms. I just wonder why it doesn't happen to work quite the same way in our aquariums, and why we are having to add additional phosphates and potassium constantly, (trimming and water changes aside).
If it works in a closed system, I would say it's either because the plants are being eaten and recycled that way, or that seasonal die off does the job. I can't think of any example of a perfectly closed system though. An oasis in the desert might be close, but even then there has to be a source of new water. Otherwise the oasis would dry up. The water feeding the spring would contain minerals from the rock it flows through. If the oasis is seasonal, which many are, then there is going to be massive die off in the dry season.

As was already mentioned, the total nutrient content of your aquarium is diminished through trimming and water changes.
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post #10 of 16 (permalink) Old 11-05-2011, 08:45 PM
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Hoppy!!! Genius! Man............ yr wasted here.......<high five>

Pediatric Psychiatrist/Child Therapist/ADHD Psychiatrist/Cognitive Hypnotherapist/Clinical Anxiety Consultant/Pediatriciac Physician.....If only plants and fish could chatter like children...
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post #11 of 16 (permalink) Old 11-05-2011, 08:53 PM
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Originally Posted by Hoppy View Post
You can, if you have the equipment and training, dry aquatic plants and measure the elemental composition of the dry plant matter. That would show you where the nutrients went - they went into building the plant matter. All of those leaves are made of carbon, nitrogen, potassium, etc. along with hydrogen and oxygen.
Can be done and actually pretty easy for many of them. LeMotte makes a test kit for plant tissue. Your local ag extension office can do plant testing too. There are numerous websites that offer tissue testing with many details.
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post #12 of 16 (permalink) Old 11-05-2011, 08:54 PM Thread Starter
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To be honest I don't tend to do much trimming, or much collecting of fallen plant leaves. I do a little when something is unsightly, but with my dense planting it's not easy to access fallen leaves in the back half of my tank and they're out of sight so I just leave them there.

From what further information I've read elsewhere, the nutrients which the plant has stored up should therefore be released back into the water or substrate as the fallen leaves melt away. And I had read some chemical analysis of what the leaves contain, but was just wondering if those chemicals go anywhere thereafter or perhaps combine into some insoluble form which collects harmlessly in the substrate.

That should all mean that in a Natural Planted Tank, so long as you don't do water changes or remove dead leaves, then you shouldn't ever have to add nutrients to the tank, (not even in the form of fish food), in order to maintain a stable plant load. The exception of course is nitrogen, which unless you have a completely sealed tank the nitrogen gas created will leak out, (or you could have nitrogen fixing floating plants such as Azolla). But I'd really have to question whether that is in fact the experience of most NPT hobbyists, that their tanks don't become nutrient depleted over time (if fish food was not added of course).
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post #13 of 16 (permalink) Old 11-09-2011, 03:33 AM
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To get a real understanding of these issues, order up a copy of The Ecology of the Planted Aquarium by Diana Walstad.

http://www.amazon.com/Ecology-Plante.../dp/0967377307
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post #14 of 16 (permalink) Old 11-09-2011, 08:53 AM Thread Starter
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Great book, changed my entire philosophy of fishkeeping or at least in part, (either that or it confirmed the direction it was going at least). And I'm running a few little mad science experiments of my own as a result.

My query as a result is in part a follow-up on one part of that philosophy which I was looking for a more detailed answer to as it didn't seem to explain it in full. For instance D.W. adds dolomite to her substrate to buffer it and prevent the system from becoming too acidic. But where is all that calcium magnesium carbonate which is being released going in time? I suppose it combines with organic acids, to form what? The calcium and magnesium atoms are still remaining in the tank in some manner, though not sure how or what is happening to them.
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post #15 of 16 (permalink) Old 11-09-2011, 05:55 PM
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If a substance cannot be released as a gas, then the only options are: getting used by plants, algae, and microbes, being removed by the filter, being removed during water changes, and sticking around in the water and the substrate. I believe that all of these things occur. I'm not a chemist, but as I understand it, Calcium is a positively charged ion, so it likes to hook up with other molecules to form chemical compounds.

Is this what you are asking? What specific compounds calcium ions form and why?

I can't answer this. If you lived in the US, I'd suggest that you contact your County Extension office and they would pass your question from a Master Gardener to a University Botanist. I don't know if you have anything like that in the UK...
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