Res Top-Off Theory

First post on rollitup, but have been on for awhile. I know some people add back nutrients with their top off water, others do not. The common reasoning for people who dont is that some nutrients can build up to toxic levels.

This comes down to the theory that plants can intake macro and micro nutrients to whatever their needs are. If this was true why are there plants that get nitrogen toxicites and other toxicities? I might be missing something, but i can only reason that plants can uptake only 2 things: h2o and nutrients(in whatever proportion the solution is mixed to), and not actually individualize salts out of the mixture.

Would love to hear back from you guys. This site has been extremely helpful before, but there are some questions that just need to be asked.

The theory that plants can uptake to whatever the needs means more-or-less that the plant will not under feed itself if the nutrients are offered properly. You take any species of anything and give it abundance it will take in more than it needs. Think of it like a fat person.

As far as topping off, I'm against it if you don't have the right equipment. If you aren't able to determine the PPM or TDS then you're rolling the dice. If you are gonna just take a crap shoot, go 1/2 to 1/4 strength on the nutes for the top off. That being said, if you have your plants and nutes dialed in and you know how they grow and feed you can top off appropriately without the meters, but I would say that considerably more growers don't know their plants than do.

thanks for the reply.

TDS meter is a must!
Let's assume that the plant is using the same ratio of nutrients every week, and you are meeting its demands with your original formula would Toping-off( I guess I should call it Adding-Back) replenish the nutrients that were absorbed? Or would this work only if your formula was exactly what the plant needed, which is almost impossible.

dukethompson said:

Think of it like a fat person.

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No, don't think of it like a fat person (human) or other animal (pig, goose), think of it as a plant.

@longdonjohnson it won't exactly replenish the same amount of each element used no, because indeed the plant doesn't take up the exact same ratio of elements as you provide it (some elements are easier to take up than others, ph influences that further). But, it still is better to top off with some nutes (assuming the ppm level dropped or drops if you were to give only water) to bring it closer to to ratio you initially provided, assuming that is what you want to give. The longer you use the same rez, the larger the difference with the initial ratio becomes, eventually risking an imbalance and nutrient antagonism. I tried to explain this before here: https://www.rollitup.org/t/aero-nft-nft.825189/page-3 which may answer your question, not sure what exactly that is

Got it. I have been trying to see both sides of this and couldn't sort it out. Thanks for the link and the breakdown. Im running a diy 4x8 240 gal dwc tray for 2 lights. Thing is a beast.

You're welcome. Some day it'll become easier and cheaper to measure the individual elements and then you would add more of the element(s) of which the ppm dropped the most to try to meet the nutrient demand more accurately.

longdongjohnson said:

Im running a diy 4x8 240 gal dwc tray for 2 lights. Thing is a beast.

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Sounds good, got pics?