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For terrestrial gardeners, a listing of the major or macro- nutrients of aquarium-useful plants comes as no surprise; they're the same, N, P, K (Nitrogen, phosphorus and potassium) and more. What is definitely different is how these elements are made available and taken up by aquarium plants.
Additional major or macro-nutrients and minor/micro-nutrients follow the same general rule; and as with land plants organic and inorganic nutrients are provided by mineral and biological sources.
As you will see, in some ways aquarium plant life is favored over the air outside; physical support, moderation of the ill effects of varying water availability... but this surfeit of water is a "double edged sword", aquatic factors often affect the usefulness and uptake of necessary chemical species.
The Major Mineral Nutrients of Plants
Allow me to present a chart of plant macro-nutrients, their form of absorption, concentration in plants as a percentage of dry weight, some notations as to function and expand in practical terms below as to sources and deficiencies.
Macro-Nutrients of Plants:
ElementForm of Concentration Some Functions
Absorption as a % of dry wt.
NitrogenNO3- (or NH4+) 1-3% Amino acids, proteins, nucleic acids, chlorophyll, coenzymes
PhosphorusH2PO4 or 0.05-1.0% High energy molecules ATP, HPO42- ADP, nucleic acids, phosphorylation of sugars, essential coenzymes, phospholipids.
PotassiumK+ 0.3-6% Enzymes, amino acids, protein synthesis, enzyme activator, stomata opening/closing
CalciumCa+ 0.1-3.5% Cell wall formation, enzyme cofactor, cell permeability.
MagnesiumMg2+ 0.05-0.7% Part of chlorophyll, enzyme activator.
SulfurSO42- 0.05-1.5% Some amino acids, coenzyme A.
IronFe2+, Fe3+ 10-1500 Chlorophyll synthesis, ferredoxins, cytochromes
Dear Reader, pardon me, but I want to be clear with you; I figure you're much like me. Reading about atoms and molecules may be exciting and impressive to relate to friends and fellow pet-fish types, but "what the Dickens good is it?" I didn't "do the science" to come up with the above table (the information was gleaned and compiled from standard works); however I do know the "nuts and bolts" of actual aquarium practice do jive with helping you to be a successful aquatic gardener. So, on with facts, attitudes and ideas of use to aquarists!
Nitrogenis provided readily by decomposing organic matter, from soil you've placed in your substrate and/or fish food. Though it is used in large quantities by aquarium plants (and microscopic aquatic life), it is rarely, and should not be purposely amended. Commercial aquarium plant fertilizers contain nil to a few percent nitrogen.
Yes, nitrogen comes in several formats (nitrates, ammonia, ammonium...); and yes, the formation and form of nitrogen compounds is influenced by bacterial action, pH, temperature and more. Know this; that nitrogen is rarely a/the rate-limiting factor in aquarium plant culture, and that a stable, "cycled" (aka "established") system will produce sufficient nitrogen for your plants use. Under varying conditions, your plants are able to utilize nitrogen from several fixed sources; high nitrates (more than a few ppm), detectable levels of ammonias are not necessary or desirable.
Phosphorusand Potassium also occur in more/less usable 'forms'; like nitrogen, they rarely have to be augmented in a 'normal' set-up. Enough comes into a system by way of tap and fish food sources to supply all but the most "boosted" plant arrangements.
In highly acidic conditions phosphate "fixing" can be a problem with this material becoming insoluble, hence the use of some carbonaceous material as substrate. Too much phosphorus as phosphate (less than 1ppm) results in algal blooms.
Potassium shortage shows up in older leaves as small spots and holes, resulting in leaf loss.
Calcium, Magnesium & Sulfur are generally not limited as major nutrients either; sufficient concentration derived from tap and food sources. In soft water, Ca and Mg may need supplementation; best achieved through the substrate. Calcium is necessary for plant growth, as is sulfur; magnesium is the central atom in every chlorophyll molecule. A lack of Ca shows in dwarfed, gnarled growth and blackened, stubby roots. Missing Mg may result in yellow to white, transparent leaves.
Ironcrosses over the border as a macro/micro-nutrient. Ferrous matter is necessary in only small concentration, but is often a nutrient deficiency cause of 'yellowing'. Your tap water may well not contain enough iron material to meet your plants needs or be too alkaline, precipitating it out of solution.
A sound approach to iron supplementation is inclusion in soil additive(s), or periodic addition in a chelated commercial prep.. Unfortunately, you can have "too much of a good thing" with iron. About half a part per million is what you want. You will have to invest in and use an iron test kit, rely on water changes and uptake of a chelated formula, and/or add iron judiciously and not worry.
Carbon: Should we mention this, the most abundant plant element in dry weight? Well, I guess so. Except in plant-crowded and otherwise boosted (lighting, chemical supplemented) systems, enough carbon as CO2 enters into aquarium systems through respiration processes and the atmosphere. Carbon can be 'forced' to become the rate limiting "minimum nutrient" factor, as can iron under intensive culture, or calcium or magnesium in soft water conditions.
Carbon dioxide infusion is useful in other ways; principally as a bicarbonate balancer in hard waters. The pH stabilization offered by carbon dioxide infusion goes a long way to promoting luxuriant plant growth.
Hydrogenand Oxygen are the remaining macro-nutrients of aquarium plants. They are obviously not in short supply.
The Minor Mineral Nutrients of Plants
ElementForm of Concentration Some Functions
Absorption in parts per million
Chlorine Cl- 100-10,000 Osmosis and ionic balance
Copper Cu2+ 2-75 Activator of some enzymes
Manganese Mn2+ 5-1500 Activator of some enzymes
Zinc Zn2+ 3-150 Activator of many enzymes
Molybdenum MoO42- 0.1--5.0 Nitrogen metabolism
Boron BO3- or B4O72- Calcium utilization, nucleic acid synthesis, membrane integrity.
Minor Nutrient Availability and Consequences:
All these elements are absolutely necessary to plant life, and readily supplied from tap, soil, and fish-food sources. Excepting specially filtered tap sources, non-soil amended set-ups, systems without fishes and feeding, the minor or micro-nutrients of aquarium plants are rarely found in limited supply... unless. Unless they are driven to being deficient by expediting growth and making the macro-nutrients alone available through supplementation.
As can be discerned from the table, boron deficiency results in the stilted growth appearances of calcium lack.
So What Does This All REALLY Mean?
In a low-intensity set up, with a mix of plants and fishes, proper substrate/soil use, of moderate lighting, few are the instances of any one nutrient becoming plant growth rate limiting. For folks with boosted lighting (thousands of lux at substrate level), possibly with carbon dioxide infusion, certainly one or more nutrients will move into this role. Carbon, if CO2 is not employed, iron, and in softer waters the alkaline earth elements calcium and magnesium.
What's an aquatic gardener to do? If an intensive one, measure and keep to a schedule of soil and water chemical supplementation; if low-key, stick with fish-food fertilization and regular partial water changes. To re-state in a slightly different manner; given that you are aiming for maximized growth with intense light and carbon dioxide you should be supplementing; fish wastes alone will not provide sufficient plant food.
About Test Kits and Testing:
Please don't panic, if on measuring for an essential nutrients in an established plant tank you find little or none detectable. Think about this; if the plants are growing, with or without intentional supplementation, where is the chemical food going? Why into your plants of course. These nutrients can be translocated to other growth, even back to the environment; but under beneficent conditions, will not show up on hobbyist test apparatus.
Instead, you might find it worthwhile to measure your sources of nutrients (the tap, soil amendment, substrate, supplement(s)) in solution outside the system with your kits; in a healthy setting these materials change, are otherwise taken up.
Plants require a number of elements for growth that they obtain from the water, soil (if any), substrate, food, fertilizer (if added), and air. In nature most of these are provided by the weathering of rock; in aquariums, depending on the rate of growth desired, they're augmented on a periodic basis.
The major chemical nutrients of land and aquatic plants are identical; the form and ways in which aquatics take them up however are different. These differences are important to the aquatic gardener in approaching methods of appropriate fertilization.
Should you utilize chemical supplementation? With most set-ups this is unnecessary to support growth. On the other hand, super-luxuriant systems do utilize soil, chemical supplementation, CO2 and intense lighting.
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