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EI nutrient targets vs. accumulated nutrient values

arcturus

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I am thinking about using dry salts and will ask for advice in a separate thread. But I have a conceptual question about EI targets that is probably irrelevant but it is bugging me :) I am aware that the targets are simply an empirical estimation. Nevertheless, EI calculators and advice are quite specific about achieving a consistent NPK weekly concentration. If we take as an example the "EI full" targets as defined in the ICF calculator, we find a weekly target of
  • 30 ppm NO3
  • 3 ppm PO4
  • 30 ppm K
  • 10 ppm Mg

However, these nutrient concentrations will increase over time until they stabilize according to the actual plant uptake and water changes. If we assume as an example that the weekly NPK uptake by the plants is 60%, and that a 50% weekly water change is performed, then after 4 weeks, the nutrient ppm will converge to these values
  • 37,50 ppm NO3
  • 3,75 ppm PO4
  • 37,50 ppm K
  • 12,50 ppm Mg

This means that after 4 weeks, we have reached an accumulated nutrient ppm that is ~25% higher than the prescribed EI weekly target (see chart below).
1639917586638.png


With the example above, if we reduce the weekly fertilizer dosage by ~30%, we will converge to the "correct" EI target values after 4-5 weeks. But if we use the prescribed EI weekly dosage, we will be basically wasting 30% of the fertilizers after the initial loading/accumulation period. So, what is the real target we are aiming at? The higher accumulated ppm, or the prescribed EI weekly target?

Am I getting something wrong with this reasoning? Thanks!
 
This means that after 4 weeks, we have reached an accumulated nutrient ppm that is ~25% higher than the prescribed EI weekly target (see chart below).


Some thoughts:
  • Would a TDS meter help to verify this? i.e. because the nutrients are not used and remain in the water, the TDS should slowly continue to climb week after week despite 50% water changes, and eventually 'converge' at a certain number?
  • Would you decide how much water to change based on TDS? For example, if you do a 50% WC but find that every week your TDS ppm is creeping upwards (towards convergence as shown in your graph), would there be benefit in doing a larger water change like 55%/ 60% etc, to keep the TDS ppm steady?
  • Also, your spreadsheet assumes consistent % uptake of each element. What if one element has an uptake of 60% but another element has a different uptake? Then the 'ending ratio' of elements in the water is going to be different from the starting ratio.
 
Hi all,
If we assume as an example that the weekly NPK uptake by the plants is 60%,
I'd agree with @erwin123. I'm <"not an EI user">, but I don't think you can make any real assumptions about plant uptake, it is going to depend on the plants.

A <"turned up to eleven plant"> might be capable of using all the added nutrients.

Conductivity measurement and water changes should allow you to remain in your nutrient "sweet spot".

cheers Darrel
 
One of our Nutrient/CO2 Gurus @ceg4048 did a little experiment and dosed his tank x3 to x4 EI dose and he only had one main comment - the plants just looked even healthier.
Like @dw1305 says just find the sweat spot that works for you :thumbup:. Trying to work out what nutrients are needed is tricky, time consuming and costly - hence the use of EI dosing and Regular Water Changes.
An excess/lack of one trace nutrient 'X' can induce a deficiency in another Nutrient 'Y' and its the latter deficiency you may see in the plants and try increasing the latter nutrients 'Y' dose when the cause of the deficiency is 'X' imbalance, it can be a bit of a mind field IMO

If you making you own ferts, the cost of excess is irrelevant, we could also use the environment 'card' - however what we send down the drains is a 'drop in the ocean' compared to what farmers add to our rivers and water supplies
 
I am thinking about using dry salts and will ask for advice in a separate thread. But I have a conceptual question about EI targets that is probably irrelevant but it is bugging me :) I am aware that the targets are simply an empirical estimation. Nevertheless, EI calculators and advice are quite specific about achieving a consistent NPK weekly concentration. If we take as an example the "EI full" targets as defined in the ICF calculator, we find a weekly target of

However, these nutrient concentrations will increase over time until they stabilize according to the actual plant uptake and water changes. If we assume as an example that the weekly NPK uptake by the plants is 60%, and that a 50% weekly water change is performed, then after 4 weeks, the nutrient ppm will converge to these values
Everything @Zeus said.... and I don't think plants consume fertilizer based on a percentage of what is available... I any event, whichever way you calculate your dosing regime, aim at being over EI rather than below. You wont go wrong by doing so. One less thing to worry about.

Cheers,
Michael
 
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With the example above, if we reduce the weekly fertilizer dosage by ~30%, we will converge to the "correct" EI target values after 4-5 weeks. But if we use the prescribed EI weekly dosage, we will be basically wasting 30% of the fertilizers after the initial loading/accumulation period. So, what is the real target we are aiming at? The higher accumulated ppm, or the prescribed EI weekly target?

Am I getting something wrong with this reasoning? Thanks!
Hello,
As mentioned by other posters above, there really are no "correct" EI values. This way of thinking, forced by the results of spreadsheets, is one of the things I dislike the most about the calculators. I suggest that you study the background information provided in the EI dosing thread:> EI DOSING USING DRY SALTS
In this article the origin of the target concentration values are provided in the following paragraphs:

The 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.

So as discussed in the article, the target values are simply the numbers that were achieved where no further significant increase in growth rates were achieved with increasing light intensity and increasing nutrient addition. The object lesson from the results, therefore is that nutrient concentration to maximize growth rate is directly proportional to the increase in light intensity, after which the growth rate levels off asymptotically to the lighting intensity. If the light intensity is lowered then the nutrient addition can also be lowered because more nutrients at a given lighting level will have only marginal increase in growth rates.

We can control the growth rate first by controlling the light intensity, and at that intensity there is some nutrient concentration level that (more or less) maximizes the growth rate at that intensity. The secondary growth rate controller is therefore the nutrient concentration.
Spreadsheets cannot, by virtue of the nature in which they present data, elucidate this concept.
We do not need to feed the plants the target value if we are not using the maximum lighting that plants can utilize.
The target values have nothing to do with "optimum", or "best", or "correct". They are merely values that cover the required nutrient load at maximum light intensity, assuming the hobbyist chooses to use maximum lighting.

Furthermore, high nutrient loading results in rapid increase in TDS and high growth rates result in high metabolism, which results in high rate of organic waste excretion into the tank by the plants. This has to be combatted by strict attention to maintenance, i.e., pruning and water changes.

EI target levels are to be used as a guide Neo. They can show you the path, but you are not required to follow the numbers blindly without understanding what these numbers actually mean.

Cheers,
 
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