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
mulm 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.
Rooting substrate
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.
Top substrates
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.
Heating cables
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.
