Notes on Continuous Plant Fertilization

Notes on Continuous Plant Fertilization

Written by George J. Reclos Friday, 30 August 2002 00:00

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This is a very controversial issue amongst those enjoying aqua-culturing.  Some prefer to add all the fertilizer in one or two doses per week, while others prefer to dose on a daily basis or even many times per day.  After discussing this matter with Pavlos Aslanis, who is much more experienced than myself in this field, it made more sense to fertilize on a continuous basis rather than once per week.  Those preferring the single fertilization dosage claim that they have standardized the needs of their plants, and since light and carbon dioxide are provided in measurable quantities, they can calculate how much fertilizer the tank needs.  They also assume that the plants will gradually consume the fertilizer during the interval between the two doses, while algae will not consume the extra fertilizer…or if it does, it will starve in the end of this period.  I am not the one to deny them the right to think so, and I am not going to claim that my approach is better than theirs.  All I am going to do is list a few points that made me follow the continuous fertilization method, and add some notes about the various chemicals I use that can be useful to other aquarists independently of which fertilization method they prefer.

One of the main reasons that I preferred continuous fertilization is that I don’t feel that my tank always has the same needs, so I want to have a greater control of what gets into my tank and when it gets there.  My 240 liter planted tank has ample light (1.2 Watts / liter or 4.5 Watts / gallon) and approximately 40 ppm CO2 – conditions that favor a quick plant growth of even the most demanding plants.  Under those conditions, I need to prune the plants 2-3 times per week.  This means that the plant mass is not constant, and since I don’t prune the same plants every time, the needs of the tank on the whole change.  There are plants that will grow 15 cm / week (Myriophyllum tuberculosum, Lemnophila sessiliflora, Micranthemoides), others that will grow half that amount (Rotala macrandra, Nymphaea lotus), and others that don’t seem to grow that much (Anubias, Microsorum pteropus).  Therefore, plants in the first category may be pruned twice per week, and plants in the second category may be pruned once per week, while the slow growers are never pruned.  I had a small amount of green algae which used to expand a bit after each heavy pruning when I was on the standard once per week fertilization regiment with a commercial fertilizer.  When the plants resumed normal growth rates, the algae would partially die.  After trying many combinations, I decided to follow Pavlos’ advice and make my own plant fertilizer – therefore I could adjust each chemical as needed.  To ensure that only a very small quantity of unused fertilizer (if any) would be present when the next dose was added, I performed large water changes daily for a month and then gradually reduced the amount of water changed – always keeping an eye on my little algae culture.

Our calculations were based on the assumption that the fertilizer would be added four times daily, 2 ml per dose = a total of 8 ml / day.  The initial fertilizer contained potassium sulfate, potassium nitrate, sodium dihydrogen phosphate, ferrous gluconate, magnesium chloride, no calcium, and power 4 (a mixture containing trace elements like manganese, boron etc).  To start with, this already makes a total of 56 ml per week, which I find it a bit too much to be added in a single dose. Moreover, the fertilizer could not be more concentrated since some elements were already close to their solubility limit (while one of them was already past it, as we will shortly see).  The supporters of the “single dose” regime claim that they don’t care that much since they add the powders directly in their tanks. Well, this may work for them, but I love my fish, and I don’t agree with this – unless I am dealing with a plant only tank.  I have had a difficult time raising my discus to their current size, and I don’t want to lose them in a cloud of concentrated phosphates.

The mixture was prepared in 2 liter batches using pure water (deionized through a mixed bed de-ionization column).  After dissolving all the elements, it was then filtered to eliminate the non-soluble part.  At this point I should be more careful… I mean why was there an insoluble part?  In order to ensure that no oxidization would take place during storage, the solution was kept in special polypropylene non-reactive bottles that were filled to the top to remove all the air, and was then stored in a dark place wrapped in aluminum foil.  Keeping chemicals in light protected containers is an old habit that dies hard…but usually prolongs the stability of the chemicals.  You can also use amber colored glass containers if you are sure that your chemicals don’t react with glass.  After using this mixture for a month or so, I noticed that the algae was now under total control (some was left as a “guinea pig” to grow in a hidden spot in the back of the tank), but the plant leaves showed some holes, which really alarmed me. After searching in the literature it became evident that this was most probably caused by a potassium (or iron) deficiency.  Iron was out of the question, since there was already more than needed in my solution, so it had to be potassium.  Potassium?  Yes, potassium.

I continued the same fertilization scheme, only adding an extra 1.5 ml of a saturated potassium chloride solution per day.  The change really astonished me.  New leaves had no holes at all, while the algae were still under control.  Why did this happen?  Simply, the potassium sulfate has a very low solubility in water (120 grams / liter – yielding less than 60 grams of potassium per liter).  Which means that 20 g of potassium sulfate was not dissolved at all (my recipe called for 140 g / liter).  In contrast, both potassium nitrate and potassium chloride are almost three times more soluble.  Increasing potassium nitrate was out of the question, since the nitrates would cause an algae bloom, so I decided to add 90 grams of potassium sulfate / liter (below the solubility limit) and add another 43.3 grams of potassium chloride (which would add the same potassium as the 50 g of potassium sulfate they would replace).  When I increased the quantity of the extra potassium to 2.5 ml / day, the algae came back green, hairy, and aggressive.  I removed some of it, reduced the amount of potassium to 1.5 ml / day again and everybody was happy.  Me, my discus, my plants and my algae.  It seems that I managed to have a tank in which the limiting factor was potassium.  Never heard of it, but after you eliminate all other possibilities, the remaining one must be the valid one, Dr. Watson.

Instead of giving you the entire maths – which will not be applicable for your tank anyway – here is the maximum solubility of some of the agents most commonly used as fertilizers.  Adding more than what is listed in the right column will just precipitate.  In short, no matter how much you add, only the quantities listed in the last column will be IN your solution.  The rest will be at the bottom of your container.  It is as simple as that.  I assume we all use deionized or reverse osmosis water, although even tap water will largely behave the same.

Notes:  Quantities are in gram / liter. Water temperature is assumed to be 25^oC.

(*) Potassium solubility is largely affected by the “common ion” issue.  Thus, if other compounds that release K+ ions are present in the same solution, the solubility of potassium sulfate gets greatly reduced.  Although this is common with all chemicals, it is far more profound in this case.  The same is true if ammonium ions (NH⁴⁺) are present in your solution (or tank).

(**) Magnesium and Calcium compounds will raise your total hardness to the degree they are not absorbed by your plants.  Regular monitoring is recommended with a GH kit, particularly if fish and plants requiring a low general hardness are kept.  You can measure the magnesium concentration separately which – by subtraction – will give you the Calcium concentration (if you want to be accurate).  When the GH remains constant (not zero) in the long run, it means that your plants consumed all the added calcium and magnesium. If it gets higher, it means you are adding more than your plants need or there is something leaking calcium or magnesium in your tank – check your stones.  If your GH is getting lower in time (and you are not using any kind of softener), then you need to add more.

(***) Exothermic reaction. If you make a concentrated solution, don’t touch the vessel till it cools down. Warning: it can get really hot.

(****) The use of a slightly acidic solution and the addition of sugar will stabilize this chemical.  Since sugar may well become the substrate for severe bacterial growth, I can only recommend avoiding it.  An acidic solution is stable for at least 6 months.

Going for an Alternative

If you want to substitute one of the agents with another (for example, Potassium sulfate with Potassium chloride) either in part or in total, all you need to know is the molecular weights of the two chemicals and the number of the “useful” ions or radicals in the molecular formula.  Thus, when considering Potassium sulfate and Potassium chloride, apart from the relative molecular weights, you have to keep in mind that each molecule of Potassium sulfate releases two potassium ions, while Potassium chloride (or nitrate) releases just one.  Just look at the small numbers at the lower right part of each ion – for radicals you must see the number after the parenthesis, as is the case for Calcium phosphate → Ca₃(PO₄)₂.  In this particular example, each molecule of calcium phosphate will give three calcium ions and two phosphate radicals.

What is the use of this table?  If you dissolve 76.5 gram of Potassium chloride in one liter of water, for instance, you will get 39 gram of potassium (K+) in this liter, which is translated in 39.000 ppm of potassium.  You will get exactly the same quantity of potassium in your 1 liter solution if you use any of the equivalent masses shown in the last column (red numbers).  If you need to dissolve quantity A from any of those chemicals and you can’t – due to solubility problems or because you don’t want nitrates for instance – you can dissolve the quantity you want, and then add the missing quantity by using an equal percentage from one of the other chemicals.  Thus, if you want to add the potassium contained in 104 gram of Potassium nitrate, but you are afraid of the too many nitrates, and you add only a third of it, you can replace the missing potassium by adding two thirds of the equivalent mass of any of the other agents (or using one third from each one).  I don’t know how much sense this will make to you … at least I tried.  I am afraid this is as far as I can go without referring to gram equivalents and moles, and I am sure most of you wouldn’t like to hear about them.