What is the “Duckweed Index” all about?

[dw1305]

What is the “Duckweed Index” all about?

I’ve never written a proper “Duckweed Index” article, but, prompted by @GHNelson, I’ve written a short introduction. I’ll break it up into a series of posts (of which this is the first), mainly to avoid author (and reader) fatigue. There is a table of contents (for this thread) is at the bottom of this post.

Fundamentally, the “Duckweed Index” is a simple technique <”to retain and improve water quality”>

Quick guide.

1. To use the “Duckweed Index”, you watch the leaf colour, and growth rate, of a floating plant, and you only<" add nutrients">* when plant leaf colour and / or growth decline.
2. “Amazon Frogbit” is now my preferred “Duckweed”.
3. You use the <"UC Leaf Colour Chart”> as your visual reference for plant colour : <” The scientific background to the "Leaf Colour Chart">
4. Plants require all <"fourteen essential mineral nutrients"> to grow, just in widely varying amounts.
5. You can use the combination of <"leaf colour and plant growth rate"> as a measure of plant nutrient availability.
6. Using the “Duckweed Index” water testing is not a requirement and you don’t need to try and accurately diagnose any nutrient deficiencies.
7. Synergistic plant / microbe biofiltration can both create, and retain, high water quality.
8. <"Bioassay"> and <"phytoremediation techniques"> have a sound scientific underpinning.
9. The “Duckweed” functions simultaneously as both the bioassay organism and your “test kit”.
10. When you thin the plants, you remove the nutrients they contain from the aquarium.

Fig 1: Healthy Amazon Frogbit (Limnobium (Hydrocharis laevigata) laevigatum), my preferred <“Duckweed”> for the “Duckweed Index”. Photo by @Cédric from <"The scientific background to the "Leaf Colour Chart"">

Please feel free to post in this thread, or PM me, with any suggestions for improvement or clarifications. I'll incorporate them into the thread as I fill in the details in subsequent posts.

Table of contents

• <"Quick Guide and ToC">.
• <"The Leaf Colour Chart">.
• <"Testing for fixed nitrogen">.
• <"The other mineral nutrients">.
• <"Fertilisers and the Duckweed Index">.
• <"The "aerial advantage" and why it is so important">.
• <"Why a rooted emergent plant would have been optimal">.
• <"Phytoremediation">.
• <"When the Duckweed Index breaks down">.
• <"and why we need to talk about iron (Fe)">.
• <"We've talked about iron deficiency, but what are going to do about it?">
• <"Plant available iron (Fe) and pH">.
• <"What is the future for the Duckweed Index?">
• <"What is the future for the Duckweed Index II, is it a mix of floaters?">
• <"References and some background reading">.

* there is an <"exception to this">.

cheers Darrel

 

[dw1305]

The Leaf Colour Chart.
The Leaf Colour Chart has its own thread - <"The scientific background to the "Leaf Colour Chart">, but I'll add in a summary here.

The Leaf Colour Chart (LCC) is a visual guide of when to add nitrogenous (N) fertilisers, originally developed for Rice (Oryza sativa) farmers.

In the "Duckweed Index" we have just substituted Amazon Frogbit (Limnobium laevigatum) for Rice, but the approach is the same, we can use the leaf colour to assess plant nutrient status.

The LCC was developed from initial research by <"Takebe & Yonoyama (1989)">* who correlated chlorophyll formation and nitrogen availability, and found that leaf colour is a good indicator for plant health and nutrition, with nitrogen deficient leaves turning to pale or yellowish green, rather than dark green. It was further developed by the International Rice Research Institute (IRRI) & University of California. Originally the chart was just four shades of green, then six, but more recently recently it has been revised to a scale of <"eight green colour shades">**.

* Takebe, Masako, and Tadakatsu Yoneyama. (1989)"Measurement of leaf color scores and its implication to nitrogen nutrition of rice plants." Japan Agricultural Research Quarterly 23:1 (1989): pp. 86-93.
** Yang, W.H., Peng, S., Huang, J., Sanico, A.L., Buresh, R.J. and Witt, C., (2003). "Using leaf color charts to estimate leaf nitrogen status of rice". Agronomy Journal, 95(1), pp.212-217.

cheers Darrel

 

[dw1305]

Testing for fixed nitrogen, the elephant in the room.
The obvious questions are:

• Why don't we just measure the amount of ammonia / ammonium (TAN) NH3 / NH4+, nitrite (NO2-) and nitrate (NO3-) in the tank water? and
• Why do we need a visual method (like the LCC) to estimate their values?

It is <"a fair question"> and I'll try and to cover it, compound by compound:

<"TAN (Total Ammoniacal Nitrogen)">.

....... The real problem is that testing for ammonia isn't as straight-forward as many forums would have you believe. Because it occurs as both a dissolved gas (NH3) and an ion (NH4+), dependent upon pH, most test kits measure "Total Ammonia Nitrogen" TAN as NH3. It should say on your test kit whether it is a "Salicylate" or "Nessler" test.

There a number of problems with this, and the measured values may have little to do with the actual ammonia levels. If the test gives a green/blue colour? it is a salicylate based test ("free ammonia reacts with hypochlorite to form monochloramine. Monochloramine reacts with salicylate, in the presence of sodium nitro-ferricyanide, to form 5-aminosalicylate"). The efficiency of these tests declines over time as chlorine is released from the sodium hypochlorite reagent.

Nessler reagent tests also have some problems, but are more stable, my suspicion would be that they aren't still sold due to the mercury issue? if it is a Nessler test it will give an orange colour.

We do some ammonia testing (as NH4+ with an ion selective electrode), but even then values are open to question due to interference by sodium (Na+) etc. Why use an ion selective electrode? because it gives you a more consistent result than the tests do. Would I make decisions based upon it? No, because it is still not reliable or repeatable.

I'm not actually going to spend too much time on TAN. We are planted tank keepers and plants are <"very efficient at mopping up ammonium (NH4+)">.

Nitrite (NO2-).
Nitrite forms a number of <"insoluble coloured compounds">, which means that <"we can test for it reasonably easily">.

...... "In the presence of an acidic buffer, the nitrite is converted to nitrous acid which diazotizes an aromatic amine, this coupled with N-(1-naphthyl)-ethylenediamine to form a red-violet azo dye")...... The alternative to the azo dye method is to use salicylic acid "the nitrite ion reacts in an acidic medium with sulfanilic acid to form an intermediate diazonium salt. The salt couples with gentisic (salicic) acid to form an amber colored solution".

Why we need to deplete TAN and NO2 as rapidly as is possible.
Both ammonia and nitrite are <"highly toxic to aquatic life - Acute toxicities of ammonia and nitrite to Clarias batrachus and their interaction to chlorides"> , but they are also <"short lived in the water column">, because <"fixed nitrogen is a scarce resource"> and both these compounds are <"low hanging fruit"> for <"plants and nitrifying microbes">.

Nitrate (NO3-) and the <"Smoking Gun">.
Nitrate is <"much less toxic to aquatic life"> than TAN or nitrite, but <"NO3 levels can build up in tank water"> where there aren't mechanisms to deplete it.

Nitrate is often the <"smoking gun"> of preceding high levels of TAN and nitrite. Unfortunately ~all nitrate containing compounds are soluble, which means that to use a colorimetric method (like in the <"API NO3 test kit">) you need to <"reduce the NO3- to nitrite (NO2-)">.

The reducing agent was traditionally cadmium (Cd), but because of cadmium's toxicity, we now use vanadium (V).

There are two options for the production of a coloured compound.

• The Azo dye method: "Nitrate is reduced to nitrite by a reducing agent in the presence of an acidic buffer, the nitrite is converted to nitrous acid which diazotizes an aromatic amine, this coupled with N-(1-naphthyl)-ethylenediamine to form a red-violet azo dye". The alternative to the azo dye method is to use
• Salicylic acid: "Cadmium (or vanadium) metal reduces nitrates in the sample to nitrite. The nitrite ion reacts in an acidic medium with sulfanilic acid to form an intermediate diazonium salt. The salt couples with gentisic (salicic) acid to form an amber colored solution".

In the lab we use an <"ion selective electrode">*, but you still needs to make up standards etc. It was actually the <"issues with nitrate (NO3-) testing"> that initially led me to the <"Duckweed Index">.

There is a much easier way of estimating the nitrate content of your tank water and that is to observe the growth rate and colour of a non-CO2 limited plant in your aquarium via the "Duckweed Index".

If your plant grows quickly, and is dark green, you have lots of nitrate.

*<https://www.nico2000.net/analytical/nitrate.htm>

cheers Darrel

 

[dw1305]

We've just talked about nitrogen (N), but what about the other mineral nutrients?
Plants require <"all fourteen of the essential mineral nutrients for growth">, just in widely varying amounts.

From <"https://wiki.groenkennisnet.nl/space/CPC/11993210/3.1.+Minerals+in+plants:+a+brief+overview">.

The primary macronutrients, the ones that plants require most of, are nitrogen (N), phosphorus (P) and potassium (K), often shortened to N : P : K. Plants needs more nitrogen and potassium than phosphorus, and more phosphorus than any of the other essential elements. <"Effects of Potassium Levels on Plant Growth, Accumulation and Distribution of Carbon, and Nitrate Metabolism in Apple Dwarf Rootstock Seedlings">.

All plants can only take up nutrients as "ions", charged particles in solution. When we refer to the element, we're really referring to an ion, K+ for potassium, PO4---, etc. for phosphorus and NH4+, NO2- or NO3- for nitrogen. Nitrogen and potassium deficiencies both cause yellowing (chlorosis) and smaller leaves and their, and phosphorus, deficiencies show up first in older foliage.

Nitrogen is the most likely limiting nutrient if the plants are small and pale green in colour and the greenest leaves are the newest leaves.

In this case (my tank, low nutrients softish water) the plants are now reasonably healthy* but you can see that leaf rosette size of the Amazon Frogbit (Limnobium laevigatum) & Water Lettuce (Pistia stratiotes) is fairly small and the Lesser Duckweed (Lemna minor) is pale in colour.

* I'll come back to this image in <"we need to talk about iron">

These are the same plants** in @Parablennius's <"Floaters"> thread, where they are on richer rations.

** @Parablennius 's plant maybe another Frogbit species

cheers Darrel

 

[dw1305]

The "aerial advantage" and why it is so important.
If we go back to the figure in the <"previous post"> we can see that plants are overwhelmingly constructed from structural carbohydrates, produced from CO2, water and light via photosynthesis.

<"File photosynthesis equation.svg - Wikimedia Commons">.

The <"aerial advantage"> is a term coined by Diana Walstad (in her book, <"The Ecology of the Planted Aquarium">) and just means a plant with leaves that have access to atmospheric gases.

This access to 420 ppm CO2 <"Global Monitoring Laboratory - Carbon Cycle Greenhouse Gases">, means that floating plants aren't CO2 limited (tank water <"contains ~ 3 ppm CO2">) and can show a really quick greening and growth response to added nitrogen (N) (as long as <"all the other essential nutrients for plant growth"> are present).

If you add nitrogen and your plants green up? They were nitrogen deficient. ​

If your plants are large, lush and dark green? They have plenty of nitrogen, and​

when you harvest a plant you remove nitrogen (largely incorporated into the photosynthetic proteins) from the tank.​

Lemna minor (Lesser (Common) Duckweed), PITA & prejudice.
The <"problems with Lemna minor"> were that it doesn't like very soft water (it stays yellow, even when nitrogen levels are high), it needs reasonably high levels of nutrients, and <"it is a PITA to harvest">. It also has a poor reputation among aquarists.

The Perfect Duckweed - <“I love you more than you will ever know”>.
After some experimentation I found that there was a "Duckweed" that didn't have these limitations, and it was Amazon Frogbit (Limnobium (Hydrocharis) laevigatum). So “Frogbit Index” would be a better name, but <”I think that ship may have already sailed”>.

Amazing Amazon Frogbit.
It will survive on low nutrients in very soft water and in very hard water, but it will also show very fast growth when light, nutrient and warmth are freely available.

Some happy Amazon Frogbit - photo by @castle

There are some issues with Amazon Frogbit being <"an invasive alien in Australia"> etc., but it isn’t cold hardy in the UK, which means that you don’t need to worry about it escaping and becoming established in the UK.

cheers Darrel

 

[dw1305]

Why a rooted emergent plant would have been optimal.
In the last section I mentioned <"atmospheric gases">, and while CO2 was the focus, all aerobic organisms require oxygen (O2) and I'll say a bit more about oxygen now. It will only be "a bit", because this is a huge subject area and requires its own thread.

Actually when I originally thought about <"plant filtration">, even <"before the development"> of the "Duckweed Index", I didn't have a <"floating plant in mind">, I had an emergent one. An emergent plant is one that would be rooted into the tank substrate, but have some leaves above the water surface. The advantage of this is that these plants have <"aerenchyma and lacunae"> that conduct oxygen into the root and from there (roots are <"leaky structures">) out <"into the root zone or rhizosphere">.

Aerenchyma (<"Schoenoplectus tabernaemontani">) : by User:Bb143143 - Self-photographed, CC BY-SA 3.0, File:Aerenchyma2.JPG - Wikimedia Commons

The original ideal was to have a rooted emergent plant, something like <"Cyperus papyrus">. This is because of their potential to oxygenate the substrate, (and filtration was via a <"planted trickle filter">). I soon appreciated that neither of these was going to be desirable, or indeed possible, for most home aquarists.

The reason for a floating plant is purely convenience. They may not be <"quite as good"> as an emergent plant, but they are much more <"plug and play">.

Floating is just more convenient. At the most basic level any plant that has access to the atmosphere will do, and you just watch the size and colour of the leaves. When growth slows and/or the leaves become smaller, or less green than they were, you add nutrients. Once growth has re-started you go back to watching and waiting.

I started with Lesser Duckweed (Lemna minor), because it is used in waste water remediation, but it doesn't like soft water (however much nitrogen it gets)., and it won't grow at very low nutrient levels. Eventually I found a "Duckweed" that didn't have these drawbacks , <"and it was Amazon Frogbit (Limnobium laevigatum)">.

Ideally it would be a rooted emergent plant like <"Papyrus (Cyperus papyrus)"> one with <"lots of aerenchyma"> ("internal plumbing") and <"extensive radial oxygen loss"> (basically a root that leaks oxygen and carbohydrates into <"the rhizosphere"> (the substrate zone surrounding the root) ).

cheers Darrel

 

[dw1305]

Phytoremediation.
I won't go into details in this thread (this also needs a thread to itself), but for those who want to know more about using plants to clean polluted water we have some threads:

• <"Amazon Frogbit (Limnobium laevigatum) phytoremediation references"> and
• <"How Floating Wetlands Are Helping to Clean Up Urban Waters"> - (original article - <"How Floating Wetlands Are Helping to Clean Up Urban Waters">).

cheers Darrel

 

[dw1305]

When the Duckweed Index breaks down.
So far it has all been <"good news">, but there are situations where the "Duckweed Index" (and in fact plants generally) don't work, either as a mechanism for improving water quality (they need to be in active growth to do this), or as a visual guide to nutrient status. The most important one of these is:

• When <"Liebig's limiting nutrient"> is nutrient that is immobile within the plant

Plant nutrients, mobile or immobile?
Most plant nutrients <"are mobile">, which means that the plant can shuffle them around to where they are needed and <"stock-pile those nutrients"> for <"a rainy-day">. For the mobile nutrients when we add that nutrient the plant can move it to the growing tissue and <"plant growth improves very quickly">. Mobile nutrients include nitrogen (N), phosphorus (P), potassium (K) and magnesium (Mg).

Mobile nutrients.

In terms of the <"Duckweed Index">? If the Amazon Frogbit (Limnobum laevigatum) plants have pale older leaves and poor growth? I just add a complete fertiliser <"Solufeed 2:1:4 and Solufeed Sodium Free TEC or Solufeed Coir TEC Combination"> and plant growth <"should pick up fairly quickly">.

Immobile nutrients.

<"Nutrient Deficiencies - MSU Extension Soil Fertility | Montana State University">

The immobile nutrients are where things become a little more problematic when you are using the Duckweed Index. We have had a <"case of manganese (Mn) deficiency">, but for most of us that is pretty unlikely and the more likely options are calcium (Ca) and iron (Fe). I'll cover calcium really quickly by saying that if you use RO water and don't remineralise it at all, but just add a complete fertiliser, calcium deficiency is a possibility, but again fairly unlikely for most of us.

This brings us on to iron (Fe), and iron deficiency is something we see a lot more of, particularly in hard, alkaline water. Because of this I'm going to give it a new post.

cheers Darrel

 

[dw1305]

........... and why we need to talk about iron (Fe).
Lack of plant available iron (Fe) is the most common deficiency among the immobile plant nutrients.

There is a complete thread (by @KirstyF) at <"Plant deficiencies and the Fe Experiment">, but I'll add a summary in terms of how it effects the "Duckweed Index".

Quick guide.

• First the most important one, while plants are iron deficient the <"Duckweed Index" doesn't work">* "to indicate nutrient status, but it can help to <"diagnose iron deficiency">".
• The visible symptoms of iron deficiency are small, yellow (<"chlorotic">) new leaves and an absence of plant growth.
• Iron ions (Fe++(+)) are difficult to keep in solution, particularly in <"hard, alkaline water">.
• Plants need a continual supply of plant available iron, this means iron availability is a binary switch between <"available" and "not available">.
• Iron needs to be supplied in a complexed, or chelated, form.
• Plants can't move iron internally, meaning that iron deficient leaves will not green up when iron becomes available again.
• It is only new leaf tissue, produced when iron is plant available, that will be healthy.

*I've edited this and I'd like to thanks @keef321 for his PM:

.... Is there any chance of you thinking about re-wording this, as it makes it sounds like you cannot use the index to help with iron deficient plants? ......... I was helped so much by your index, and simply want others to realise it can help them with iron. I used the duckweed index to a great extent to help me diagnose iron deficiency, as without the chelated iron the duckweed simply melted away, and as i moved over to stronger & stronger chelates it improved lots.....

Further edit: I'm going to add in @keef321 's post <"Cloudy Water Hazy Water and Algae!">, because <"a picture is worth a thousand words">.

This iron deficient Amazon Frogbit (Limnobium laevigatum) (thank-you to @jameson_uk for the photo, from <"Duckweed Index says Nitrogen please?">). I'm sure @jameson_uk won't mind me mentioning that he is <"red-green colour blind"> and colour blindness does cause an extra level of complexity in judging "greeness".

<"Earlier in the thread"> I said I'd come back to this photo and specifically to the white-ringed leaf.

This is also iron deficiency, just at a stage where the iron supply has been turned off during leaf formation. These <"netted, reticulated">, <"striped"> or <"snake-bite"> lesions are how iron deficiency looks in a monocotyledon, the different appearances being dependent on leaf shape.

Image from <"Frogbit taken a turn">.

This is stripy, iron deficient Maize (Zea mays), but you also see this effect in Hygrorhyza aristata -<"Slightly sad floating plants"> and a recovery.

To stop this post becoming too unwieldy I'll continue in a new post: "We've talked about iron deficiency, but what are we going to do about it?"

cheers Darrel

 

[dw1305]

We've talked about iron deficiency, but what are going to do about it?
Because of the <"lag phase">, between adding plant available iron (Fe) and <"new healthy leaves growing">, I now use a tweak to the original Duckweed Index concept, where I add iron (Fe) (as FeEDTA, & magnesium (Mg)*,) on a regular basis.

I've called this approach the <"Hybrid Duckweed Index">, mainly because it borrows from <"Estimative Index"> the concept of <"ensuring that nutrient deficiencies don't occur">.

This is @keef321's "before" picture, <"with iron deficiency"> .

and "after" without iron deficiency:

This <"Bacopa caroliniana"> belongs to @Lapul and <"shows iron availability being turned back on"> after a period of deficiency. I'll link in the <"Should i start to fertilize?"> thread as it is relevant to iron and well worth a read. You can also see some healthy Amazon Frogbit (Limnobium laevigatum) in the photo.

I'll start a new post, but we need to carry on talking about iron and the mechanisms for <"keeping it in solution">.
* I'll come back to this later.

cheers Darrel

 

[dw1305]

Plant available iron (Fe) and pH.
Iron (Fe) is a <"Jekyll and Hyde"> element, where <"both deficiency and toxicity"> can be issues for aquatic plants. Iron is <"abundant in the earth's crust">, so <"issues of deficiency"> are very much ones of solubility.

The problem with ferric iron (Fe+++) is that it <"combines with dissolved oxygen"> & hydroxide (OH-), and with other (multivalent) anions, like <"phosphate (PO4---)"> and bicarbonate (HCO3-), to form <"insoluble compounds">.

.......... it (iron) is a very strong hard lewis acid which easily forms insoluble salts with many of the hard lewis bases within our hydroponic solutions. When iron is added to a nutrient solution in its “naked” form (for example when adding iron (II) sulfate) the ion easily reacts with carbonate, phosphate, citrate, oxalate, acetate or hydroxide ions to form insoluble compounds that make the iron effectively unavailable to our plants.....The solution to this problem is actually easy and comes in the form of chelating agents that “wrap” around the iron ions and make them disappear to anions that may want to form stable salts with them. There are many of these chelating agents with the most commonly used being EDDHA, EDTA and DTPA.......

Chelators and complexors
Because of this <"tendency to go out of solution"> ferric iron (Fe+++) ions are combined with an organic acid, <"a chelator like FeEDTA">, which stops this happening. The iron ions are actually so tightly bound by the chelator (FeEDTA etc.) they would be unavailable if the chelator wasn't broken down (photodegraded) by light - <"EDTA - MOTM">.

As well as chelates there are also compounds where the iron is less tightly bound and "complexed". Some <"unscrupulous vendors"> will try to suggest that this <"makes their product superior"> and they may also suggest that ferrous iron (Fe++) offers advantages , but <"neither of these statements are true in practice">.

Which chelator you use will depend on the <"pH of your water">.

Decrease in soluble iron upon addition of iron chelates is caused by fixation and by replacement of the chelated iron by calcium at high pH...........
At high pH (pH ‐ H2O 7.25 and 7.85) this order is: Fe‐EDDHA ≫Fe‐DTPA > Fe‐HEEDTA > Fe‐EDTA. In cultivating plants at low pH, Fe‐EDTA and Fe‐DTPA will be the more efficient iron sources. At high pH, however, only Fe‐EDDHA supplies sufficient soluble iron for a long period .........

from: . Boxma (1981) "Effect of pH on the behaviour of various iron chelates in sphagnum (moss) peat", Communications in Soil Science and Plant Analysis, 12:8, pp. 755-763.

For most aquarists <"Chempak Sequestered Iron"> would work as a an iron source: <"Chempak® Sequestered Iron with Magnesium & Manganese | Thompson & Morgan">. We think it is probably <"https://www.gardenworld.co.uk/resou...equestered Iron Plant Tonic 4x20g 15019v1.pdf"> rebranded.

I'll move onto fertilisers for the aquarium generally (and the Duckweed Index specifically) in the next section.

cheers Darrel

 

[dw1305]

Fertilisers and the Duckweed Index.

Substrate
In nutrient terms rooted plants have access to the substrate, and that is a world of <"unknown unknowns">, with a huge number of <"potential variables">. Nutrients that aren't available in the water column <"may be plant available"> within the substrate, but not to floating plants and epiphytes (or <"algae">).

What the "Duckweed Index" allows us to do is side-step the thorny, multifactorial question of what happens in the substrate (should you have one) and just concentrate on the water column, and whether all the nutrients are plant available within it. Bryophytes (mosses and liverworts), plants grown as epiphytes (like <"most Aroid and ferns">) and floating plants are all reliant on nutrients in the water column, but all plants <"can take up ions via their foliage">.

Fertilisers
All plants can only take up mineral nutrients as "ions", <"charged particles in solution">. This means that there are no <"specific aquarium fertilisers">, there are <"just fertilisers">. UKAPS member @Happi has written a very useful article and that would be my suggested starting point: <"Solufeed 2:1:4 and Solufeed Sodium Free TEC or Solufeed Coir TEC Combination">.

If people want to tailor a specific mix, they can use dry salts to <"make their own bespoke fertiliser mix">. We have our own super-duper spreadsheet <"IFC Aquarium Fertilizer Calculator"> and there is also the <"Rotala nutrient dosing calculator">.

The Duckweed Index can be used for all dosing regimes from <"running on petrol fumes"> to <"Triffid">, just by varying <"the amount of nutrient added">.

I said <"I'll come back to magnesium (Mg)">, and I'll just mention it now. Some recipes for the <"Estimative Index don't include magnesium">, and this is because the <"extensive limestones in the mid-west of the USA"> have undergone <"dolomitization". where some the calcium (Ca) has been <"replaced by magnesium">.

This isn't the case in most of the UK, and water from our limestone aquifers has a <"very high Ca : Mg ratio">. I don't see a downside to adding a small amount of magnesium, even though it is mobile within the plant and a rapid greening occurs when it ceases to be Liebig's limiting nutrient.

cheers Darrel

 

[dw1305]

What is the future for the Duckweed Index?
The Duckweed Index has been developed over ~15 years, and while I don't expect any major changes, but there will be <"continuing tweaks to it">.

The Duckweed Index has definitely gained a bit of traction, particularly among <"German Apistogramma keepers"> and with <"Tropica"> alluding to it in their product description for Amazon Frogbit - <"Limnobium laevigatum - Tropica Aquarium Plants">. They say:

.......... When the nutrient level in the water decreases, it will grow much slower which makes it a great indicator of the available nutrients.

Alternative plants for the Duckweed Index.
In terms of the floating plant I've concentrated, originally on Common (Lesser) Duckweed (Lemna minor) and subsequently, on Amazon Frogbit (Limnobium laevigatum) because they have <"leaf green"> leaves, but in terms of improving water quality any floating plant would do.

Water Hyacinth (Eichornia (Pontederia) crassipes) also has green leaves, but is a <"turned up to eleven plant"> and needs plenty of everything (light, heat, nutrients) to grow.

There are some other options that are more amenable to cultivation, but they have leaves that aren't grass green, mainly because of the presence of hydrophobic hairs.

Below, these are two floating ferns, <"Azolla caroliniana"> and <"Salvinia "auriculata group"> (the plant above), their major disadvantage is their leaf colour. The <"flowering plant is Utricularia gibba">, which is a sub-surface floater, but not suitable for the Duckweed Index, <"not having any leaves"> and being insectivorous. "Insectivorous" matters, because that is an alternative nitrogen (N) source, not available to other plants.

This is <"Pistia stratiotes (Nile Cabbage or Water Lettuce)"> (below) it would be just as suitable as Amazon Frogbit, if it had a green leaf. It also shows <"great plasticity in growth form">.

I'll give a Phyllanthus fluitans "Red-root Floater" <"a post to itself">, because a Phyllanthus - Limnobium mix may offer some advantages for the Duckweed Index.

cheers Darrel

 

[dw1305]

What is the future for the Duckweed Index II, is it a mix of floaters?
You might remember @castle's Amazon Frogbit (Limnobium laevigatum) photo from <"earlier in the thread">. The other plant in the photo is Red-root Floater (Phyllanthus fluitans) - <"Phyllanthus fluitans - Tropica Aquarium Plants">.

The Lithuanian fertiliser company "VIMI" - <VIMI> have a slightly different take on the Duckweed Index, where they've used the <"intensity of red pigmentation"> in Red-root Floater (Phyllanthus fluitans ) as their index of nitrogen content - <"What are the causes of diatoms?">.

Nitrogen limitation also gets a runout on the 2Hr Aquarist web site <"How to make red root floaters (Phyllanthus fluitans) redder?">

RRFT METHOD.

VIMI has perfected a certain method RRFT (RED ROOT FLOATER TEST) which can maintain optimal parameters for many plants without using tests, but using one plant, Phyllanthus fluitans.

The essence of the method is to constantly test the water with the color of the Phyllanthus fluitans plant.

At higher than 5 mg/L (ppm) NO3 this plant grows green:

At 0-1mg/L NO3 (ppm) values, this plant turns red:

So, we can follow the situation in the aquarium without tests, since this plant grows quite quickly, is convenient for observation, and may not take up much space in the aquarium, it is perfect for seeing the current and following the previous situation in the aquarium, especially with the difficult to determine N amount, which varies a lot in the aquarium.

This plant is one of the few floating plants that can turn red. It only turns red in strong light and if the water is very minimal in nutrients. If the water has a lot of nutrients, it always grows green, but only when they start to decrease, this plant changes color from green to brown, finally even red. It blooms with small white flowers. Total hardness: 0 - 20°dGH Carbonate hardness: 0 - 15°dKH Temperature: 18 - 28°C pH limits: 6-7.5 Color: green to red Growth rate: fast - medium Use: water surface, for determining the nutrient content of aquarium water .....

Personally I don't think that Phyllanthus fluitans, <"on its own">, is as useful as Amazon Frogbit (Limnobium laevigatum) but I am going to try @Wookii and @castle's mix as the next development for the Duckweed Index.

cheers Darrel

 

[dw1305]

References and some background reading.
There is a list of (mainly) Amazon Frogbit (Limnobium laevigatum) wastewater phytoremediation references at: <"Amazon Frogbit (Limnobium laevigatum) phytoremediation references">. I'll carry on updating this link as I get new references.

Books
Diana Walstad's "The Ecology of the Planted Aquarium" is now in its 4th Edition*. <"Planted Aquariums">.
I'm unashamedly a fan, and I wouldn't have ever come to create the <"Duckweed Index without her book">, but <"there are certain things"> in the earlier editions of the book that she has subsequently revised:. <"Walstad revises">

*I possess the 2003 edition and I haven't seen the newest edition.

cheers Darrel

 

[Genahanney]

How does the "duckweed" index account for nitrogen developing in nutrient soil? The soil may be rich in organic matter, but the test for nitrate and other nitrogenous compounds will show 0..So this index is only suitable for keeping a plant aquarium on neutral ground (not for soil)?

If the plants are pale green in color, there may be nitrogen in the water, but its consumption may be reduced due to a lack of other micro and macro elements, right? It turns out that this index works not only with unlimited CO2, but also with an unlimited amount of all other elements?

 

[Mike Singh]

Hi Darrel
Will Salvinia natans be a viable alternative? I'm am considering it in a very shallow (15cm depth) Paludarium. The roots of the Frogbit will be to long cosmetically.

 

[dw1305]

Hi all,

Will Salvinia natans be a viable alternative?

Yes, any floating plant works in terms of growth rate.

The problem with Salvinia natans (auriculata group) (and Pistia stratiotes) is just <"that the leaf hairs"> make it difficult to accurately assess leaf colour.

This plant had been grown in very warm water and high light, it has been enjoying itself, but may still be nutrient deficient, you just can't tell.

The advantage of Amazon Frogbit (Limnobium (<"Hydrocharis laevigata">) laevigatum)) is that its leaf remains "leaf green" under high light etc., which allows you to use the Rice "nitrogen" <"Leaf colour chart"> as a datum - <"The scientific background to the "Leaf Colour Chart"">.

cheers Darrel

 

[dw1305]

Hi all,

How does the "duckweed" index account for nitrogen developing in nutrient soil?

No, it doesn't directly, although <"fixed nitrogen compounds are highly soluble">, and there will be transfer from the substrate to the water column, but to an unknown extent.

Substrate
.... In nutrient terms rooted plants have access to the substrate, and that is a world of <"unknown unknowns">, with a huge number of <"potential variables">. Nutrients that aren't available in the water column <"may be plant available"> within the substrate, but not to floating plants and epiphytes (or <"algae">).

The original idea was to have a <"rooted emergent plant">. This would have the dual benefits of Diana Walstad's <"Aerial Advantage"> and oxygenating the substrate (via <"Radial Oxygen Loss">). The problem was just that a rooted emergent plant, like a <"Cyperus"> or <"Nelumbo"> spp., isn't suitable for every tank, while every tank <"can have a floating plant">.

If the plants are pale green in color, there may be nitrogen in the water, but its consumption may be reduced due to a lack of other micro and macro elements, right?

Yes, you need all fourteen of the essential mineral nutrients to be present, this is due to the <"assembly line aspect of plant growth">. This is the same for all plants, just with <"nutrient requirements"> that span <"several orders of magnitude">.

Personally I actually favour a <"lean dosing approach"> and my Frogbit <"never looks like this"> plant, which belongs to @Cédric .

Deficiencies of mobile plant nutrients are <"difficult to diagnose">, but you can use probability to some degree. Because fixed nitrogen is the mobile mineral nutrient that plants require most of, it is a good starting point. You also get a very quick growth response when you add fixed nitrogen (if it was deficient).

Deficiencies of non-mobile nutrients take longer to rectify, <"but are easier to diagnose">.

It turns out that this index works not only with unlimited CO2, but also with an unlimited amount of all other elements?

No, not at all, a floating plant takes CO2 out of the equation, but one of the great advantages of Amazon Frogbit is that it has a <"very flexible growth response to nutrients"> and you can use the <"Duckweed Index"> with both <"lean dosing"> or <"with EI"> type nutrient levels.

cheers Darrel

 

[simon_the_plant_nerd]

Hi all,

Yes, any floating plant works in terms of growth rate.

The problem with Salvinia natans (auriculata group) (and Pistia stratiotes) is just <"that the leaf hairs"> make it difficult to accurately assess leaf colour.

This plant had been grown in very warm water and high light, it has been enjoying itself, but may still be nutrient deficient, you just can't tell.

View attachment 217866

The advantage of Amazon Frogbit (Limnobium (Hydrocharis laevigata) laevigatum)) is that its leaf remains "leaf green" under high light etc., which allows you to use the Rice "nitrogen" Leaf colour chart as a datum. <"The scientific background to the "Leaf Colour Chart"">

cheers Darrel

I have Salvinia Natans in my tank which was given to me by @dw1305 and it no longer looks anything like his (please excuse Mr Snail) due to the different, but equally healthy, growing conditions.

The Salvinia grows so quickly in my tank that it covers the water surface long before I get those hair covered, large leaves and they’re always bright green. It’s difficult to keep track of which is new growth and which is old. I’ve got Pistia and Frogbit in there too and it quickly gets crowded. Everything is healthy in the tank and I think the Frogbit does the best job of representing tank nutrients. Pistia comes second as the leaves will turn yellow if it’s unhappy.

RRF is probably least useful as the leaves are easily crowded and damaged causing them to curl and fade which could be mistaken for nutrient deficiency. I grow them in a separate tank now where I can control the amount of nitrates and light they get. I just grow RRF for fun now.