he original EI experiments followed this line of reasoning and light was continually added to drive higher and higher uptake rates. At some point, the physical limitations of uptake were reached such that adding more light no longer produced higher uptake rates or higher growth rates. This limit was on the order of 5-6 watts per gallon (wpg) so that for a 20 Gallon tank, adding more than about 100-120 watts T5 failed to produce increased growth rates. The value 5-6 wpg can be considered Unlimited lighting because the plant can no longer make use of lighting intensity higher than this value. At this unlimited lighting we can then measure the uptake rates of the various nutrients. Since adding more light did not result in any higher uptake of these nutrients, the uptake rates measured under the unlimited lighting can also be considered to be unlimited nutrient uptake. On a weekly basis, these measurements were recorded to be the approximate values:
Nitrate (NO3) 20ppm per week
Potassium (K) 30ppm per week
Phosphate (PO4) 3ppm per week
Magnesium (Mg) 10ppm per week
Iron (Fe) 0.5ppm per week
These numbers tell us that if we can provide the weekly ppm listed next to each nutrient then we will be providing the maximum nutrients that the plants can eat, even if that tank (any size) is illuminated by 5-6 WPG. Few people have a 5 wpg tank and as we saw, a lower lighting level creates a lower uptake demand so if we had a tank lit by only 2 wpg it would consume less than the numbers above. The problem is that we are not sure how much less because the tests were not done for every possible lighting level. The good news is that it does not matter. If the tank has lower lighting than 5 wpg then it simply means that there is a greater margin of error and less chance of consuming more that we are dosing. After a bit of experience and careful monitoring we will be able to determine, if we wish, how much we can lower the dosing to match the lower lighting. This is where the “Estimative” part comes in.
In order to achieve 20ppm Nitrate per week we could assume that the tank has an uptake rate of 20ppm/7days a week = 3ppm per day (more or less). We could dose 3ppm per day but the developer of this procedure decided at the time to simply divide the ppm in thirds and dose three times a week, i.e. 7ppm three times per week – Mon-Wed-Fri (presumably easy to remember). The 3X dosing per week arbitrarily became standard procedure but this is not carved in stone. The chemistry has been already done to convert ppm into teaspoons of powder. Here is a sample of how the reference 20 US gallon tank could be dosed:
Sunday – 50% or more Water Change then dose [3/16 teaspoon KNO3] + [1/16 teaspoon KH2PO4] + [½ teaspoon MgSO4]
Monday – 1/16 teaspoon CSM+B
Tuesday - [3/16 teaspoon KNO3] + [1/16 teaspoon KH2PO4] + [½ teaspoon MgSO4]
Wednesday - 1/16 teaspoon CSM+B
Thursday - [3/16 teaspoon KNO3] + [1/16 teaspoon KH2PO4] + [½ teaspoon MgSO4]
Friday – Rest
Saturday - Rest
You can see that if the tank were lit with 5wpg and if the uptake rate were maximum (3ppm) you would just about run out of nutrients the morning of the water change. If you skipped a day of dosing you would really be playing with fire. Since most of us don’t use this much light there is a surplus by water change day. Some people really fret about this surplus, or “Nutrient Build-up”. Really though it just doesn’t matter. If anything, it gives you an extra cushion if you miss a day or two like being out of town for a long weekend for example.
