AQUATIC HORTICULTURE		BY KAREN RANDALL

Plant Nutrition — Part 1

KAREN RANDALL

Plant growth will be limited by the nutrient in least supply. The trick is to provide enough of all nutrients to satisfy the needs of the higher plants, while limiting excess nutrients that could feed unwanted algae.

Some of these nutrients are abundantly available in the average aquarium, while others are not. Leibig’s law of minimum applies to them all: Plant growth will be limited by the nutrient in least supply. The goal of the aquatic gardener is to see that all nutrients are available in sufficient quantities for good higher plant growth, while avoiding excessive amounts of any substance that could either be toxic or promote algae growth.

The amount of nutrients that will be needed is highly dependent on the amount of light supplied to the tank. A tank with low to moderate lighting may receive enough nutrients just from water changes and fish food, while a strongly lit tank will almost certainly need supplementation of at least some nutrients.

Plants are able to move some nutrients from older, dying tissue to areas of new growth. This allows the plant to maintain plant mass in the absence of new supplies of the minerals in question. In times of plenty, plants are also able to store these nutrients for later use. These “moveable” nutrients are nitrogen, phosphorus, sulfur, iron and potassium. Plants can store enough nitrogen to triple their weight before slowing down, and enough phosphorus to increase their weight four or five times! This is one of the reasons why plants can appear to be “doing great” for the first few weeks in a hobbyist’s tank, only to fail when their internal stores are used up.

Some plants are better at storing and moving nutrients than others. This accounts, at least to some extent, for the competition between some plants in a closed system. If a plant is much better at capturing and storing a nutrient than a neighboring plant, it can deplete the water of that nutrient, causing the less adaptable plant to fail.

In the average heavily planted community display tank, there is probably plenty of nitrogen and phosphorus available just from the fish and their feeding. If there is an adequate supply of these two substances from fish food, potassium is probably also in adequate supply.

In the case of an aquarium with few fish and/or very strong growth, it may be necessary to supplement with these substances, particularly nitrogen and potassium. It is truly a rare instance in which phosphate becomes limiting, and any excess phosphate can lead to algae problems. The easiest “low-tech” way to add macronutrients is to add water from a fish tank to the planted tank.

     

Minerals Necessary for Optimal Plant Growth*
Macronutrients
Mineral		Approximate concentration
Carbon (C)		43 percent
Nitrogen (N)		1 to 3 percent
Potassium (K)		0.3 to 6 percent
Calcium (Ca)		0.1 to 3.5 percent
Phosphorus (P)		0.05 to 1 percent
Magnesium (Mg)		0.05 to 0.7 percent
Sulfur (S)		0.05 to 1.5 percent
Micronutrients
Mineral		Parts Per Million
Iron (Fe)		10 to 1500 ppm
Chlorine (Cl)		100 to 300 ppm
Manganese (Mn)		5 to 1500 ppm
Zinc (Zn)		3 to 150 ppm
Copper (Cu)		2 to 75 ppm
Boron (B)		2 to 75 ppm
Molybdenum (Mo)		Trace
*Dry weight in entire plant

For those of a more scientific bent, or those trying to troubleshoot a specific problem, it is definitely worth buying good-quality test kits for phosphate and nitrate. Beware of inexpensive test kits, however. In many cases you get what you pay for, and inaccurate measurements can be worse than none at all if you base decisions on them. Nitrate levels in a low-light tank can often be as high as 20 milligrams per liter (mg/L) without causing algae problems, while phosphate can sometimes be as high as 2 mg/L under the same conditions without causing algae problems. In a well-lit tank, nitrate usually begins to be a problem if it approaches 10 mg/L, while phosphate can be troublesome even at 0.5 mg/L.

Under most circumstances, phosphate levels will be lower than nitrate in an established tank (unless the phosphate is being inadvertently introduced by the aquarist). If you find that you have no measurable nitrate, and still have measurable phosphate, it is a very good sign that nitrogen is limited in your tank. Whether or not something needs to be done about this imbalance depends on the health of the plants and whether there are unacceptable levels of algae in the tank. In tanks with a moderate fish load and plenty of fast-growing plants, it is not uncommon to have levels of nitrogen and phosphate that are unmeasurable with standard hobbyist test kits. In most cases, this does not mean that the plants are starved for these nutrients. Plants are extremely efficient in the uptake of the nutrients, and are able to use levels that are undetectable with our kits.

Sometimes, however, particularly when there is an imbalance between nitrogen and phosphate in the tank, algae — the more efficient scavenger — is able to scrounge up enough nitrogen to get the upper hand while thriving on the phosphate that is available because the higher plants are unable to use it. In this case, it often makes sense to supplement nitrate. Likewise, if your plants are showing clear signs of a nutrient deficiency even when you know their trace element needs are being met, it may make sense to supplement.

In both cases, my choice would be to boost nitrogen in the substrate by using a slow-release, solid houseplant supplement that is as high as possible in nitrogen and as low as possible in phosphate. Jobes makes a slow release fertilizer for ferns and palms that fits this description. Pieces of this type of fertilizer can be placed deep in the substrate where it is available primarily to the plant roots, limiting its availability in the water column. Some people like to make “laterite balls” to encase the fertilizers to further slow their release within the tank. This works very well in a tank in which you are primarily trying to rectify a nutrient imbalance. If there are algae problems in the tank, I would use caution in adding anything that contains even small amounts of phosphate to the tank.

Other people prefer to add liquid nitrate solutions directly to the water column. For those interested in this approach, there is a very useful paper on the subject written by Paul Sears and Kevin Conlin. The paper, entitled “Control Of Algae In Planted Aquaria”, is available on the world wide web.

When reading the paper, remember that the important aspects in terms of macronutrient supplementation are not the trace element product that is used. While you can buy trace element mixes from hydroponic sources, and some people are having very good results with them, you can also use the “poor man’s dosing drops” concept (detailed near the end of the paper) when supplementing trace elements in the form of a good commercially prepared aquarium plant trace element supplement.

There is no easy way for a hobbyist to determine the amount of potassium in the water. The good news is that excess potassium has not been implicated in excess algae growth the way both nitrate and phosphate have. So, the absolute amount of this mineral in the tank water is not critical. Good aquatic plant supplements contain potassium because there is the possibility of this element becoming deficient in the aquarium. Otherwise, as with nitrate, it can be supplemented using the poor man’s dosing drops approach in the form of KNO₃. Unlike phosphate and nitrate, it is necessary that potassium be available to the plants from the water column, not the substrate. Potassium in the water is needed for the proper development of cell walls.

Phosphate is rarely a concern because of the tremendous rate of turnover of this substance in a body of water. Studies have shown that plants can take up the total amount of phosphate available in a lake in two minutes, while it is replaced at an equivalent rate by decaying organic matter. If there are any fish in the tank, it is extremely unlikely that the plants will be phosphate limited. In a tank with no fish it can happen. In these cases it is important to try to keep the supplemental phosphate in the substrate and out of the water column. Again, terrestrial plant fertilizer sticks may be used.

Calcium and magnesium are probably adequately supplied from tap water as long as the water is not too soft and the aquarist does regular water changes. If the carbonate hardness is below 2, it may be necessary to supplement both. Magnesium is easily supplied by adding small amounts of magnesium sulphate (Epsom salts) to the tank, which will also fill any sulfur needs. Calcium can be added in the form of calcium carbonate. Another method of raising the calcium content of the water is to put shell grit in a media bag in the filter. It is best not to include calcium-bearing materials in the substrate because they dissolve at a rate that cannot be controlled by the aquarist, and may raise the carbonate hardness and pH too much. Sulfur compounds are usually available in adequate supply indirectly in the aquarium and rarely need to be supplemented separately by the aquarist. Next month we will discuss trace element supplementation.