This is a dosing related thread that is focused strictly upon plant uptake of fertilizers that we add (although, drifting off-topic is fine). As most realize, we don’t have a good handle on this, but it is an important part of trying to maintain targeted levels of nutrients. It is not so important to the EI user, since that concept simply slops on more-than-enough fertilizer and resets with large and frequent water changes and the wide swings in fertilizer levels between water changes are ignored. Many of us prefer a more tightly controlled approach, and the tighter we try to control it, the more important the need to identify plant uptake becomes so that we don’t bottom-out at any point.
Several of us have been discussing what has always been something of a futile effort to measure plant uptake of nutrients. To our knowledge, we have not seen a consensus on a good method for doing this. There are plenty of random comments about it scattered throughout this forum and the Internet generally. We were hoping to use this thread to try to gather opinions and methods used by members to measure/determine uptake and this can include simply stating what uptake numbers you might use based upon studies that you have found elsewhere (please provide sources, if possible). To this end, I open this thread to any member willing to express themselves on the subject.
There are many, many variables that impact uptake. The primary variables that come to mind are plant mass and type of plants, nutrient interactions (a la Mulder’s charts), pH levels, light, CO2 and [you add some]. Since each tank has it’s own mix of these, stating an uptake number may not be as useful as providing the method for determining it. With a method identified, any of us can then run to our own tanks to try it. However, citing a number, while less useful, might also be revealing if we find a reasonable range that seems to be consistent among tanks. This is where citing sources might be useful, e.g.; I’ve found many useful studies that originate in the aquaponics and hydroponics area.
My input:
The basic way that I determine uptake is to simply use the before and after approach: take readings after a water change, take another reading just before the next water change, add what was dosed and the result is the uptake. Of course, the quality of test methods and tools will increase or decrease accuracy. I’ve found this to be a reliable method for K, Ca, Mg and Fe, as well as the parameters pH, dGH and dKH. However, the other elements become less firm, beyond this.
NO3 and PO4 are a little more difficult for gauging uptake, because we have animals and other things adding to it that we can’t measure well. So, measuring these two elements one day, then measuring them x-number of days later, doesn’t give us a great idea about uptake. Further, in the case of NO3, it would be good to know how much Nitrogen (N) is actually used and, in this case, it involves all of the N sources (NH3-N, NH3-N, and NO3-N). We usually, without thinking about it, use NO3 as a proxy for all N. One link that I’ve found that might provide some use in this area is here: N uptake(?).
Hobby-grade tests for measuring traces (other than iron) don’t exist, so this leaves us mostly blind and, like all of the elements, heavily reliant upon trial and error by observing plants. Currently, I am working with a lab that has just introduced the ability to measure in ppb (a mass spec test). I will report on my findings over the next month or two on that.
Several of us have been discussing what has always been something of a futile effort to measure plant uptake of nutrients. To our knowledge, we have not seen a consensus on a good method for doing this. There are plenty of random comments about it scattered throughout this forum and the Internet generally. We were hoping to use this thread to try to gather opinions and methods used by members to measure/determine uptake and this can include simply stating what uptake numbers you might use based upon studies that you have found elsewhere (please provide sources, if possible). To this end, I open this thread to any member willing to express themselves on the subject.
There are many, many variables that impact uptake. The primary variables that come to mind are plant mass and type of plants, nutrient interactions (a la Mulder’s charts), pH levels, light, CO2 and [you add some]. Since each tank has it’s own mix of these, stating an uptake number may not be as useful as providing the method for determining it. With a method identified, any of us can then run to our own tanks to try it. However, citing a number, while less useful, might also be revealing if we find a reasonable range that seems to be consistent among tanks. This is where citing sources might be useful, e.g.; I’ve found many useful studies that originate in the aquaponics and hydroponics area.
My input:
The basic way that I determine uptake is to simply use the before and after approach: take readings after a water change, take another reading just before the next water change, add what was dosed and the result is the uptake. Of course, the quality of test methods and tools will increase or decrease accuracy. I’ve found this to be a reliable method for K, Ca, Mg and Fe, as well as the parameters pH, dGH and dKH. However, the other elements become less firm, beyond this.
NO3 and PO4 are a little more difficult for gauging uptake, because we have animals and other things adding to it that we can’t measure well. So, measuring these two elements one day, then measuring them x-number of days later, doesn’t give us a great idea about uptake. Further, in the case of NO3, it would be good to know how much Nitrogen (N) is actually used and, in this case, it involves all of the N sources (NH3-N, NH3-N, and NO3-N). We usually, without thinking about it, use NO3 as a proxy for all N. One link that I’ve found that might provide some use in this area is here: N uptake(?).
Hobby-grade tests for measuring traces (other than iron) don’t exist, so this leaves us mostly blind and, like all of the elements, heavily reliant upon trial and error by observing plants. Currently, I am working with a lab that has just introduced the ability to measure in ppb (a mass spec test). I will report on my findings over the next month or two on that.
