from an article about heating cables - posted as a response to someone saying they were going to use laterite without a heating cable:
I'm curious as to why you would go through the expense of setting up an "almost" optimum aquarium. The laterite and the heating coils are intimate components.
Visualize if you will a biotope of your choice. In your mind take along your 100 gallon tank. Imagine that it does not have the bottom in place, just the 4 sides. press this into the SOIL of your biotope so that the you will have 3 inches from the top of the soil to the bottom of the sides.
Carefully slide in the base from a side so you include the soil. Let's just say the bottom is now fused again to the sides so we have taken a piece of nature, straight out of the biotope, including water, soil, plants, fish, etc. A perfect scenario? Nope. It does not take a expert on ecology to see that less than optimum plant and fish health will result, even in natural light.
What has happened is that the movement of water through the soil substrate has been blocked. Plant roots need oxygen to metabolize. There is a soil substrate/water/root hair interface of tremendous import that is disrupted.
The authors of "The Optimum Aquarium" tell you the coils are there to supply slow steady convective movement through the substrate. In my opinion, this is the only method on the market which provides water movement slow enough to keep the laterite in place.
The laterite in the substrate serves a critical function in a planted aquarium. The laterite has many negatively charged sites; ammonia in the form of ammonium ions is positively charged. The laterite attracts and holds the ammonium ions like a magnet until a plant root hair exchanges another positively charged ion for the ammonium (adsorption) and takes it in to metabolize into amino acids and ultimately protein. No laterite, no negative charges, no easy ammonium uptake.
You have the additional benefit of removing ammonia. Instead of just the "ammonia to nitrite to nitrate" cycle in biologic filtration, you will bypass the "nitrogen cycle" and its accumulating nitrate level. The nitrogen ends up being removed from the tank as you cut and prune excess plant tissue because plant tissue is partly made of protein, which is 14% nitrogen.
The scientists at Dupla figured out that since you have to heat the tank anyway you might as well do it by putting a coil in the substrate. The gentle currents emulate the ground water/surface water movement. It isn't just that the roots like to be warm. Some experiments on terrestrial plants show marked increase in growth if you heat the soil higher than the surrounding air
The heating coil provides just the right velocities to circulate the tank water with its ammonium throughout the substrate. Some of this is exchanged for H+ (protons) or other cations (positive ions like K+, Ca++, Mg++, Na++, Al+++).
) how can the roots get cold? the substrate will surely equalise with the coolest surround temperature - either the air around the tank or the water ? 
