Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
When you do the full EI method, and the substrate has a high CEC then some of the fertilizers you add to the water will get temporarily locked up in the substrate. When you skip dosing the plants have that reserve.
Since you are now adding substrate fertilizers you do not need to depend on the EI to load the substrate, so just for that reason you could use a bit less fertilizer.
On the other hand, a high light, pressurized CO2 tank could use all the fertilizer you can add. Perhaps the full EI recipe is right for your tank, even with the substrate fertilizers.
The EI method is not cast in stone. You are free to vary it in any way that suits you.
Start with the full EI recipe and monitor the NO3. If it climbs too high by the end of the week, then dose less of everything in proportion and see how it goes the following week.
Continue to monitor things. As the Osmocote becomes available then gets used up the plants may start taking in more ferts from the substrate, then switch back to water sources.