AQUATIC HORTICULTURE		BY KAREN RANDALL

Plant Nutrition — Part 2

KAREN RANDALL

While slower growing plants in this moderately lit tank do not show the effects as strongly, the fast-growing Hygrophila difformis (center front) clearly shows signs of iron deficiency. When iron was supplemented, the plant quickly reverted to its normal bright-green color.

Some trace elements are stable in the tank and remain available in a form that can easily be used by the plants for a long period. Others, like iron, quickly oxidize into forms that are not easily used by plants. For this reason, most commercial trace element supplements contain “chelating” agents. These are organic “coatings” that keep the nutrients from binding with other substances to form insoluble compounds. Plants are able to break down and use these chelators, and there are bacteria in the tank as well that break down the chelators, releasing small amounts of the nutrient into the water over a period of time. An anaerobic substrate also tends to keep minerals in a form that plants are more able to access. This is part of the reason why soil and peat substrates can be highly successful under the proper circumstances. Another reason is that the humic acid produced by peat substrate acts as a natural chelator.

In low-light tanks, 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. As the rate of plant growth increases through the availability of more intense light and added carbon dioxide (CO₂), the chances become greater that the trace elements available through these sources will become exhausted and the tank will require supplementation.

Common Symptoms of Nutrient Deficiencies (and Excesses) When consulting this list of deficiences and excesses, please remember that many symptoms are possible, and that the ones given here should only be used as a clue in tracking down a problem in the planted tank, not as an absolute diagnosis. Carbon dioxide deficiency — Small leaf size, slow growth, rough whitish deposits on plant leaves (from biogenic decalcification). Nitrogen deficiency — Old leaves turn yellow, or occasionally reddish, due to the production of anthocyanin (this reddish pigment is only seen in tanks with very strong light levels). Phosphorus deficiency — The symptoms here can be similar to those in nitrogen deficiency. There is leaf loss and small dead areas on older leaves. Potassium deficiency — Yellow spots on older leaves, and withering of leaf tips and margins (also on older leaves first). Calcium deficiency — Young leaves often have yellow margins. New growth is often deformed. Magnesium deficiency — Yellow spots, first on older leaves. It can look very similar to iron deficiency. Sulphur — Yellowing of the new leaves first. Red color from the production of anthocyanin possible in strong light. Iron deficiency — Yellowing leaves, first at the growing tip. Leaves become brittle and transparent before they eventually disintegrate. Fast-growing plants, like Hygrophila sp., will show iron deficiency the fastest. Manganese deficiency — Yellow areas between veins, while the veins themselves remain green. The tissue between the veins then dies, producing elongated holes in the leaves. These symptoms can also be seen in the presence of excess iron because this condition can block the uptake of manganese. Copper deficiency — The tips of the leaves die and margins wither. Excess copper will kill a number of plants, including Vallisneria, Ludwigia, Sagittaria and others. Zinc deficiency — Yellow areas appear between the veins, on the margins and at the leaf tip on older leaves. Boron deficiency — Growth tips die. The plant produces side shoots, but these aslo die quickly. Molybdenum deficiency — Yellow spots between veins on old leaves first. This is followed by brown areas along leaf margins. Flowering is inhibited.

Iron is the mineral most likely to be in short supply. Some people find that in their moderately lit tanks, a chelated iron supplement is all that is necessary. For a brightly lit tank, a balanced trace element supplement is probably the best answer. If you have decided that with your tank’s level of lighting (assuming more than 2 watts per gallon, and definltely when above 3 watts per gallon) that trace element supplementation is necessary, there are several ways to go about it.

There are several very good trace element supplements on the market, and more are becoming available all the time. My advice is to read the labels. The best companies will state the ingredients on the label or at least provide a chemical analysis upon request. A good aquarium plant fertilizer will not contain nitrate or phosphate, but it will contain supplemental potassium and possibly magnesium, in addition to all trace elements.

Trace element supplements are available in both liquid and tablet form. The liquid is for leaf feeding and floating plants, while most tablets are meant to be placed in the substrate for root feeders. The composition of the fertilizers is not always the same. Some companies, like Dupla, produce their fertilizers as part of a total “system” of plant nutrition. In these cases, it is often necessary to use several products, both solid and liquid, in order to provide all nutrients.

The nutrients available in your tap water and the makeup of your substrate will largely dictate how much and what kinds of supplementation you need. A tank with a soil-based substrate will not require supplementation as soon as a tank with a gravel/laterite substrate. Even when supplementation does become necessary in a tank with a soil-based substrate, it is likely that a liquid supplement alone will be adequate for quite some time.

In a tank with a gravel/laterite substrate, particularly in a tank without substrate heating, most people find that solid substrate fertilizers of one sort or another are beneficial. This is even more important in tanks containing heavy root feeders, such as Echinodorus, Cryptocoryne or Anubias spp.

When you begin to use a trace element supplement, keep in mind that the manufacturer can only give very general guidelines on the amount needed because every tank is different. Also, manufacturers have to try to produce a product that will help plants in a wide variety of water conditions. The approach of most manufacturers is to make a product that, if used as directed, will provide enough of each mineral to allow healthy growth, while at the same time avoiding quantities that might be toxic to either fish or plants. Just as with fertilizers for terrestrial plants, different manufacturers will have differing formulations based on their own perceptions of the “ideal” product.

Which product works best with your specific water supply and substrate is, in many cases, a matter of trial and error. Probably any of the better brands will produce an improvement in an unsupplemented tank (assuming adequate light and CO₂ levels), but finding the best supplement for your particular tank may require some experimentation. Unless you see clearly negative effects after using a product, give it a trial of at least two to three months before deciding whether it is a good choice for your tank. Frequent switching between products makes it impossible to tell how any of them work. Some novices attempt the “shotgun” approach of using several different products at the same time. This is unwise, because although each individually may be carefully formulated to avoid excesses of any particular mineral, their combined effects could produce toxic levels of some elements in the aquarium

Repeating from last month, I will mention the Sears/Conlin “poor man’s dosing drops” approach to aquarium fertilization. A number of aquatic gardeners enjoy finding the very least expensive way of caring for their plants. They are willing to go to the trouble of obtaining hydroponic trace element mixes in order to formulate their “poor man’s dosing drops.” If you decide to try this approach, please be aware that not all of these trace element mixes are the same. Some are higher in copper than we would want, while others are too low in iron. This type of project is fine if you are interested in experimentation, but for the average aquatic gardener the commercial trace element mixes are probably safer and more effective.

A heavily planted, strongly lit tank will need more trace element supplementation than a tank with more modest growth. The amount of trace element supplementation needed will also be dependent on the types of plants you are growing, the fertility of the substrate and the minerals available in your water supply. It is always best to err on the side of caution. Start with half the recommended dose and work up.

The only trace element that we can easily test for is iron. Get a good iron test kit — some of the less expensive ones are inaccurate at the low end, and excess iron can lead to algae problems. Two companies known to produce reliable test kits are Hach and LaMotte. I have also used the Dupla kit with satisfactory results.

If you are using a well-balanced trace element supplement, and you able to get a reading for iron in your tank, you probably are providing adequate amounts of all trace elements. At the same time, do not be fooled into overdosing your tank if you do not get an iron reading in the first few weeks of supplementation. Plants can store quite a bit of iron. If they have been iron deprived for a length of time, they will suck up all available iron until they have stored as much as they can. Eventually, they will reach a state of equilibrium in which they are only taking enough to maintain their growth rate. During this period, if you keep adding more and more of a broad-spectrum trace element supplement in order to achieve a specific iron level, you run the risk of oversupplementing with minerals other than iron.

Watch your plants. Let them be your guide. If they are growing well and looking healthy, they are receiving the nutrients they need. If not, take a close look at where they might be deficient. In the accompanying sidebar you may note that there are a number of different deficiencies that produce similar symptoms. Add to that the fact that excesses of one nutrient can block the uptake of another, and you begin to see just how complicated aquatic plant nutrition can be!

References Baensch, H. A. and R. Riehl. 1993. Baensch Aquarium Atlas. Volume 2. Mergus-Verlag GmbH. Melle, Germany. Pp. 1212. ISBN 1-56465-114-2. Jacobsen, N. 1979. Aquarium Plants. Blanford Press, Poole, Dorset, England. Pp. 16. ISBN 0-7137-0865-4. Scheurmann, I. 1993. Aquarium Plants Manual. Barrons Pub., New York. Pp. 93. ISBN 0-8120-1687-4.