Nutrient Availability Vs. Ph


Lifetime Members
Lifetime Member
Nov 8, 2015
I have been researching nutrient uptake according to PH and stumbled across some interesting links which I thought I would share. The discussions in this post kicked it off: (it discusses mulders chart, a chart showing nutrient uptake according to PH, and the opinions of burr740, solcielo lawrencia and daniel deacu on zinc and chelates: all excellent input, love reading your stuff guys, thanks!)

The nutrient uptake charts I found on line introduce a lot of confusion. The one shown in the above post appears to focus on land crops, but there are others which focus on hydroponics. Hydroponics of course considers plants with their body in air and their roots in solution, and our interest is in plants submerged totally in solution, so neither soil nor hydroponic charts are really representative. However the differences between the two charts are substantial enough to get me wondering.
Here are two links comparing charts for soil and hydroponics together:
To increase confusion, the following link discusses hydroponics, but uses the chart for soil as the basis.

Of course, such charts are often on the sites of manufacturers who want to sell more products, and the fact that there is no y-axis labelling does not help either. I also have not been able to localise the original sources of these charts, so I can not tell if this is only based on root uptake or whatever. Of all the articles I found, the following article was the most interesting for me. It explains the hydroponics chart in clear language, and describes the effects of the individual nutrients in laymans terms. For someone (like me) who is not a chemist or biologist, it should make very interesting reading:

Now a few thoughts (or rather where I am most confused):
- Is the hydroponic chart any more helpful to us than the soil chart, given that our plants are normally totally submerged? Seems to me that hydro is a little closer to our needs, or am I just persuading myself to believe that?
- If we can at least lean a little towards the hydro chart, then the availability of certain elements falls off dramatically at higher PH (mainly micros, but also Phosphorous). It seems to me that large PH swings as CO2 comes and goes could introduce large instabilities in nutrient uptake. At the lower end micros suddenly become much more available. Or is the PH swing from CO2 not relevant here? Is the degassed PH the one which counts in such considerations?
- The article in the last link specifically states that the hydro-chart represents the uptake for non-chelated nutrients, and that the range can be extended with chelated nutrients. This goes against what I have understood in recent years, that non-chelated nutrients seem to work better (whether this is because of the availability of the nutrients or the abscence of the chelator, I have not quite got yet).
- But if I consider this chart with respect to the recommendation of high CO2 levels (and thus low PH), it seems to me that just pushing the CO2 to the limit could just release the nutrients plants need in aquariums where the degassed PH is higher. And since this limit is being pushed, only small changes, can knock the balance out. For example: I have an overflow and sump, and it seems that within a week, the sieves and media can get clogged just enough to raise the water level in the aquarium a little, which canges the gas exchange effect. And since I am not one of those who is willing to push my fish to the point of gasping at the surface (I go for 1 PH drop), I am thinking that maybe my nutrient levels are bordering on available / not available (esp. Zinc). This together with the mulder chart considerations is seriously putting my sanity to the challenge.

So, if anyone out there has some logical explanation for my confusion, can tell me what I have missed, or can tell me that I am seriously overthinking it all, I would love to hear from you ... and sorry if I have confused anyone to my level, that is not my intention. I am just trying to understand the issues better myself.

Meanwhile, I am going to try to start lowering my deassed PH a little. If I see improvements, I will add a post to my journal.