Fertilizers.

Some basics of plant growth and nutrition. Before you make decisions about fertilizing your plants or diffusing CO₂ in your tank, please check out Dave Huebert's sensible article "Water Plants 101: a basic introduction to the physiology and ecology of aquatic plants" at Bruce Hallman's website. It's a very brief and sound introduction to plants and carbon dioxide, and mineral nutrients. Dave Huebert offers a counterbalance to CO₂ faddism, and anxieties raised by questions of "full-spectrum" lighting and achieving photosynthetic saturation.

His advice on the easiest way to increase the availability of carbon dioxide is characteristic:

"For the aquarist, the supply of CO2 can be augmented in two ways. Both methods work by increasing the rate of diffusion of CO2 into the plants. First, the rate of water movement in the aquarium can be increased. This will decrease the thickness of the boundary layer and ensure that CO2 levels are at air equilibrium. This method is inexpensive, easy to implement and will produce excellent growth of aquatic plants under most conditions..."

Jim Kelly pulled together some sensible, well-explained summaries of plant-growing basics in two articles. One is "Great aquarium plants, real cheap," once again at Bruce Hallman's site. The other is "How to grow beautiful aquarium plants on a student budget," at theKrib.com. Go there, and at least skim over the headings that Jim offers, to get the lay of the land. Then come back here.

About fertilizer. You can't make plants grow with fertilizer. They have to be growing strongly already, spurred by strong light, a long enough "day," suitable pH and an enriched substrate. "Liquid fertilizers, added to the aquarium water, ... are no substitute for a nutrient-rich substrate," says Christel Kasselmann, in Aquarium Plants (2003, p. 52). Get your substrate right, with sufficient cation exchange capability (CEC) from lateritic clay and with minimal organics that will rot. Once the plants are actively growing, you will want to replace nutrients before any become exhausted.

First of all, there are some things not to worry about at an initial stage: don't worry yet about nutrient levels. Some people peering into their aquaria worry about nutrient "levels." What are appropriate levels for all the nutrients in there, they want to know; are any essential nutrients missing? Generally, a good level for all nutrients is a detectable level. If your moderately sensitive test kit can detect any iron at all, for example, it's enough for today. Don't worry about tomorrow. First of all, vascular plants take up more iron--— and other micronutrients--— than they can use and store them for future need. Algae can't do this. This advantage "higher" plants have is a major tactic you'll exploit in your battle against algae.

And second, the micronutrients are mostly toxic at enriched levels. No one ever poisoned their tank by not fertilizing. So if your tank has just been set up, or if you've only just added plants to it, hold back with the fertilizers. First give your plants some time to get settled in and start putting on new submerse growth. Whether you've bought them at your LFS or ordered them from an e-source and received them in the mail, they have been growing with emerse leaves in the moist air of a greenhouse. They need to start putting on some submerse growth. When plants put on new growth in the first couple of weeks in your aquarium, they aren't pulling nutrients from the water or substrate so much as using nutrients previously stored in their tissues. Karen Randall has compared this process with onions sprouting in the crisper drawer. So, you don't even have to wonder about fertilizers for the first six weeks. About twelve hours a day of adequate light will do.

There is no one fixed, parts-per-million concentration of fertilizers that will encourage plants yet discourage algae. Frustratingly, phytoplankton-laden "green water" conditions can be just as stable as clear water conditions, over a wide range of values. Ratios among fertilizers may be every bit as important as absolute amounts. N:P is one pivotal ratio, often recommended to be kept at 4:1, that is to say, four parts nitrogen to one part phosphate. And if Ca:Mg ratios are too low, magnesium may compete with calcium for uptake, and plants may show symptoms ordinarily associated with calcium deficiency. Low ratios of calcium to potassium may have similar results.

Reading the leaves. Reading the signs is more an art than a science. It takes a skilled eye to recognize the symptoms caused by a deficient nutrient in each kind of plant.

The Skeptical Aquarist can think of four easy ways to run afoul here:

1. Deficiencies of any one of several nutrients may share commonplace symptoms, like premature yellowing of the leaves. How are you to judge "premature?"

2. Deficiency of one nutrient may become a limiting factor preventing the uptake of others, a concept that has featured among techniques for controlling algae.

3. Additionally, some cations compete with one another for uptake, so that an imbalance of ratios may prevent uptake of a competing nutrient.

4. Finally, since most micronutrients are toxic in high concentrations, symptoms of toxicity may be confused with symptoms of deficiency.

Recognizing nutrient deficiencies. The more unmistakable nutrient deficiencies tend to show up in new growth. Deficiencies that are manifested in mature growth, resulting when mobile nutrients in scarce supply are translocated from mature growth, are hard to interpret. It's easy for an amateur to be misled. Mobile nutrients, which can be translocated, include the three macronutrients, nitrogen, phosphorus and potassium, as well as magnesium, and some micronutrients that I think you shouldn't fret over. Chuck Gadd points out that nutrient deficiencies tend to show up first and most vividly in fast-growing plants.

Non-mobile nutrients, which must stay where they are first laid down, include calcium and sulfur, which are built into plant structures and proteins, as well as various micronutrients used in plant metabolism, as catalysts or in chlorophyll, e.g. copper, iron, manganese, boron. These non-mobile nutrients can't be freed and redirected by the plant to where they are needed in new growth. So deficiencies will show at growing points and in the developing new leaves.

Nutrient deficiencies are less likely to be an issue when lighting is not intense (not above two watts/gallon) and when additional CO₂ isn't being diffused into the water. Karen Randall, in her "Aquatic Horticulture" series at Aquarium Frontiers, Dec 1997, was of the same opinion as Diana Walstad, namely "it is often possible to meet the trace element needs of the plants through regular water changes and the normal feeding of the fish in the tank." She gives a list of deficiency symptoms in a sidebar.

Chuck Gadd posts a chart of symptoms of nutrient deficiencies, expanded from one posted by Neil Frank to the Aquatic-Plants Digest, that is a case in point. Some symptoms in the chart are said to appear in mature growth. When symptoms appear in mature leaves instead of new growth, I turn skeptical. The nutrient in question must be one of the translocatable ones, which a plant can shift to new growth in times of need. Has the mature leaf reached the natural end of its cycle anyway? Can you be sure you're not seeing symptoms of fertilizer toxicity instead? And if you're assessing symptoms, you'd better take into account the competition between some micronutrients for assimilation and the mutual dependence of other micronutrients. Potentially toxic micronutrients include manganese, copper, even iron. The trace elements, such as boron, have an even narrower range between deficiency and toxicity.


Estimating from iron tests. Iron is the only one of the micronutrients that is easily tested for, which gives rise to many misleading estimates of micronutrient concentrations, extrapolating from iron test results. However, iron is highly reactive and may be untraceable within twenty-four hours of dosing. That may lead some fertilizer-dependent aquarists to redose all the micronutrients.

Fishfood as a complete fertilizer. All the micronutrients and trace elements can be expected to be in organic residues, where the mineral elements are incrementally freed and made available, as the chelating organic structures break down. Organic remains provide the best slow and steady release of nutrients, I feel.

The ultimate source of these organic residues? Remember plant guru Diana Walstad's core message in Ecology of the Planted Aquarium: "Fishfood is the perfect fertilizer, whether it's live, frozen, freeze-dried or processed into flakes or sinking wafers, because it is composed of organisms or their processed remains." Life processes have already scavenged and assembled and concentrated the essential micronutrients, or the food organism would not have survived.

Go easy. When you do come to add fertilizer, add about one-third the amount that the label suggests. Watch and wait. You can cautiously increase the dosage next time. As you can tell, I'm not selling fertilizer.

In general, I'd advise you to add the fertilizer to the make-up water. This is a really useful suggestion. Don't add the nutrients directly into the aquarium until you're very confident that you can judge how much fertilizer is needed, merely by the look of the plants. In the meanwhile, if you add fertilizer to the make-up water instead, you won't inadvertently build up toxic levels of some micronutrients by adding fertilizer faster than your particular plants in that particular aquarium are using them. Catch my drift? If you have a minimal level of fertilizer in the make-up water, why, you're still free to survey all your aquaria, to judge whether additional fertilizer levels should be adjusted. Another benefit of adding fertilizer to the make-up water is that if you find that you're suffering from fertilizer anxiety, you can get relief by doing water changes! How excellent! It isn't unusual at all to see aquarists whose algae have got seriously out of hand and who have posted an urgent cry for help at an aquarium web board, who don't even think to mention that they haven't begun their anti-algae counterattack by stopping fertilizing. Others use three different fertilisers weekly, to be sure they aren't missing that one trace element, yet they add the full dosage recommended by the manufacturers for each one! Their plants are shedding leaves, and they wonder what they can add.

An alternative approach, endorsed by Tom Barr, is appropriate for highly-buffered water where the aquarist is diffusing carbon dioxide and running intense lighting. The "hard water" method espoused by Tom Barr, keeps K about 20-30ppm. In water with high levels of calcium, there are never the symptoms of calcium "deficiency" that in actuality are a result of potassium competing with calcium (and with magnesium) for uptake. In soft waters, by contrast, such enriched levels of K quickly manifest themselves as calcium "deficiency."

Tom Barr described the method in a nutshell: "You maintain a stable level of nutrients by doing large weekly water changes that prevents anything from becoming in excess and dosing up to 3-4x a week (depending on light intensity/plant biomass) to make sure nothing runs out and causes a deficiency." He outlined his procedure of dosing trace elements and estimating their levels by the iron level in the Aquarium-Plant Digest.

Macronutrients. Nitrogen (N), in the form of ammonia/ammonium (NH₃/NH₄) is continually added to the system by fish metabolism and the processes of decomposition. If your plants were ever starved for nitrogen, you could add more fish. "Tropical fish! the amusing plant fertilizer." Fish also supplement the system's carbon dioxide.

Among the macronutrients, nitrogen is metabolically cheaper for plants in the form of ammonium NH_4. Metabolizing nitrate requires the enzyme nitrate reductase. Typically, plants virtually exhaust the available supplies of NH₄ and NH₃ before they begin to take up nitrates (NO₃).

Carbon (C) is normally derived by plants from carbon dioxide. In water of higher pH values, CO₂ is increasingly locked up as carbonate. Some "hard water" plants are able to unlock the carbonate in the process called "biogenic decalcification." CO₂ diffusion is designed to supplement carbon in the form most easily available to plants.

The major aquarium source of phosphate (P) is the fish meal content in flake feed.

Besides carbon, the only other macronutrient that is likely to be in short supply in your aquarium is potassium (K). Though nitrogen is available day and night, in the form of ammonia, there is no comparable source for potassium, which enters only through live, freeze-dried or flake foods. So, though terrestrial plants, and doubtless aquatic plants too, may use N and K in a ratio of 1:1.43, it doesn't logically follow that there is a nutrient deficit that needs to be met in that same ratio. Once the scarce potassium is supplemented and is no longer the limiting factor, readily available nitrogen can be utilized.

Every system has an ideal K (Potassium) level. It's not a fixed figure, it's a ratio of Ca:K. Potassium competes with calcium for uptake. If you reduce the ratio by adding K to any system, you're likely to see perplexing effects of Ca deficiency. In Ca-rich waters of Florida or New Mexico, you can pretty much add K ad lib. Calcium is so plentiful in those alkaline waters that you could never unbalance the ratio. However, in Ca-poor waters-- such as mine-- you can easily upset the Ca:K ratio.

I dose make-up water with potassium so that the plants will use up the nitrogen that the system already has. For several years I dosed with potassium sulfate (KSO₄) in plain old Tetra FloraPride. I like this fertilizer because it has no nitrogen and no phosphate. The potassium sulfate content of FloraPride is 3%, signalled by the nitrogen-phosphate-potassium (NPK) rating of 0-0-3. Phosphate is always the middle number in the N-P-K rating (it's alphabetical), and the two zeros signal "No Nitrogen/No Phosphate." So the rating is 0 (Nitrate) 0 (Phosphorus) 3 (Potash). That's a good rating. Phosphates aren't even in laundry detergent any more, but aquarists mysteriously burdened with algae problems sometimes turn out to have been adding pinches of sodium phosphate to lower their pH. There's no nitrate either in FloraPride, of course; that would be the first number in the N-P-K rating. FloraPride also contains some iron. But recently several companies have marketed potassium-based fertilizers without any iron.

In my very soft water, plants can exhaust the small amounts of available calcium. I recognize the effects of calcium deficiency when new growth is deformed. Java Fern fronds may be contorted to one side of the central rib, and leaf edges may be pinched or scalloped. A teaspoonful of crushed coral (aragonite: the usual marine aquarium substrate), scattered over the substrate, may do the trick, or in a 10-gallon tank, a single "escargot" snail shell could be all that's required, in my case. Some extra calcium in my soft water permits me to dose some potassium, and the carbonate stabilizes my pH.

Micronutrients are required by plants in the merest traces. Before you spend money for a designer fertilizer, ask your local water company for a recent analysis, which they will be glad to mail you. Check and see if there's a missing micronutrient element in your water. ("Got selenium?") It's on the far edge of a possibility, but it's not very likely. Don't pay good money for trendy additives like vitamins, which belong in food, not in the water, where fish can't take them up.

Among the micronutrients, the two that aquatic plant growers seem most concerned with are copper and iron.

Plants don't need copper to build their structure, just the merest trace, which they use as part of some of their enzymes. Those ions are used over and over as catalysts, so they are not rapidly used up. My hunch is that your water is unlikely to be so utterly depleted in copper that any supplement is needed for the plants' benefit. This is not the majority view, however. If your pH is over 7.0, much of your system's copper is precipitated in insoluble forms that are unavailable to organisms-- until your pH drops, through bio-acidification or for whatever reason, and soluble ionic copper is released. Since copper sulfate is sometimes used as a medication, you'll find more about the chemistry of copper among the "Treatments" in the Health & Diseases" folder.

Iron likewise is required by plants at traces levels. Issues of iron deficiency may be exaggerated. Certainly chelation of the iron is essential to the bioavailability of this reactive element.

Seachem's Flourish Excel. Seachem aver that their Flourish Excel provides "a simple source of readily-available organic carbon" that is a "photosynthetic intermediate" with iron-reducing properties that promote the ferrous Fe(II) form of iron, rather than the virtually unavailable Fe(III). In this aspect Excel seems to be chelating the iron like a humic substance, is it not?

The named ingredient is "polycycloglutaracetal," which isn't mentioned anywhere on the web except as this ingredient in Excel. (Sure I checked!) Seachem describes the ingredients as "relatively simple organic compounds" such as "photosynthetic intermediates." Scott Hieber started a series of posts to the Aquatic-Plants Digest, 12 June 2002 etc., inquiring how this ingredient differed from simple sugars such as glucose or sucrose, and "Nestor" explained it this way (in part): "The base compound is the acetal, which is formed by adding alcohol molecules to aldehyde molecules. Glutaric aldehydes in general are used for disinfectants. They are also used interchangeably with plant tannic extracts to cure gelatins for specialized applications. Glutarals are carbon-hydrogen-oxygen compounds - C₅H₈O₂ is the dialdehyde." and the next day he went on to add: "I would imagine that the glutaracetal provides those in an aqueous solution, and that absorption of these compounds in low concentrations might allow the plant to assemble what it needs while saving energy by skipping a step or two." --which seems a sensible explanation, good enough for us non-chemists.

In Robert Hudson's interview with Dr Greg Morin of Seachem, Dr Morin additionally called Flourish Excel "a low molecular weight organic compound" similar to certain "ribulose biphosphates" like phosphoglycerate, which are produced by enzymatic action from Excel. Simple sugars, like glucose and sucrose, do quickly break down in the aquarium, but bacteria are too quick to scavenge them for plants or algae to be able to use them. Overdosing with Excel does cloud the water.