The ADA dosing approach is one of many commercial systems available, but it's very commonly used and stand in stark contrast to the EI dosing approach because of the nutrient levels used - much much lower. It forms a good comparison to the EI system because we can find many successful tanks that are run on either system - so the variables and outcomes are well known and can be easily compared. Many commercial systems lie between ADA's approach (which is very lean where water column dosing is concerned) and the EI dosing approach (which doses more heavily than any commercial system).
So studying them both approaches gives a sense of the range of which tanks can run in. Some tanks run better with one approach than the other.
The ADA system relies on a rich substrate as a base, while the amount of nutrients dosed into the water column is far more lean. The water column dosage is high in potassium and contains iron, but contains much smaller amounts of NO3/PO4. Many of such tanks will measure 0 ppm NO3 in their water column when water is tested. Plants will mainly draw their nitrogen supply from the ammonia rich soil. As the soil pewters out (6-12months), such tanks become naturally nitrogen limited and growth slows down (may not be a bad thing). Additional new soil & root tabs are added to enrich the soil as it ages.
The lean dosing system is rooted in Asian principles that aim at reducing excess, with the idea that while higher NO3/PO4/Fe levels alone are seldom a trigger for algae (whose main trigger are organic waste/debris, unhealthy plants, excessive light etc), it can indeed exacerbate existing algae issues. Potassium has no effect on algae even at high levels, hence the system takes liberty with dosing it.
A weekly cumulative dose of nutrients using ADA's liquid fertilization system would give approximately:
NO3 - up to 0.7ppm mix of NO3/NH3
PO4 - up to 0.6ppm
K - up to 25ppm
Fe/traces - up to 0.06 ppm
The decimals have been checked and there is no error. According to the above, EI's rate of dosing NO3 into the water column is about 28 times the dosage of ADA's system. This is partly due to the very rich substrate ADA aquasoil provides as a counter balance to the lean dosing in the water column.
Having a very lean water column makes it easier to maintain algae free tanks in tanks that are sparsely planted. This is why hardscape focused, sparsely planted tanks use in Aquascaping competitions do well under this regime. It is also an easy path for beginners. Growth is slower compared to tanks that run rich water column fertilization, but the tanks generally are much more stable. This methodology has proven successful enough that ADA has become one of the largest brands in the planted tank world.
Through substrate feeding, each plant has access to nutrients in its own plot of soil, whereas water column nutrients are free for all. The aggressive, faster growers will snatch up what they can when they can.
The downsides of being so reliant on substrate fertilization are costs (large volumes of aquasoil become very costly quickly), and the need to manage and enrich the soil over time.
A point to note is that the success of ADA tanks as a whole is not because of their nutrient dosing approach. (which is not particularly sophisticated) It is mostly due to their tip-top maintenance routine ; regular water changes, cleaning and siphoning of detritus, and husbandry skill to trim and position plants correctly. ADA tanks generally also choose easy growing plants that grow well in a large range of conditions; many of their tanks feature java fern and anubias, cryptocoryne species. The usage of easy plants coupled with good design results in tanks that are aesthetically pleasing yet easy to maintain.
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The short answer is that you should start dosing water column fertilizers as soon as you have plants in the new tank and no later.
New plants have no established root system, which limits their ability to draw nutrients from the soil.