The substrate

Since the substrate forms the anchor and growing medium for plant roots, it is vital to choose a material, or combination of materials, that will not only sustain the plants both mechanically and nutritionally, but also look attractive as part of the tank decor. Here, we consider the options available for successful plant growth.

Gravel

Gravel is the universal substrate in aquarium circles. It can be used as the sole substrate in planted aquariums, although it is best employed as a top layer covering areas or 'pockets' of nutritious growing medium beneath.

The most popular type of gravel is low-calcium beach gravel with a particle size of 2-3mm (0.08-0.1 in). This grade allows roots to penetrate easily and yet provides a firm support for the plants.

Baked clay gravels are also available. These are larger in particle size, brown to deep brown in colour and, since they do not contain calcium, they do not affect the hardness or pH value of the water. These gravels are pre-packed and are often impregnated with trace elements by the suppliers, thus making them an ideal plant growing medium. Their highly porous structure also provides an excellent 'home' for nitrifying bacteria in biological filter systems.

Gravel additives

The iron-rich clays known as laterite that are a common feature of tropical regions, often imparting a reddish colour to the bed of streams and pools, can be especially valuable in promoting lush plant growth. The iron compounds dissolve from such soils and combine with organic acids produced by the plant roots. This 'organic iron' is not only more easily absorbed by plants, but it also enables other trace elements that the soil contains (and which are normally 'locked up' in stable compounds) to be made available for plant growth.

In open water biotopes the clays are scarcely overlaid by humatic materials and the plants that inhabit these situations will thrive well in an aquarium with just the clay as a gravel additive. However, the latest research shows that those species coming from heavily forested areas grow in a thicker layer of material formed as a result of the leaf fall from the canopy above. Incorporating such material in commercial preparations of laterite has been shown under controlled laboratory conditions to promote more luxurious growth in plants such as aroids and echinodoras.

These tropical clays are prepared for the aquarium in the form of a fine screened powder. It is mixed with 75 per cent of washed lime-free gravel and the remaining gravel used as a top cover. These products also include a powdered fertilizer mixture containing growth hormones to promote the formation of roots. For those who wish to add these clays to an already established aquarium, the material is also available in the form of cylindrical sun-dried pellets. These

Below: Aquarium plants grown in slatted hydroculture pots. These help plants to become established and allow easy removal and repositioning.

Above: Tropical iron-rich clays -laterite - are available for aquarium use both as fine-screened powder and as sun-dried pellets, which are simply pushed into the gravel.

are simply pushed into the gravel at the base of the plants.

Rock wool hydroculture

Rock wool - a fluffy greyish substance - is an ideal growing medium for plants set in hydroculture pots. These are plastc pots with an open lattice structure that allows the plant roots to extend outwards into the main tank substrate. The rock wool, impregnated with a suitable fertilizer, acts as an inert support for the roots as they absorb essential nutrients. For aquarium use, wrap the roots in rock wool and insert each plant into a pot before setting it into the substrate. Small pots of 4cm (1.6in) diameter are ideal.

Below: Various grades of aquarium gravel plus two colours of baked clay, fertilizer-impregnated substrates. The two finer grades of gravel are usually the most suitable to support good growth in aquarium plants.

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