Last update: October 17, 1998
The substrate in a plant tank has three purposes. First and foremost, it provides a source of nutrients for the plants via adsorption through the plant roots. Second, it provides an anchor for the plants. Third, it provides an attractive and natural looking background for both the plants and fish.
There are some things the substrate should not do in a plant tank. For one thing, the substrate should not alter the water chemistry in the aquarium. Some types of commonly sold aquarium gravel contain limestone or calcium and can lead to uncontrolled hardening of the water. This may be fine for African Rift Lake cichlids, but is a severe problem for the plants and fish typically found in heavily planted tanks.
Another important factor in selecting the substrate material is the grain size. It should be selected such that it is neither too fine nor too coarse. Too fine a material will tend to become solid over time, due to biological waste material acting as a "mortar" between the substrate grains. As reported in "The Optimum Aquarium", this is a long process, taking on the order of many years to become a problem. The overall effect is to prevent nutrients from reaching the plant roots.
Gravel that is too coarse has two obvious problems. First, it will not offer a very good foothold for the plants, especially plants that have delicate roots. The second is that excess food and fish waste can settle into the rather large gaps in the gravel, making it difficult for scavengers to get to it. This leads to excess nitrate buildup as the material decomposes and will lead to water quality problems in the long run.
Fortunately, the most common kind of aquarium gravel available in the fish shops in this area meets all these criteria. It is a common quartz gravel sold as "sandblasting gravel". The brand we use is named "Tex-Blast" and is light brown in color with a grain size of between 1/16" and 1/8" (2-3 mm).
The amount of this gravel needed for a tank can be calculated by determining the volume of gravel needed (for example, 47" x 17" by 3.5" for a 90 gallon tank) and dividing by 19 cubic inches. per pound. Also note that this gravel displaces 2.4 gallons per 50 pounds. This is useful when determining the true water capacity of the aquarium.
Most plant books recommend that some additives be put in the substrate to enhance plant growth. Substances like peat, potting soil, sand and clay are often mentioned. "The Optimum Aquarium" recommends the use of laterite in the lower one third of the substrate. The book is lacking in specific reasons for using laterite, but leaves the impression that the main reason is to supply iron to the plant roots.
One clue to the real function of laterite (in conjunction with substrate heating coils) is the claim that they "integrate the substrate into the aquarium". The following information is credited to Jeff Frank and provides more insight into what this may really mean. Hopefully, we haven't introduced too many errors in our paraphrasing of his comments.
Laterite is a remnant of volcanic rock which has been highly weathered by exposure to tropical temperature, precipitation, and forest derived humic acids over geologic time. Laterite, or any clay for that matter, has a crystalline structure which has many negatively charged sites which are important for plant chemistry. Except for decomposed organic matter there are no negatively charged sites in the aquarium. Soils from temperate regions (clay fractions of which are relevant for comparison to the tropical laterite) not exposed to the accelerated weathering of the tropics retain too much Ca++ and Mg++ which will adversely affect hardness and pH in a plant tank.
Many sources agree that ammonium is the preferred form of nitrogen for plant utilization. Ammonium, and many other positively charged ions like Fe++, K+, Ca++, Mg++, and Na++, are attracted by the negatively charged sites provided by the laterite.
The negative sites attract and hold the ammonium ions like a magnet until a plant root hair exchanges another positively charged ion for the ammonium (adsorption) and takes it in to metabolize into amino acids, and ultimately, protein. By providing this readily usable source of nitrogen, the additional benefit of removing ammonia (due to ammonia/ ammonium equilibrium) is realized. Instead of just the "ammonia to nitrite to nitrate" cycle in biologic filtration, the "nitrogen cycle" and its accumulating nitrate levels is avoided altogether. The nitrogen ends up being removed from the tank as you cut and prune excess plant tissue because plant tissue is partly made of protein, which is 14% nitrogen.
Substrate heating coils are an important element in the substrate for a number of reasons:
Once the laterite and gravel layer is in place, the rest of the gravel is added on top, effectively containing the laterite in the lower layer. We generally make no provisions for terracing the substrate. We have found that aquarium gravel has a very strong tendency to become level over time. Any visual layering will be accomplished with different sized plants.