Last Update

Experiment #1

Nutrient uptake
in a planted aquarium


1. Summary of the article
(in a nutshell)

This article seeks to find out the answer to the question of how much nutrients the plants consume in the normal planted aquarium, or what nutrient uptake can be expected there on a weekly basis. Based on several tests in densely planted and strongly lit aquariums (with the light irradiance of ~100 µmol PAR at the substrate), which was artificially fertilized with CO2 (~35 ppm) with a regular supply of all essential nutrients [in the so-called high-tech aquariums], I have verified that the level of nutrient uptake by aquatic plants largely depends on the external concentration of these nutrients (whether they be in the substrate or water column). In other words, at higher external nutrient concentrations the plants uptake more nutrients (under otherwise identical conditions) than at lower external nutrient concentrations. At extremely high external nutrient concentration in the water column (30 ppm NO3, 20 ppm K, 5 ppm PO4, a 0.5 ppm Fe) in an extremely densely planted aquarium without substrate, without filter media and without fish, the weekly nutrient uptake by aquatic plants was approximately:

10 ppm NO3, 5 ppm K, 1 ppm PO4, and ~0.01 ppm Fe

It should be noted, however, that the external concentration of some nutrients (mainly N and K, and possibly some microelements) represented a luxurious uptake of nutrients far beyond what they needed for optimal growth or condition (which was confirmed by laboratory analysis of nutrient content in dry matter).

2. Keywords

1) nutrient uptake in aquatic plants; 2) decrease of nutrients; 3) planted aquariums; 4) fertilization

3. Introduction

Every aquarist who wants to have prosperous plants in his aquarium, inevitably deals with the issue of ensuring a sufficient supply of nutrients (in addition to the issue of appropriate lighting). Without light and nutrients plants simply can't do well. Unfortunately, instead of spreading reliable information among aquarists, there is often a lot of delusions or speculations, the most common source and seedbed of which are Internet discussion forums. If you prefer reliable information verified by the scientific research, focus rather on the scientific literature → see the Study section. A good source of scientific information that is presented in a fairly accessible and comprehensible form to aquarists is Diana Walstad's Ecology of the Planted Aquarium (although the author often pass her assumptions off as scientifically proven facts without proper backing it up, which is an evil called "confirmation bias" today). Other excellent publications, featuring lots of useful information about the ecology of both aquatic and terrestrial plants, are Robert Wetzel's Limnology: Lake and River Ecosystems, and Petra Marschner's Mineral nutrition of higher plants. From time to time, interesting information about aquatic nutrition can also be found in popular-aquaristic magazines, where some scientists sometimes publish their findings.

So, in this article, I will outline the possible ways to find out how much nutrients the aquarium plant are consuming under the given conditions, and at the same time describe the exact way I have done it, and what I base my knowledge on in order for everyone to be able to verify (replicate) it.

4. Methods and data

Methodology → how I proceeded:

The uptake of nutrients in a planted aquarium can be measured in the following ways:

  1. By measuring the decrease of nutrient levels using drop test kits

    • Procedure:
      • At the beginning of the reference period (e.g. at the beginning of the week) we measure the concentration of some important nutrients in the aquarium (NO3, PO4, Fe) using drop test kits. We will do the same at the end of the reference period (e.g. at the end of the week). We then subtract both measured values, and the difference represents the approximate uptake of nutrients in the tested aquarium over a given period.
    • Example:
      • On Monday, I measure 30 ppm NO3, 1 ppm PO4 and 0.50 ppm Fe in the aquarium. On Sunday, I measure 25 ppm NO3, 0.7 ppm PO4 and 0.49 ppm Fe. This means that over the period of one week the concentration of nutrients in the water column decreased by 5 ppm NO3, 0.3 ppm PO4 and 0.01 ppm Fe.
    • Pros of this method:
      • Simplicity (it only requires to buy the drop test kits, and to measure the nutrient concentration).
    • Cons of this method:
      • The least accurate determination method. With commonly available drop test kits (e.g. JBL or Sera), it is possible to measure only selected nutrient types → most often NO3, PO4, and Fe (CO2 uptake cannot be measured by this method at all). In addition, drop test kits are rather used for approximate (rough) determination only → they are not intended to determine the exact concentrations. For this reason, the measured results may be very inaccurate, and even if by the calibration of these tests we ensure more accurate measurements, the results may result in an overestimation, as this method does not actually detect direct nutrient uptake by plants, but only the rate of nutrient decrease in the water column, which is not necessarily the same, because part of the nutrients that we add to the water in the form of fertilizer may not only end up in plants, but also in the substrate or filter media, or microbes may eventually break them down, or they may partially precipitate or otherwise degrade. How big a part it will be, however, is hard to quantify with this method.

  2. By measuring the depletion of nutrient levels using laboratory analysis

    • Procedure:
      • On Monday, we take a water sample from the aquarium and take it to the laboratory for analysis. We will do the same on Sunday (or the next Monday). Based on the difference, we find a relatively accurate weekly depletion of nutrients in our aquarium.
    • Pros of this method:
      • A relatively reliable way to find out the total depletion of nutrients in the entire system (aquarium). Thanks to laboratory analysis we can know the values of all the elements of interest (except CO2).
    • Cons of this method:
      • As with the previous method, we do not actually find out the nutrient uptake but the depletion of nutrients here. Thus, the same restrictions apply here as in the previous method. In addition, the absorption capacity of the substrate as well as of the filter media can significantly influence the results. If a larger amount of the added nutrients "soaks" (absorbs) into the substrate, we get a much higher depletion of nutrients. As already mentioned, it won't be possible to determine precisely what amount of nutrients pertain to the plants themselves. However, I suppose that in aquariums with inert substrates the use of this method may make sense (see results below).

  3. By reducing the dose of fertilizer until the symptoms of nutrient deficiency begin to appear

    • Procedure:
      • First week we dose unlimited amounts of nutrients to the aquarium (we can start at the recommended dosage of the Estimative Index method). Second week we decrease the dose from the first week by 20%, and we monitor the condition of plants. Third week we decrease the dose from the second week by 20%, and we monitor the condition of plants. This way we continue until the plants begin to show any signs of nutrient deficiency. When this happens, we record the last known "trouble-free" dose that represents the real uptake of nutrients in our aquarium. The invervals between dose adjustments is better to set to 2 weeks so that the plants have enough time to respond to the change.
    • Pros of this method:
      • Simplicity → no measurements or complicated calculations or laboratory analyzes are required.
    • Cons of this method:
      • Practically identical to the second method. In addition, when the nutrient deficiency symptoms begin to appear in plants after a reduction in the fertilizer dose, we will have no certainty about which nutrients are still sufficient and which are already lacking. So in this way, we will only find out when nutrients in a given ratio cease to suffice to plants. In addition, this method does not count in the reserve of nutrients that plants can create at the time of "abundance" and which can then be used for their growth for different periods of time at a time of "lack".

  4. By weighing plant biomass increments (over a longer period of time)

    • Procedure:
      • This method consists in regular and long-term weighing of the increments of plant biomass → i.e. plant parts (or surpluses), which are removed from the aquarium during regular care for plants (consisting mainly of trimming and replanting). We remove the trimmed plants' excess from the aquarium, wipe the excess water from its surface (preferably in a salad spinner dryer or in a towel), and then weigh it on a digital scale with a minimum resolution of 0.1 g. We record the weighing results (in grams) for at least 2-3 months, and then calculate the average weekly yield of "fresh weight". Then we convert this result to the unit of bottom area of the aquarium (g/dm2). Example: I have been recording plant biomass gain in my aquarium for three months. In total, it was 543 g in 123 days. Thus, the average weekly yield was 30.9 g (543 g / 123 days * 7 days). My aquarium had a bottom area of 31x14 inches (80x35 cm), i.e. 28 dm2. Thus, the average weekly gain in the tested aquarium was 1.1 g / dm2 (30.9 g / 28 dm2). Based on the resulting nutrient ratio in dry matter (see the "Data I base on" section) we calculate the amount of nutrients required to produce a given amount of plant biomass. However, the increments measurement alone does not lead us to the goal, as it usually does not take into account the increments of root biomass, photosynthesis waste products, or the efficiency of distribution and uptake of nutrients (which never makes 100%). And because the influence of these factors on the final result can only be estimated, the calculated amount of nutrients consumed will always be just an estimate (albeit qualified, and probably the best we will be able to come home with).
    • Pros of this method:
      • In home conditions, this is perhaps the most accurate method of determining the amount of nutrients our plants consume over the reference period (one week). This method is used for rough determination of nutrient uptake in aquatic plants even by experts from the Institute of Botany of the Academy of Sciences of the Czech Republic in Třeboň.
    • Cons of this method:
      • In order for this determination of uptake (using this method) makes any sense, it is necessary to determine the percentage of nutrients in the dry matter of the test plants, which is only possible by laboratory analysis (which is not a cheap thing → a complete analysis of most essential nutrients in the dry matter costs about 1,500 CZK = $70). While calculations may be based on a general average, the calculated results may not be too accurate in such a case. Moreover, if root biomass increments are not taken into account, the results may be slightly underestimated (my estimation: by 20%).

Factors affecting the result

  1. Root biomass
    • When weighing plant biomass increments, it was not always possible to take into account the amount of root biomass, although some plants were removed from the aquarium with roots from time to time, and new shoots were planted in their place. However, submerged plants have – with some exceptions – a small proportion of roots to shoots (only 10-30% of the total biomass), and some species (e.g. bladderworts or hornworts) are even completely rootless. In addition, it should also be borne in mind that during the regular trimming of new plant growths, the plants will again produce new shoots after trimming, while the root system will no longer grow significantly after trimming the shoots and will remain more or less the same; so it can play a bigger role probably only in expansive plant species. This will in practice mean that the greater the number of trims, the smaller share the root biomass will have in the total fresh weight.
      • Example: If we assume that on average 20% of the plant biomass accounts for the root system, then for the plant weighing, for example, 10 g (fresh weight), the root system will make 2 g. But if we cut (trim) the plant by half every 14 days, we get about 4 g of fresh weight every 10 weeks (10 g = whole plant, of which 2 g = roots + 8 g = shoots and leaves; if we cut half of the shoots every 14 days, we get 4 g). So for example, in 3 months (12 weeks) we get 24 g of fresh weight (6 trims x 4 g). However, the root biomass will still be roughly the same, which means that it will still only have these 2 grams. So for every 24 grams of fresh weight of the shoot biomass is only 2 grams of root biomass (which is only ~8%). Of course, actual numbers may vary.
      • Counted effect: 20% (for every 100% of plants trimmed the new root biomass makes 20% at the most)
  2. Waste products of plants (photosynthates)
    • About 1-2% of carbon (C) from gross photosynthesis are excreted by aquatic plants back into water, most often in the form of acids.
      • Counted effect: 0% (if plants produce a certain biomass [dry matter], the amount of mineral nutrients is derived directly from it)
  3. Substrate
    • Large amounts of nutrients can be also absorbed from the water column by porous (especially clay-based) substrates. However, it can be assumed that such absorbed nutrients will be largely accessible to plants in the substrate.
      • The effect cannot be estimated (if a substrate with a high sorption capacity is used, it would be probably better to somewhat increase the nutrient dosage)
  4. Other organisms in the aquarium
    • It has been reported in the literature that up to 60% of nitrogenous substances (N) is converted by microbes in an aquarium to nitrogen gas (N2), which then bubbles out of the aquarium. However, it is not primarily nitrogenous substances in the water column, but rather in the substrate that are formed there when decomposing organic matter, or denitrification in deeper anaerobic layers. For this reason, I will not count on this effect in my calculations.
      • The effect on nutrients in the water column is likely to be negligible
  5. The ratio of nutrients in dry matter
    • The ratio of nutrients in the dry matter, which I use to calculate the actual uptake of nutrients in aquatic plants, was determined by means of laboratory analysis at the Research Institute for Soil and Water Conservation in Prague. If someone wanted to do a similar test and would not be able to have a precise laboratory analysis of the plant material done, he/she could come out of the range for good growth reported in the literature (2.5-4.5% N, 0.25-0.75% P, 1.5-5.5% K, 1.0-4.0% Ca, 0.25-1.0% Mg, 0.01-0.03% Fe). Of course, for different plant species (and even for the same plant species under different conditions), the specific nutrient ratio in the dry matter will vary, and thus the final results may also vary. This effect can play a relatively large role.
      • Counted effect: none (I base my numbers on accurate laboratory analysis)
  6. Dry matter content
    • The dry matter content of aquatic plants can range from 5-15% of fresh weight, although it will likely be around 10% for the vast majority of aquatic plants grown in aquariums. I myself tried to weigh the dry matter for three different samples of a mixture of commonly used aquatic plants (Anubias, Echinodorus, Pogostemon, Rotala, mosses, etc.), and the percentage (share) of dry matter on a fresh weight was 8-11%. In addition, I had the dry matter content determined by laboratory analysis, and it came out 9.88%.
      • In my test I will count on 10% dry matter content
  7. Efficiency of the distribution and intake of nutrients → supplied energy and plant metabolism
    • Although there will be relatively few nutrients in the water, if the water circulates in the aquarium, the plants will gradually draw all the nutrients they need. However, the rate at which plants will displace these nutrients from water (or substrate) will depend primarily on the amount of energy flowing into the aquarium (mainly in the form of light, but also of heat and nutrient concentration). This means that with stronger lighting, higher temperatures and higher nutrient concentrations, photosynthesis (and thus growth and nutrient uptake) will proceed faster as plant metabolism accelerates.

Selected methods

To find the real nutrient uptake by aquatic plants I chose the following three methods:

  1. Weighing of plant biomass increments in planted aquarium
  2. Weighing of plant biomass increments in test aquarium
  3. Measuring the decrease of nutrients (in aquarium without substrate and filter media) by laboratory analysis

Data on which I base [the weighing of biomass increments]

[sušina = dry matter, rostlina = plant, voda = water, organické látky = organic substances, popel = ash]
Aquatic plants contain 90% of water and 10% of dry matter on average. The dry matter consists of organic and inorganic (mineral) substances, of which approximately 10% remains in the form of ash after burning. Approximately 85-95% of the dry matter consists of organic substances on the basis of carbon (C), hydrogen (H), and oxygen (O). The remaining 5-15% is then attributable to other substances which can generally be found in the following range (relative to total dry matter):
2.5-4.5% N, 0.25-0.75% P, 1.5-5.5% K, 1.0-4.0% Ca, 0.25-1.0% Mg, 0.01-0.03% Fe
(The upper limit being characteristic of nutrient-rich eutrophic waters or wastewaters.)

According to Robert D. Munson 1), the average optimal nutrient content 2) in dry matter for terrestrial plants is roughly the following: 3.5% N, 0.48% P, 3.5% K, 2.5% Ca, 0.63% Mg, 0.63% S, 0.02% Fe. Unfortunately, no one has investigated what external nutrient concentration these values would correspond to in aquatic plants yet. Of course, the exact values will vary from species to species, and specific environmental conditions, plant age or other effects will play a role. However, if the nutrient content in the dry matter of our plants is within a certain optimal range (see the blue bar with white values), we can assume that our plants have sufficient (optimal) amounts of nutrients. On the other hand, if the nutrient content in the dry matter of our plants is significantly lower or significantly higher, it can be assumed that they have insufficient or excessive amounts of nutrients at their disposal. The first case will most likely be manifested by various symptoms of nutrient deficiency, while the latter may exhibit various effects of toxicity (e.g. growth deformations).

  • 1) KALRA, Yash P. Handbook of reference methods for plant analysis. Boca Raton: CRC Press, c1998. ISBN 1-57444-124-8.
  • 2) The optimum nutrient content in the dry matter corresponds to such an external nutrient concentration that results in normal plant growth. In other words, it is the amount of nutrients in the plant tissue that is sufficient to provide the plant with all the essential nutrients needed for normal (unrestricted) growth. If the plant has less nutrients than this optimum range in its tissue, it means that it does not have enough nutrients for normal growth in its environment, and that it may have various symptoms of nutrient deficiency (from growth retardation to severe chlorosis or necrosis). Conversely, if a plant has more nutrients than this optimum range in its tissue, it means that it has an excess of nutrients in its environment, so it stores nutrients to the reserve (above what it needs) and, if there are too many nutrients, the symptoms of nutrient toxicity may begin to show up.
  • GERLOFF, G. C. a P. H. KROMBHOLZ. Tissue analysis as a measure of nutrient availability for the growth of angiosperm aquatic plants. Limnology and Oceanography. 1966, vol. 11, issue 4, s. 529-537. DOI: 10.4319/lo.1966.11.4.0529. Available from: http://www.aslo.org/lo/toc/vol_11/issue_4/0529.html

Nutrient content in dry matter:

Plant speciesWaterDry matterSource
∑NPKCaMgFe
fresh weight
(%)		dry weight
(%)
Eichhornia crassipes90.29.81.030.421.810.02^1
Myriophyllum hetorophyllum0.161.251.470.260.0020^2
Ceratophyllum demersum0.264.010.770.420.0011
Najas guadalupensis0.153.490.980.470.0007
Eleocharis acicularis0.242.860.530.330.0030
Utricularia inflata0.261.980.670.210.0021
Potamogeton diversifolius0.273.081.140.190.0012
Eleocharis sp.1.40.130.6^3
Myriophyllum pinnatum2.10.204.0
Potamogeton americanus2.00.352.2
Sagittaria cuneata2.00.231.0
Spirogyra sp. [algae]1.00.106.79
Elodea sp.87.712.32.60.433.12.60.29^4
Lagarosiphon sp.88.111.92.90.351.62.50.43
Ceratophyllum sp.91.58.53.30.475.90.660.52
Eichhornia crassipes1)2.200.383.83^5
Eichhornia crassipes2)2.550.424.40
Eichhornia crassipes3)1.110.162.15
Alternanthera philoxeroides1)3.190.274.74
Alternanthera philoxeroides2)1.760.144.23
Alternanthera philoxeroides3)0.880.080.99
Justicia americana1)3.250.212.65
Justicia americana2)3.200.344.20
Justicia americana3)1.000.231.12
Anacharis canadensis0.021.091.640.560.00001^6
Ceratophyllum demersum0.221.292.460.690.0005
Eleocharis smalli0.171.001.950.400.0005
Lemna minor0.171.202.580.460.0002
Myriophyllum exalbescens0.261.350.920.210.0002
Potamogeton pectinatus0.341.993.760.200.0012
Potamogeton richardsonii0.171.334.030.500.0002
Sagittaria cuneata0.552.820.780.350.0019
Sagittaria rigida0.311.820.990.430.0021
Vallisneria americana0.163.751.820.300.0003
Azolla pinnata + japonica93.36.73.990.472.331.080.32^7
Alternanthera philoxeroides1.3-3.5^8
Ceratophyllum demersum2.7-3.0
Eichhornia crassipes1.3-3.7
Elodea canadensis2.2-6.3
Lemna minor2.5-5.0
Myriophyllum spicatum1.8-4.1
Pistia stratiotes1.7-3.9
Potamogeton sp.1.1-3.5
Salvinia auriculata0.8-1.8
Myriophyllum heterophyllum3.51^9
Cabomba canadensis2.52
Elodea canadensis3.12
Utricularia sp.3.57
Potamogeton pulcher2.11
Lemna minor3.38
Myriophyllum brasiliense86.313.7^10
Myriophyllum spicatum87.212.8
Myriophyllum heterophyllum90.010.0
Potamogeton diversifolius90.29.8
Potamogeton crispus88.211.8
Potamogeton nodosus84.215.8
Elodea densa90.29.8
Ceratophyllum demersum94.85.2
Najas guadalupensis92.77.3
Hydrotrida caroliniana93.66.4
Cabomba caroliniana93.07.0
Eleocharis acicularis88.911.1
Alternathera philoxeroides85.514.5
Sagittaria latifolia85.015.0
Spirogyra sp. [algae]95.24.8
Pithophora sp. [algae]85.114.9
Eichhornia crassipes94.15.92.50.434.41.01.1^11
Pistia stratiotes94.15.92.10.303.52.41.0
Hydrilla sp.92.08.02.70.282.94.50.9
  • ^1 ABDALLA, A. A. a A. T. ABDEL HAFEEZ. Some aspects of utilization of water hyacinth ( Eichhornia crassipes ). International Journal of Pest Management: Part A. 1969, vol. 15, issue 2, s. 204-207. DOI: 10.1080/04345546909415116. Available from: http://www.tandfonline.com/doi/abs/10.1080/04345546909415116
  • ^2 BOYD, C.E. Chemical analyses of some vascular aquatic plants. Archiv für Hydrobiologie. 1970, vol. 67, nr. 1, pp 78–85.
  • ^3 HARPER, Horace J. a Harley A. DANIEL. Chemical Composition of Certain Aquatic Plants. Botanical Gazette. 1934, vol. 96, issue 1, pp. 186-. DOI: 10.1086/334455. Available from: http://www.journals.uchicago.edu/doi/abs/10.1086/334455
  • ^4 LANCASTER, R.J., M.R. COUP a J.W. HUGHES. Toxicity of arsenic present in lakeweed. New Zealand Veterinary Journal. 1971, vol. 19, nr. 7, pp. 141-145.
  • ^5 LAWRENCE, J. M. a W. W. MIXON. Comparative nutrient content of aquatic plants from different habitats. In: Proceedings 23rd annual meeting. Atlanta, Georgia: Southern Weed Science Society, 1970, s. 306-310. 1)sewage, 2)agricultural pollution, 3)unfertilized ponds
  • ^6 LINN, J. G., R. D. GOODRICH, D. E. OTTERBY, J. C. MEISKE a E. J. STABA. Nutritive Value of Dried or Ensiled Aquatic Plants. Journal of Animal Science. 1975, vol. 41, nr. 2, pp. 601-609. Available from: http://http//www.journalofanimalscience.org/content/41/2/601; LINN, J. G., R. D. GOODRICH, D. E. OTTERBY, J. C. MEISKE a E. J. STABA. Nutritive Value of Dried or Ensiled Aquatic Plants. Journal of Animal Science. 1975, vol. 41, nr. 2, pp. 610-615. Available from: http://http//www.journalofanimalscience.org/content/41/2/602
  • ^7 MOORE, A. W. Azolla: Biology and agronomic significance. The Botanical Review. 1969, vol. 35, issue 1, s. 17-34. DOI: 10.1007/BF02859886. Available from: http://link.springer.com/10.1007/BF02859886
  • ^8 PIRIE, N. W. Weeds are not all bad: Water hyacinths and other pests can also be good animal fodder. Ceres (FAO Review). 1970, vol. 3, nr. 4, pp. 31-34.
  • ^9 RIEMER, D. N. a S. J. TOTH. Nitrification of aquatic weed tissues in soil. Hyacinth Control Journal. 1971, vol. 9, nr. 1, pp. 34-36.
  • ^10 BOYD, Claude E. Fresh-water plants: a potential source of protein. Economic Botany. 1968, vol. 22, issue 4, pp. 359-368. DOI: 10.1007/BF02908132. Available from: http://link.springer.com/10.1007/BF02908132
  • ^11 BOYD, Claude E. The nutritive value of three species of water weeds. Economic Botany. 1969, vol. 23, issue 2, pp. 123-127. DOI: 10.1007/BF02860614. Available from: http://link.springer.com/10.1007/BF02860614

5. Results

1) Weighing plant biomass increments in nature aquarium

In this test, I regularly trimmed overgrown plants (roughly every week) in a well-established, cycled and densely planted aquarium, which I then (after their harvest) wiped dry and weighed out (as "fresh weight"). You can see the mass of plants I harvested from the aquarium during the monitored period (= few months) in the following table. At the end of the test, I calculated the average weekly increase (yield) of plant biomass per unit area of bottom surface, so that I can compare the results of different sized aquariums.

Test #1 → Bottom area: 14.4 dm2

Aquarium parameters:

Size:15G (62ℓ) → l15 x w15 x h17 inches (l38 x w38 x h43 cm)
Filtration:canister filter, 110 gph = 440 ℓ/h (7x per hour, decent water circulation),
spray bar, filter media volume 3ℓ = 0.75G (5%)
CO2:~35 ppm → cylinder + CO2 reactor (11-20h → 9h per day)
Substrate:ADA Aqua Soil Amazonia Powder
Fertilization:Test A: sporadically (15 ppm NO3, 0.5 ppm PO4, 5 ppm K, 0.2 ppm Fe)
Test B: heavily (30 ppm NO3, 3 ppm PO4, 8 ppm K, 0.5 ppm Fe)
Aditives:ADA Green Gain (bacterial preparation) → 10 drops after water change
Lighting:~100-120 µmol PAR at the substrate (strong light),
13-21h → 8h per day in total
Source water:remineralized water from reverse osmosis (3°dKH, 6°dGH)
Amount of plants:medium
→ Alternanthera, Pogostemon, Ludwigia, Anubias, Eleocharis, mosses
Animals:10x Neocaridina davidi var. Sakura

Test A: sporadic fertilization + nutrient-rich substrate

DateFresh weightIntervalNote
2014-08-02Beginning of the 1st test
2014-08-1030.03 g8 days
2014-08-236.10 g13 days
2014-08-3010.70 g7 days
2014-09-063.70 g7 days
2014-09-2010.44 g14 days
2014-10-0126.17 g11 days
2014-10-1040.44 g9 days
2014-10-1844.12 g8 days
2014-11-0998.00 g22 daysincluding ~80% of all roots
Total:269.70 g99 days
19.07 gØ weekly increase
1.32 g/dm2Ø weekly increment per area

Test B: heavy fertilization + nutrient-rich substrate

DateFresh weightIntervalNote
2014-11-09Beginning of the 2nd test
2014-11-2259.82 g13 days
2014-12-1211.00 g20 days
2014-12-2145.93 g9 days
2014-12-2898.50 g7 daysdrastic mosses trimming
2015-01-0413.30 g7 days
2015-01-1020.00 g6 days
2015-01-1788.25 g7 daysPogostemon & Alternanthera
2015-01-2412.30 g7 days
2015-01-3112.20 g7 daysmech + Rotala
2015-02-1476.44 g14 days
Total:437.74 g97 days
31.59 gØ weekly increase
2.19 g/dm2Ø weekly increment per area

Test #2 → Bottom area: 28.0 dm2

Aquarium parameters:

Size:28G (112ℓ) → l32 x w14 x h16 inches (d80 x š35 x v40 cm)
Filtration:canister filter, 312 gph = 1250 ℓ/h (11x per hour, decent water circulation), lily pipe, filter media volume 1.6G = 6.5ℓ (6%)
CO2:~35 ppm → cylinder + CO2 reactor (5:15-19:30 → 14h per day) + 2 mℓ EasyCarbo per day
Substrate:ADA Aqua Soil Amazonia
Fertilization:Easy-Life → 7.3 ppm NO3, 6.2 ppm K, 0.8 ppm PO4, 0.8 ppm Fe (weekly)
Aditives:ADA Green Gain (growth medication → plant hormones) → 10 drops after water change
Lighting:~70-100 µmol PAR at the substrate (strong light), 8-20h[12h] at 50% intensity, 13-15h[3h] at 90% intensity
Source water:leaved-to-stand tap water →
5 ppm NO3, alkalinity 11°dKH, hardness 14°dGH (95 ppm Ca + 4 ppm Mg)
Amount of plants:medium → Hemianthus, Eleocharis, Hydrocotyle, Echinodorus, Fissidens
Animals:Paracheirodon simulans (~30pcs), Caridina japonica (~20pcs), Crystal Red Shrimp (~15pcs), Otocinclus affinis (9pcs)

Results

DateFresh weightIntervalNote
2014-07-03Beginning of the test
2014-07-0638 g3 days
2014-07-1362 g7 days
2014-07-3022 g17 days
2014-08-0786 g8 days
2014-09-017 g25 days
2014-09-07145 g6 daysComplete liquidation of Glossostigma elatinoides (even with roots)
2014-10-03150 g26 days
2014-10-265 g23 days
2014-11-0328 g8 days
Total:543 g123 days
30.90 gØ weekly increase
1.10 g/dm2Ø weekly increment per area

2) Weighing plant biomass increments in a test aquarium

In this test, I did the same as in the above one, except that I did not use any substrate or filter media in this densely planted aquarium to see how plants would grow in an aquarium without a nutrient-rich substrate where they were solely dependent on the feeding from the water column.

Experimental aquarium

Aquarium at the beginning of the test

Aquarium parameters:

Size:15G (62ℓ) → (d38 x š38 x v43 cm)
Filtration:canister filter, 110 gph = 440 ℓ/h (7x per hour, decent water circulation), spray bar → without filter media
Skimmer:Eheim Skim 350 (15-30 minutes a day to remove surface scum)
CO2:~30-50 ppm → cylinder + CO2 reactor (24/7)
Substrate:without substrate (only small inert pebbles in small plastic baskets)
Fertilization:30 ppm NO3, 3-5 ppm PO4, 20 ppm K, 0.5-1.0 ppm Fe → weekly dosage
Aditives:eSHa Pro-Phyll (growth medication → plant hormones) → 10 drops after water change
Lighting:~85-100 µmol PAR at the substrate (strong light), 8h per day (6-14h)
Heater:Juwel 100W heater → temperature: 77°F (25°C)
Source water:remineralized water from reverse osmosis (5°dKH, 8°dGH)
Amount of plants:large

Plant species

  • Alternanthera reineckii 'Mini' (5x) x
  • Alternanthera reineckii 'Splendida' x
  • Eleocharis vivipara
  • Hygrophila difformis
  • Lilaeopsis novae-zelandiae x
  • Limnophila aquatica x
  • Limnophila hippuridoides 'Red'
  • Limnophila sessiliflora
  • Ludwigia palustris
  • Ludwigia sp. 'Red' x
  • Pogostemon erectus x
  • Rotala wallichii x
  • Sagittaria platyphylla
x Plants that did not perform well in this experiment.

Results

DateFresh weightIntervalNote
2014-11-09Beginning of the test
2014-11-156 daysAdding submerged plants
2014-11-2913.14 g14 days
2014-12-0643.47 g7 daysOne shrimp added
2014-12-1114.23 g5 daysLimnophila sessiliflora
2014-12-2142.20 g10 days
2014-12-2751.20 g6 daysAbout half were roots
2015-01-0419.70 g8 days
2015-01-100.00 g0 days
2015-01-1722.82 g7 days
2015-01-2437.60 g7 days
2015-01-3136.62 g7 days
2015-02-0512.50 g5 daysLudwigia sp. 'Red'
2015-02-09107.00 g4 dnyPlants for analysis
2015-02-140.00 g5 days
Total:400.48 g97 days
28.90 gØ weekly increase
2.00 g/dm2Ø weekly increment per area

Photogallery

Picture 1 Picture 2 Picture 3 Picture 4
Photos documenting poor growth of some plants at high external concentration of nutrients: 1) Top view, 2) Alternanthera, 3) Ludwigia sp. 'Red', 4) Rotala wallichii

Nutrient content in dry matter:

Dry matterExternal concentration of nutrients
(mg/ℓ)
%Nutrient content in dry matter (%)
NPKCaMg
2-60.15-0.81.2-70.5-4.00.15-1.2 ← sufficient range
9.88%4.620.598.370.850.37 30 NO3, 3-5 PO4, 20 K, 0.5-1.0 Fe

At least potassium (K) seems to be well beyond the toxicity threshold. Unfortunately, I did not have the microelements analyzed, but I assume that they were also in the toxic zone (and were probably the main cause of the poor condition of some plants).

How many nutrients are there per 1 gram of fresh weight
(What is the average nutrient uptake in a model aquarium?)

Nutrient uptake calculator:
Nutrient:NPKFe
Content: % % % %
Note:   To get the above values (% of N, P, K, Fe), you'll need to have
a lab analysis done on the nutrient content in your aquarium plants.
Fresh weightTime periodTank's dimensions
g days [l] x [w] x [h] cm
g/week
( g/dm2)		 ℓ
  Account for root biomass (+ 20%)

Results:

Weekly uptake of nutrients in the tank
NutrientUptake
(mg/ℓ)
CO2
NO3
PO4
K
Fe

3) Measuring nutrient decrease by laboratory analysis

In this test, I used a densely planted aquarium without a substrate with an empty filter without any filter media that only served to circulate the water. I added carbon dioxide gas to the aquarium, which I dissolved using a glass diffuser so that the resulting CO2 concentration in the water was about 30-50 ppm. I kept the temperature at 77°F (25°C) using an aquarium heater. Every day I removed the surface scum using the skimmer. For lighting the aquarium I used LED lighting with Cree XP-G2 chips with a total power of 20W and irradiation of 100-400 µmol.m-2.s-1 (100 µmol at the bottom, 400 µmol at the surface level).

Test progress:

On Sunday, I exchanged 50% of the water in the aquarium and dosed the fertilizer there in the following amounts: 30 ppm of nitrate and 20 ppm of potassium (as KNO3), 5 ppm of phosphate (as KH2PO4). Some nutrients were already in the demineralized water which I used (drinking water treated with reverse osmosis) and which I adjusted the alkalinity and hardness to 5°dKH (using NaHCO3) and 8°dGH (using CaSO4*2H2O a MgSO4*7H2O) at. On Monday morning I added 0.5 ppm of iron to the aquarium + other trace elements (in the form of the commercial Easy-Life ProFito fertilizer). Subsequently, on Monday (1st day), Wednesday (3rd day), Friday (5th day) and Monday (8th day) a water sample (1.0-1.5ℓ) was taken from the aquarium in the morning, and carried to the lab for analysis. Laboratory analysis results can be seen below.

Experimental aquarium

Aquarium at the end of the test

Aquarium parameters:

Size:15G (62ℓ) → (d38 x š38 x v43 cm)
Filtration:canister filter, 110 gph = 440 ℓ/h (7x per hour, decent water circulation), spray bar → without filter media
Skimmer:Eheim Skim 350 (15-30 minutes a day to remove surface scum)
CO2:~30-50 ppm → cylinder + CO2 reactor (24/7)
Substrate:without substrate (only small inert pebbles in small plastic baskets)
Fertilization:30 ppm NO3, 5 ppm PO4, 20 ppm K, 0.5-1.0 ppm Fe → weekly dosage
Lighting:~85-100 µmol PAR at the substrate (strong light), 8h per day (6-14h)
Heater:Juwel 100W heater → temperature: 77°F (25°C)
Source water:remineralized water from reverse osmosis (4-5°dKH, 8°dGH)

Results of laboratory water analysis:

ParameterAbbr.UnitDay 1Day 3Day 5Day 8difference
per week
pHpH7.017.007.006.75
conductivityµS/cm60452480
alkalinityKNK4.5mmol/ℓ1.531.68-0.15
conversion°dKH4.284.70-0.42
chemical oxygen demandChSKMnmgO2/ℓ<0.50<0.50<0.50<0.50
ammonium ionsNH4mg/ℓ<0.05<0.05<0.05<0.05
nitritesNO2mg/ℓ0.010.010.01<0.01
nitratesNO3mg/ℓ29.230.125.016.312.9
phosphatesPO4mg/ℓ5.073.563.242.932.14
potassiumKmg/ℓ18.412.95.5
sodiumNamg/ℓ40.739.90.8
calciumCamg/ℓ37.737.30.4
magnesiumMgmg/ℓ14.114.4-0.3
sulfatesSO4mg/ℓ150151-1
chloridesClmg/ℓ220
ironFemg/ℓ<0.10<0.10
hardness [recalculated]°dGH8.58.50

Graphical representation of the depletion of selected nutrients (in mg/ℓ):

6. Discussion

Weekly nutrient uptake in planted aquarium

In several independently conducted tests, the average (Ø) weekly yield per unit of planted area ranged from 1 to 2 g/dm2.

This means that on the area of 4 × 4 inches (10 × 10 cm) about 1.5 grams of plant biomass (fresh weight) per week will grow in a heavily illuminated planted aquarium.
Because it is a "fresh weight", it is roughly 10% = 0.15 grams of dry matter (after conversion).
Assuming there is about 45% carbon (C), % nitrogen (N), % potassium (K), % phosphorus (P), and % iron (Fe) in the dry matter, the plants in the 28G (112ℓ) aquarium with a ground plan of 28 dm2 will need the following amount of nutrients to increase by 1.5 g/dm2 per week (i.e. 42 grams in total): g CO2, mg NO3, mg PO4, mg K, and mg Fe .2)

2) Caution: Remember to distinguish between mg and ppm (mg/ℓ) !
The specific percentages of individual nutrients in the dry matter will vary depending on the external concentration of the nutrients in the aquarium, the plant species composition, and other factors.

Example:
Suppose I have a 28G (112ℓ) heavily illuminated planted aquarium with a floor plan of 21 × 14 inches (80 × 35 cm) = 28.0 dm2. So, based on the above tests, I can more or less expect to have weekly yield of about g of fresh weight, which means that per week the plants will consume (under Estimative Index fertilization method) roughly the following amounts of nutrients: 
  mg CO2[  mg x ] → converting this number to mg/ℓ does not make sense
   mg NO3[   mg x ] =    mg/ℓ [  mg / ℓ]
    mg PO4[    mg x ] =    mg/ℓ [   mg / ℓ]
   mg K4 [    mg x ] =    mg/ℓ [  mg / ℓ]
    mg Fe4[   mg x ] =  mg/ℓ [   mg / ℓ]

This gives us a weekly nutrient uptake of approximately:

[ g CO2], mg/ℓ NO3, mg/ℓ PO4, mg/ℓ K, mg/ℓ Fe

Plant demand ≠ fertilizer depletion ≠ nutrient uptake

We should distinguish between the amount of nutrients:

  1. that we add to the water
  2. that disappear from the water
    • through precipitation, adsorption, filtration, etc.
  3. that plants uptake (absorb)
    • sometimes they uptake even what they don't want, or in quantities they don't want
      • and while some plant species can effectively neutralize (or excrete) excesses, other species can be poisoned by them
  4. that plants actually need

Let's say we add 1 mg/ℓ Fe to water. Most of it may precipitate or end up in the filter media, but part may remain dissolved in the water [for a certain period of time] and accessible to plants. Plants can uptake (absorb) part of this remaining dissolved portion, but if this portion is disproportionately high for them, they may be able to absorb it, but they may not be able to utilize it (for example, they may store part of it in vacuoles).

What does this mean for us? It means that if we want to estimate the amount of nutrients needed for optimal plant growth, it is not appropriate to rely on what we add to the water, what we measure in the water, or even what the plants actually uptake. The most reliable indicator is to compare the current nutrient content in the dry matter of our plants with some universal standard → i.e., the optimal range of nutrients in dry matter.

Comparison of methods

MethodNitrates
(NO3)		Phosphates
(PO4)		Potassium
(K)
Method #2 (weighing biomass increments)11.4 mg/ℓ1.03 mg/ℓ4.6 mg/ℓ
Method #3 (measuring nutrient depletion)12.9 mg/ℓ2.14 mg/ℓ5.5 mg/ℓ
Difference13%107%20%
  • While the difference between the second [weighing biomass increments] and the third [measuring nutrient depletion] method of determining nutrient uptake for nitrates and potassium is not so high (13-20%), for phosphates the difference is more than double (107%).
  • Thus, from the measured difference in nutrient concentration in the water column at the beginning and at the end of the week, no real uptake of nutrients by aquatic plants can be derived. Even in an aquarium, where the substrate or filter media does not affect the results.

Comparison of results of daily nutrient uptake in planted aquarium:

Daily uptakeNitrates
(NO3)		Phosphates
(PO4)		Potassium
(K)
T.Barr1-4 ppm0.2-0.6 ppm---
Goliaš (method #2)1.5 ppm0.13 ppm0.6 ppm
Goliaš (method #3)1.8 ppm0.3 ppm0.8 ppm
  • T.Barr (the author of Estimative Index fertilization method) states in his pivotal article on the Estimative Index method (2005) that plants normally consume 1-4 ppm of nitrates, 0.1-0.6 ppm of ammonia and 0.2-0.6 ppm of phosphates in planted aquarium per day, and that he got to these values by measuring nutrient decrease using high-quality LaMotte and Hach test kits.
  • I have found through laboratory analysis that in a heavily lit, CO2 fertilized tank with an extremely high external nutrient concentration (30 ppm of nitrates, 5 ppm of phosphates and 20 ppm of potassium), the average daily nutrient depletion is about 1.8 ppm of nitrates, 0.3 ppm of phosphates and 0.8 ppm of potassium.
  • And with an even more precise method of weighing plant biomass increments, I refined these results to approximately 1.5 ppm of nitrates (NO3), 0.13 ppm of phosphates (PO4), and 0.6 ppm of potassium (K) per day
    • or converted to weekly uptake:
      10.4 ppm NO3, 0.94 ppm PO4, 4.3 ppm K and 0.01 ppm Fe
  • But this is an utter extreme that is difficult to meet in ordinary planted aquarium. Under moderate light and adequate (non-extreme!) nutrient levels, total nutrient uptake will be much lower. The fact that these values apply to extreme (not normal) conditions must be kept in mind.

7. Conclusion

How does the nutrient uptake mechanism work?

The primary goal of my tests was to determine the nutrient uptake on the selected sample of planted aquariums with different nutritional and light conditions. I dare say that I have achieved this goal.

As soon as we know how much nutrients our plants really need to grow well, we will be able to judge for ourselves which fertilization methods are appropriate and which less. Some methods focus on plant nutrition through a nutrient-rich substrate (e.g. ADA), while other methods focus on plant nutrition through the water column (e.g. Estimative Index).

In light of the above tests, it is clear that finding the actual uptake of nutrients by aquatic plants under specific conditions is only half the answer to the question of how much nutrients the plants need to grow well. Indeed, as we have found, the uptake of nutrients by plants from the environment depends (among other things) on the concentration of nutrients in the environment. Plants uptake nutrients through various obstacles (e.g. boundary layer, concentration gradient, cuticle, cell membrane), while the general rule is that the higher the concentration of nutrients in the environment, the more nutrients (and in less time) the plant can uptake.

However, this general rule has its limits, which means that the plants cannot absorb nutrients in an unlimited extent. Each plant has a certain limit, and when it is exceeded the nutrients accumulated in its tissue begin to act inhibitory or even toxic (destructive). Therefore, to assume that providing unlimited amount of nutrients is always (i.e., under all circumstances) optimal for plants, from which they can only benefit, cannot be correct. Plants can indeed cope with an excess of nutrients to a certain extent, but only at the cost of accumulating these nutrients in their body (the tissue), which cannot in principle be optimal for them. If the optimum nitrogen content in the body is, for example, 3.5%, but the plant has twice as much nitrogen in it, then it is more likely that the plant is already oversaturated (nearly poisoned) with nitrogen. And while it may not necessarily be outwardly apparent, inside the plant body it may be manifested by increased effort to compensate for this suboptimal state by the activation of various energy-intensive mechanisms.

Back to Top