Your substrate is not merely the stuff that holds plants down. It is a dynamic part of their health. Many think it's the simplest component of the aquatic garden, but it's a very complex part and often the most intensley debated.
One often-overlooked component of a substrate is it’s bacterial biofilm. After you place your substrate into the aquarium, it almost immediately begins to be colonized by bacteria. It can take several months for the bacteria to establish, but once they do, they help break down various materials and make nutrients available to the roots. Also plant roots secrete chemicals and enzymes that break down substances around them making additional nutrients available.
Cation exchange capacity or CEC is occasionally used to measure the merit of substrates. CEC is the sum total of the exchangeable cations that a soil (substrate) can adsorb. Cations are positively charged particles. Basically a substrate with a high CEC will have more positive charged particles for your plants to absorb. Since many nutrients used by plants fit this description, this is good.
|A section of a planted aquarium with some of the
Seachem Flourite substrate exposed.
In the old days, the only substrates available were colored quartz gravel, sand, or soil. Now things are considerably more complex. Substrates are hard to classify into groups, but for this article I have tried to create some organization by dividing them into inert, soil-based, laterite-based, nutritive substrates, and other.
The inert group would contain the colored quartz gravels and sands sold in most pet shops. Plants can derive no nutrients from these type substrates. After a long time in your tank, once a bacterial biofilm has had time to develop, the plants establish a little, and the gravel becomes filled with decaying organic matter, these substrates become viable. This usually takes many months and again, the nutrients don't actually come from the gravel or sand itself but from the material accumulated in it.
Soil is actually a good substrate. It has a high CEC, it probably already has a biofilm, and usually has lots of nutrients. Among its detractors are that it's messy, and anaerobic areas can develop. Anaerobic processes can give off gasses which are harmful to plants and fish. Also you can’t really know what toxins, fertilizers, or parasites might be present in soil. Many people feel strongly about soil as a substrate, and its use has increased in recent times. A possible way around the risks of having potentially harmful soil is to use commercial potting soil. For more information on soil substrates I recommend you read Ecology of the Planted Aquarium by Diana Walstad.
Laterite is a gravel additive. It is clay or soil that is placed in the bottom layer of your aquarium's substrate. It is usually used with the inert substrates to make them fertile. Its good points are that it works great, is inexpensive, has readily available nutrients, and has a high CEC. Its negatives are that it can be messy like soil, and some claim that it gives out over time unless you use substrate heating cables. There are several other gravel additives as well, but a discussion of them and a discussion of the substrate heating cables mentioned above goes beyond the scope of this article. Like most everything else in this hobby, some people love ‘em. Some people think they are useless.
The nutritive substrates are various gravel type products that, unlike plain quartz, actually have some fertilizing capacity-some more than others. Compared to the other groups, these products are relatively new to the market. A few of these are Flourite, Onyx, Eco-Complete, Flora Base, and the ADA products. The debate is fierce regarding their merits or lack thereof. A detractor to these is their high price, but their advocates remind us that this is only a one time expense for most of them. Individually they all have their various pros and cons. Overall they are good substrates for your tank.
A couple substrates used that were not originally intended for the aquarium are Turface, which is used on baseball fields and kitty litter (clay based and containing no added chemicals). Some complain that Turface is too light weight and that kitty litter can get soft. However, many people have had success with both of these.
A good study on some of the above substrates and a few others can be found on Jamie Johnson’s web page.