# Liebig's Law of the Minimum



## hoppycalif (Apr 7, 2005)

From Wikipedia: "*Liebig's law of the minimum, often simply called Liebig's law or the law of the minimum, is a principle developed in agricultural science by Carl Sprengel (1840) and later popularized by Justus von Liebig. It states that growth is dictated not by total resources available, but by the scarcest resource (limiting factor).*" This "law" should have a lot of influence on how we fertilize our planted aquariums.

When we don't get as good a growth rate as we expected we are often told to increase the fertilizing we are doing - rarely good advice. Plants need nitrogen, potassium, phosphorous and carbon in relatively large amounts, and many other elements, like iron, calcium and magnesium, in very small amounts. Liebig's law says that if our plants are lagging in growth rates, that is due to the shortage of one of those elements, not a lot of them. Ideally we would figure out which element is lacking and dose only that element. But, we have no good way to determine which element that is.

What we can do is pick one of those elements and try to double the amount available to the plants, then watch the plants to see if that was the one that was slowing down the growth, as noticed by an increase in plant growth after that one element increase. When that doesn't increase the growth rate, we should stop dosing that element and increase another element, again watching for increased growth rates. If we do this until we get the added growth rate we will have found which element was in short supply.

Of course, once we increase the element that was in short supply, another element will then be the one limiting the plant growth at some higher growth rate. So we could possibly increase several of the elements until we get maximum growth rate (limited by how much light intensity we have).

One thing we know is that carbon is very likely to be the element that will eventually be what determines what the maximum growth rate can be, because the normal source of carbon for all plants is CO2, from the atmosphere or from some other source. That is why starting to add CO2 to a planted aquarium will very often result in much faster growth rate of the plants.

I'm not sure where this is taking me![smilie=b:


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## mistergreen (Mar 3, 2007)

I thought of the EI dosing method where you throw in all the resources into the tank.


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## hoppycalif (Apr 7, 2005)

The EI method does make sure none of the nutrients that it uses can be low enough to restrict the growth rate of the plants. But, carbon will still be most likely to be restricting that growth rate, unless you are adding CO2 to the water. And, there will be enough surplus nutrients to keep lots of algae growing.

My experience with planted aquariums was:
10 gallon tank with vals as the plants, but a weak light on the tank - little growth.
Change to fluorescent light - good growth.
29 gallon tank with variety of plants and fluorescent light - some plants grew, others didn't.
Dosed per the EI method - more plants grew and grew faster.
Added pressurized CO2 - all plants grew and faster, and algae grew very well.

When CO2 was becoming popular, it was mostly DIY CO2, so it wasn't even close to 30 ppm of CO2. But, the improvement in plant growth made people willing to pay for pressurized CO2. Very likely carbon is the "minimum" nutrient in most, if not all planted aquariums that don't get injected CO2, so tinkering with fertilizing will not be helpful. El Natural aquariums will be much more successful because the soil in the substrate can produce CO2 and possibly make another element be the "minimum", like calcium/magnesium, for example.


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## Michael (Jul 20, 2010)

This also explains why new Walstad tanks often show explosive growth, then slow down as the months pass. Levels of macro and micro nutrients (except CO2) are maintained by input of fish food. Natural CO2 production slows as organic matter in the soil decomposes into more stable forms, like humus. Of course natural production of CO2 never entirely ceases because plants, animals, and bacteria respire all the time. And we manage natural CO2 with lighting schedules and using circulation or filtration methods that do not cause off-gassing.

It is a common misconception that soil becomes "depleted" of nutrients in a Walstad tank. If the soil is composed of materials that have high cation exchange capacity, the substrate absorbs and releases nutrients that come from fish food.

But this does not happen with carbon. In a mature Walstad tank, bacteria in the substrate do continue to contribute CO2, but not at the levels when the soil was new and organics were decomposing at a rapid rate. When managed correctly, the mature tank achieves a steady state in which inputs of nutrients (fish food) and carbon (respiration and atmospheric diffusion) roughly balance the requirements of plants. When you add the great biofiltration capacity of plants, the tank becomes remarkably stable. These are the tanks you hear about, "All I ever did for 3 years was feed the fish and top off evaporation!"


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## mistergreen (Mar 3, 2007)

Yeah, I remember growing Beard Algae farms using the EI method. Plants were fine and healthy but massive beard algae growth on hardscape and hardware.


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## hoppycalif (Apr 7, 2005)

mistergreen said:


> Yeah, I remember growing Beard Algae farms using the EI method. Plants were fine and healthy but massive beard algae growth on hardscape and hardware.


Too bad we don't think dark gray blobs are beautiful


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## mistergreen (Mar 3, 2007)

hoppycalif said:


> Too bad we don't think dark gray blobs are beautiful


If bba was red or green, I'd welcome it.


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## Michael (Jul 20, 2010)

I've watch videos of our aquarium fish in their natural habitats, and they often have lots of algae. I've tried really hard to find it beautiful, but when it shows up in my tanks all I think is "YUCK!"


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## hoppycalif (Apr 7, 2005)

I"m wondering if light is a nutrient? If so, using a higher intensity light would not increase growth rate if carbon or iron, etc. was the least available nutrient. And, increasing the light intensity would only improve growth until another nutrient became the minimum. I think this is how it works, and is why you need to add CO2 if you want a higher intensity light to help the plant growth.


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## mistergreen (Mar 3, 2007)

I wouldn't say light is a nutrient. It's more like a driver or metabolism. Different species have different levels built in their DNA like Anubias vs Rotala.

Low light plants will usually get beard algae grow on them in a high light tank given all the available nutrients.


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## hoppycalif (Apr 7, 2005)

A complication with Liebig's law of the minimum is that different plants have different needs for nutrients and light. If we had nothing but one species of plants in our tank it would be possible to adjust everything so we would get the maximum growth, with no leftover nutrients to feed algae. Maybe this means we shouldn't put anubias in the same tank with Amazon sword plants.


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## mistergreen (Mar 3, 2007)

hoppycalif said:


> A complication with Liebig's law of the minimum is that different plants have different needs for nutrients and light. If we had nothing but one species of plants in our tank it would be possible to adjust everything so we would get the maximum growth, with no leftover nutrients to feed algae. Maybe this means we shouldn't put anubias in the same tank with Amazon sword plants.


We can put low light plants in with high light plants, mainly in the shade of the bigger plants.


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## Dude (Nov 14, 2018)

mistergreen said:


> I wouldn't say light is a nutrient. It's more like a driver or metabolism. Different species have different levels built in their DNA like Anubias vs Rotala.


This makes me wonder what plant would have the most genetic potential for fast growth. Say someone cranked everything up to 11 (lights, fertilization, and CO2), what is the biological limit of growth for various plants? Surely it has been done before. In my own limited experience in much more modest conditions I've observed some plants growing several inches per day.


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## mistergreen (Mar 3, 2007)

Yup, there are weeds that grow very fast like hornwort, elodea, H. Polysperma.


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## Seattle_Aquarist (Mar 7, 2008)

mistergreen said:


> I thought of the EI dosing method where you throw in all the resources into the tank.


Hi All,

I'm not sure I totally embrace Liebig's Law, there have been several cases I have seen where more than one nutrient was effecting healthy plant growth. I would be more inclined to say that there is usually a primary cause and likely secondary causes.

As for the EI Method it does provide a surplus of all three macro-nutrients (N, P, K), and if the pH is correct (i.e. below 7.0) sufficient ETDA iron, but very little of the other secondary nutrients magnesium, and no calcium whatsoever. It does contain manganese, boron, zinc, and molybdenum.

Living in the Seattle area our water if very, very soft and has 9 ppm of Ca and 0.6 ppm of Mg, if I don't add additional secondary nutrients to my tanks my plants just don't grow. -Roy


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## hoppycalif (Apr 7, 2005)

Seattle_Aquarist said:


> Hi All,
> 
> I'm not sure I totally embrace Liebig's Law, there have been several cases I have seen where more than one nutrient was effecting healthy plant growth. I would be more inclined to say that there is usually a primary cause and likely secondary causes.
> 
> ........... -Roy


I think it isn't that some of us don't embrace Liebig's Law, but that we just don't understand it. I find it hard to grasp, as simple as it is. There is certainly a lot of evidence to support it. One good example is CO2. Before people used CO2 with their planted aquarium they tried to get good plant growth by just getting more intense lighting, which is understandable because there were few ways to get more intense lighting - incandescent bulbs and fluorescent bulbs were the only choices. When we heard about using a yeast/sugar mix to get CO2 into the water it was almost miraculous. It always caused the plants to grow much faster, with whatever lighting we had. That was because carbon was the nutrient that was least available to the plants. The Walstad method worked so well largely because it used CO2 generated by nature to increase the amount of carbon available to the plants.

I think there is a benefit to trying to think about Liebig's Law as we figure out how to make out aquatic plants grow. Of course it isn't the magic bullet, but it is still important.


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