Threshold On Plant Nutrients
I was wondering if there is a threshold on available nutrients that plants have less difficulty in taking them up. For instance; if we take two identical tanks with the same size plants and one tank had 5 ppm nitrates and the other tank had 15 ppm, then the question would be:
Would the plants in the tank with 5 ppm nitrates have a harder time taken up this nutrient then the tank with 15 ppm?
No to that specific question, but the take up of nutrients is a bit more involved.
Aquatic plants have very specific needs for 17 nutrients in relative proportion. An excess of some nutrients can cause the plant to "shut down" assimilation of certain other nutrients. I won't bog things down on this point, but move on.
As for nitrogen, most aquatic plants prefer ammonium, and they will take this up [provided all other necessary nutrients are available, and light is sufficient to drive photosynthesis] until it is exhausted. Only then will they turn to other forms of nitrogen--and again only if everything else is still sufficient--and studies indicate their next preference is nitrite. Only after this (i.e., ammonium and nitrite are exhausted) will they then turn to nitrate--and again provided everything else is still available.
The reason is the inward workings of the plant. Ammonium is easiest to assimilate, requiring minimal effort. Nitrite requires more effort from the plant, and nitrate even more. This is because both nitrite and nitrate have to be converted by the plant back into ammonium, and this takes considerable energy.
There are some aquatic plants that do take up nitrate better, but they are few. And before you ask, I would have to dig through my research to name them.:-)
I was thinking that the higher ppm might affect the uptake much faster and the lower ppm much slower. So actually, the plants in both tanks if identical, are taken up the nutrients at the same rate. In other words; If the input and output of both tanks remains the same, then the ppm stays stable provided other parameters are in balance.
I have a variety of plants in my aquarium and my nitrates do not go up measurably. I have yet to see them over 5ppm even after a three week span between water changes (Florida trip). I couldn't tell which ones are the best for this but I might guess the duckweed, then swords and maybe the hygrophila .
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