[Brian]

Fertile Substrates

"Rooted aquatic plants grow best on a mineral soil such as a silt loam with low organic matter. Rooted aquatic plants require no N, P, S, or micronutrients in the water column when grown on a fertile substrate. Rooted aquatic plants grow best when the substrate is anaerobic ... in fact some roots will not produce root hairs UNLESS the substrate is anaerobic. In most cases, it appears that inorganic carbon limits growth of submerged aquatic plants... not because it is too low in concentration or because the uptake mechanisms are inefficient in aquatic plants (in fact both factors are comparable to terrestrial plants), but because the diffusivity of CO2 in water is about 10,000 times slower in water than in air.

"Aquatic plants undergo cyclical growth, even under constant conditions. This means that even under optimal conditions your plants will slow down and maybe even die back every once in a while.

"Rooted aquatic plants, unlike their terrestrial counterparts, can absorb mineral nutrients both from the water through their leaves and from the sediment through their roots. Unfortunately, it is often assumed that rooted aquatic plants can satisfy all of their mineral nutrient requirements from the water through leaf absorption. This is, however, incorrect. As early as 1905 a researcher by the name of Raymond H. Pond stated that, " ... a soil substratum is requisite for normal growth." and that, " [rooted aquatic plants] make a better growth on a good loam soil, just as many land plants do." Since then, the dramatic and consistently superior growth of plants rooted in soil compared to plants rooted in sand has been shown repeatedly for many different aquatic plant species from many different types of habitat.

"While the reasons for this superior growth are not completely understood, certain facts are clear. First, submerged soils are generally lacking in oxygen. This is of benefit to rooted aquatic plants since under anoxic conditions Fe, P and N are more readily available than under aerobic conditions. Second, nutrient concentrations are higher in a fertile soil than in the overlying water. Third, there is no competition with phytoplankton for available nutrients." - DH

We call these "fertile" substrates; however, all of our efforts amount to one thing and one thing only: conspiring to provide a safe, low level of nutrients for plants at the root zone. A fertile substrate simply means that we attempt to provide a greater proportion of nutrients to the roots.

Taken from: Substrates for Aquarium Plants