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 of his approach: "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..."
 
His second method of increasing the rate of CO₂ availability is through diffusing CO₂ into the water, a method he characterises as "expensive, and if done improperly, can be lethal to fish." I'd agree with him there. 
 
Chuck Gadd's "Introduction to fertilizers for a planted tank" is the clearest brief primary orientation I've seen. Jim Kelly pulled together some sensible, well-explained summaries of plant-growing basics in two articles. One is "Great aquarium plants, real cheap", archived from rec.aquariance again at Bruce Hallman's site. The other is "How to grow beautiful aquarium plants on a student budget" at The Krib. Go there, and at least skim over the headings that Jim offers, to get the lay of the land. Then come back here.
 
"Losing" CO₂. Nowadays many aquarists with planted tanks are diffusing carbon dioxide into their water, in order to compensate for the carbon that's locked up in carbonates in their "hard" water and provide  a carbon source their plants require. They minimize splash where the water is returned from the filter and reduce surface agitation generally, in order not to lose this added CO₂. In less labor-intensive planted systems like mine and perhaps yours, however, this is not an issue: CO₂ is at equilibrium with the atmosphere, from which it diffuses in, though rather slowly.  My plants are "carbon-starved", I'm sure. But I'm beginning to hear misguided warnings extended even to aquarists without injected CO₂, to avoid turbulence at the water surface. Reflect a moment: the only CO₂ that could be "lost" at the surface is artificially added CO₂.
 
About fertilizer. You can't make plants grow with fertilizer. They have to be growing strongly already, spurred by sufficiently intense light in usable wavelengths, 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 capacity  (CEC)  from lateritic clay and with minimal organics that will rot.
 
Cation exchange capacity is a measure of the substrate's capability of holding cation (that is, positively charged)  nutrient ions. Washed fine gravel and coarse silica sand of typical aquarium substrates have low capacities for cation exchange, whereas colloidal clay baked as laterite has high CEC. Humus and peat also have high CEC values.
 
Once the plants are actively growing, you will want to replace nutrients before any become exhausted. But 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 planted 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: 'luxury uptake' the ecologists call this activity. Algae can't do this as effectively, though they can polymerize and store some phosphate within their cells. This advantage that "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 most likely 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 they are 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. Ten to 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. In his "Introduction" I mentioned and linked to, Chuck Gadd points out that nutrient deficiencies tend to show up first and most vividly in fast-growing plants. 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 to mature growth, are hard to interpret. In those cases 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 also some micronutrients that I think you shouldn't fret over. 
 
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 in these instances 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 posted 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 perhaps 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 be tempted to 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." I can't locate his outlined procedure of dosing trace elements and estimating their levels by the level of iron, the "Estimative Index" (EI), which was formerly archived at the Southwestern Aquatic Plant Enthusiasts site, but an article "The Estimative Index (EI): dosing with dry salts" at the UK Aquatic Plant Society site gives a synopsis, with some brilliant photographs of planted aquaria.