Substrates For Plants
Plants obtain a large proportion of their nutrients through the roots and from the surrounding substrate. The substrate in the aquarium must therefore be a suitable rooting medium and a provider of nutrients
The majority of aquaria often have a simple gravel substrate of a few centimetres deep. This standard substrate provides almost nothing of use to aquarium plants; it has no nutrient content, little ability to hold nutrients, allows too much water movement and is not sufficiently deep to provide support for larger plants. A number of alternative substrates are available for aquariums and it is a combination of these that can be used to create a suitable substrate mix for aquarium plants.
Substrate size and depth
The 'grade' size of substrate has a large effect on the provision of plants nutrient requirements. Most gravel substrates have a grade size of around 3-4mm which is useful when using gravel cleaners, as debris can be easily separated. This grade also allows a large amount of water movement to occur.
The problem with water movement through the substrate is that it also introduces oxygen, which binds to nutrients, making them difficult for plants to use, as well as simply 'washing' nutrients away. Larger grade substrates can be used as a thin, visible top layer although the main substrate should consist of a 1-2mm grade compact substrate.
Nutrient storage in substrate
Nutrients circulate within the aquarium and are taken up by plants through the leaves and roots. An established substrate can act as a 'nutrient sink', holding nutrients in a concentrated area where they can be easily obtained.
Nutrients which travel into the substrate will 'bind' with organic material, such as the debris produced from fish and plant waste. Providing the substrate is relatively compact, the nutrients will then stay in the substrate, bound to organic material. Over long periods of time, the bonds are broken down and the nutrients are slowly released at a steady rate and taken up by the roots of plants.
To create a good nutrient holding substrate, a compact (1-2mm) main substrate must be used and the substrate should be around 5-10cm deep (2-4"). Larger substrates allow water movement to remove nutrients and introduce oxygen, creating oxygen bonds (oxidation) rather than organic bonds with the nutrients.
The rooting substrate will take up the majority of substrate space and should be considered as the main, or even only, substrate. A good rooting substrate for a planted aquarium must be able to hold nutrients and also provide support for roots that anchor the plant.
If a substrate is too large, it will not hold nutrients and may hinder the growth of roots. If it is too fine, it will compact and stop water flow entirely, which causes stagnation and the production of harmful chemicals.
A grade size of 1-2mm should create a compact and firm substrate whilst still allowing a gentle flow of water.
The rooting substrate does not have to contain nutrients, these can be provided by specialised substrate additives and fertilisers but it should be inert. An inert substrate will have no effect on the water quality in the aquarium.
A typical substrate which is commonly available to act as a good rooting substrate is lime-free or quartz gravel. This type of substrate is normally a little more expensive than 'standard' pea gravel, but creates a far better environment for the aquarium plants.
Nutrient rich substrates
Plants use a wide variety of nutrients and many of these nutrients will have to be artificially introduced to the aquarium. Because plants obtain most nutrients through the roots, and the substrate acts as a nutrient sink, the substrate is an ideal place to situate nutrients.
The advantage of using nutrient rich substrates or substrate additives is that the low oxygen conditions within the substrate allow nutrients to be released over long periods of time. Nutrient rich substrates are commonly available and most are designed to be used in small amounts, as an additive to a main or rooting substrate.
In many cases, a good nutrient rich additive will last for a number of years before it begins to run low on nutrients. If the aquarium is supplemented with regular liquid fertilisers, the substrate will continually 'recharge' itself and may never need replacing. A nutrient rich additive should be placed around two-thirds below the top of the substrate, or mixed in with the lower half. This area is where oxygen levels are lowest and where roots will take up most nutrients.
Most suitable rooting substrates are a sandy brown colour and 1-2mm in diameter. In your aquarium you may wish to have a different substrate appearance to match the decor or style of aquarium. In this case a thin top layer of a different substrate or gravel can be used. A 1-2cm top layer of substrate will not affect the plants providing it is not calcareous. Normally only substrates designed for marine aquaria are calcareous, but it is always best to check with your retailer first.
In aquariums that are very heavily planted, the use of heating cables may significantly benefit plants. A heating cable is a long insulated cable, which is placed in a winding fashion along the base of the aquarium, beneath the substrate. The cable produces a very gentle heat, which causes convection (heat) currents to circulate in the substrate. The convection currents carry nutrients, allowing a more even distribution of nutrients and allowing plants to obtain nutrients from their entire root system. To distribute heat quickly and evenly from the cables, a layer of very fine substrate, such as sand is placed around the cable, beneath the main rooting substrate.
A typical planting substrate
For aquariums with only a few plants, or with a selection of hardy varieties, the substrate need not be too complex. The use of a lime free 1-2mm substrate on its own is enough to provide a significant improvement over pea-gravel or other larger substrates. The addition of a nutrient rich substrate additive will also provide many benefits, even if only for a few plants.
For more heavily planted aquaria, a structured substrate should be used which may include; A heating cable covered by up to 5cm of silver sand, 2-3cm of rooting substrate, a thin layer of nutrient rich substrate, and another 3-5cm of rooting substrate. On top of this can be placed a top layer of any substrate, if desired.