Choosing Substrate for a Planted Aquarium: Aqua Soil vs Inert

Active aqua soil versus inert sand and gravel, the ammonia phase of fresh soil, how deep to layer it, and when to cap.

Active aqua soil vs inert substrate

The 2Hr Aquarist splits substrates into two families. Inert substrates are made from rock minerals or hard-fired clay; they last essentially forever, decompose extremely slowly, and hold no significant nutrients even when marketed as if they do, so they always need fertiliser or root tabs. Active aqua soils can bind mineral elements — including ammonia — and hold them in a form plants can use; many also contain peat that lowers carbonate hardness (KH) and buffers pH to just below 7.

The ammonia phase of fresh soil

Not all soils behave the same. The 2Hr Aquarist notes some aqua soils are heavily enriched with ammonia (for example ADA Aquasoil) while others are not (for example Dennerle). High-ammonia soils need frequent water changes for the first few weeks and/or pre-cycling the tank for 1-2 weeks before planting. The soil's own buffering (holding pH below 7) actually reduces the toxicity of that ammonia.

How deep, and capping

Tropica recommends a nutrient-rich bottom layer of about 1/2-1 cm beneath gravel of 0.8-4 mm grain, sloping the total depth from roughly 3-4 cm at the front up to 6-8 cm at the back, with about 1 cm of substrate where strong-rooted plants will sit. 'Capping' — placing an inert layer of sand or fine gravel over an enriched base or soil — is a common way to combine the nutrient store of a rich base with the tidy look of sand.

Sources: The 2Hr Aquarist, Complete planted-aquarium substrate guide (www.2hraquarist.com); Tropica Plant Guide, Hardscape and bottom layer (tropica.com) and Aquarium Soil (tropica.com).