Root tabs and nutrient substrates: feeding through the roots

Roots or leaves?

Submerged plants can take nutrients from the water through their leaves, which is how mosses, floating plants and plants tied to wood are fed. Rooted plants can also feed through their roots. Tropica’s guide states that aquarium plants can absorb macronutrients via both roots and leaves, so fertilizer can be added to the water or as capsules in the substrate; it adds that liquid fertilizer is the only option for mosses, floating plants and plants on rocks and roots ^[1].

In soil science, nutrients reach roots in two ways: carried in water moving toward the root (mass flow) and by diffusion. Montana State University Extension notes that mass flow accounts for about 80 % of nitrogen movement to roots but only about 5 % of phosphorus, which moves mostly by diffusion ^[6]. That research is on soils and crops, not aquarium gravel, but it explains why less mobile nutrients are usually placed close to roots rather than spread thinly.

Capsules and tabs: what the labels say

	Tropica Nutrition Capsules	Seachem Flourish Tabs
Declared contents	nitrogen, phosphorus, iron, manganese and micronutrients	per gram: N 0.4 %, K2O 0.15 %, Ca 0.15 %, Mg 0.1 %, S 15 %, Fe 2 %, Mn 0.1 %
Placement	close to the roots of large solitary plants and plants with sturdy roots	one tab per 10–15 cm radius; 6 tabs for a 10 US gal tank
How long	released over 1–2 months (product page)	replace every 3–4 months
Other notes	resin balls remain in the gravel and are biologically harmless	will not alter pH; slightly acidic in very soft water

Sources: ^{[2, 5]}.

Tropica recommends its capsules for stem plants, Cryptocoryne, Echinodorus, Vallisneria and other plants with sturdy roots ^[2]. Large rooted plants are the obvious candidates; the Amazon sword and Cryptocoryne profiles cite Tropica on the same point.

Two points to read carefully

• How long a capsule lasts. Tropica’s product page says the capsules release their nutrients over 1–2 months ^[2]; its fertilizer guide says 6–9 months ^[1]. The product page is the more specific source, but the two do not agree, and neither explains the difference.
• “No phosphate or nitrate.” Seachem describes Flourish Tabs as containing no phosphate or nitrate, while the guaranteed analysis on the same page lists 0.4 % total nitrogen ^[5]. Both can be true if the nitrogen is in a form other than nitrate, such as the hydrolyzed yeast listed among its ingredients; the label does not say.

Nutrient substrates

The alternative to adding capsules later is a substrate that carries nutrients from the start. Tropica sells two kinds:

• Aquarium Soil, a complete substrate made of volcanic minerals with a 2–3 mm grain, used on its own. Tropica states that it lowers pH and slightly affects water chemistry, and recommends changing 25–50 % of the water at least twice a week for the first four weeks ^[3].
• Plant Growth Substrate, a concentrate laid as a 1 cm layer under ordinary gravel. Tropica describes it as neutral, not affecting KH or pH, and says it absorbs nutrients and makes them available to plant roots ^[4].

The pH effect of a soil matters beyond the plants: it is one of the buffers that make the CO2-from-KH-and-pH estimate read high. Tropica’s soil page also gives the quantity formula, length × width × depth ÷ 1000 for liters ^[3], which the substrate calculator uses with a front-to-back slope.

Water column or substrate?

The sources do not set the two against each other. Tropica sells both and describes capsules as an extra supplement next to its liquid products ^[2]. Seachem’s FAQ on Flourish Tabs suggests using either the tabs or its liquid Flourish, and both where some plants feed more by root and others by leaf ^[5]. The practical split is by plant: rooted plants can use the substrate, and everything else has to be fed through the water.