Nutrient availability and pH: Are those charts really accurate?

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When growing plants, either in soil or hydroponically, we are interested in giving them the best possible conditions for nutrient absorption. If you have ever searched for information about plant nutrition and pH, you might remember finding a lot of charts showing the nutrient availability as a function of the pH – as shown in the image below – however, you might have also noticed that most of these images do not have an apparent source. Where does this information on pH availability come from? What experimental evidence was used to derive these graphs? Should we trust it? In this post, we are going to look at where these “nutrient availability” charts come from and whether or not we should use them when working in hydroponic crops.

A google search in 2021 showing all the different versions of the same nutrient availability plots.

Information about the above charts is not easy to come by. People have incessantly copied these charts in media, in peer reviewed papers, in journals, in websites, etc. Those who cite, usually cite each other, creating circular references that made the finding of the original source quite difficult. However, after some arduous searching, I was able to finally find the first publication with a chart of this type. It is this white paper from 1942 by Emil Truog of the University of Wisconsin. The paper is titled “The Liming of Soils” and describes Truog’s review of the “state of the art” in regards to the liming of soils in the United States and the differences in nutrient availability that different pH levels – as set by lime – can cause.

The paper is not based primordially on judicious experiments surrounding nutrient availability but on Truog’s experience with limed soils and the chemistry that was known at the time. He acknowledges these limitations explicitly in the paper as follows:

I also emphasize that the chart is a generalized diagram. Because adequate and precise data relating to certain aspects of the subject are still lacking, I had to make some assumptions in its preparation and so there are undoubtedly some inaccuracies in it. There will be cases that do not conform to the diagram because of the inaccuracies, or special and peculiar conditions that are involved, e. g., conditions that are associated with orchard crops.

“The liming of soils” by Emil Truog

It is therefore quite surprising that we continue to use this diagram, even though there have been more than 80 years of research on the subject and we now know significantly more about the chemistry of the matter. Furthermore, this diagram has been extended to use in hydroponics, where it has some very important inaccuracies. For example, Truog’s decision to lower nitrogen availability as a function of pH below 6 is not based on an inability of plants to absorb nitrogen when the pH drops, but on the observations done in soil that showed that below this value, the bacteria present in soil could not effectively convert organic nitrogen into nitric nitrogen, the main source of nitrogen that crops can assimilate. In hydroponics, where nitrate is provided in its pure form, nitrate availability does not drop as the pH of the solution goes down.

Several other such assumptions are present in his diagram. Since the changes in pH he observed are associated with lime content, the drops in availability are as much a consequence of pH increase as they are of increases in the concentration of both calcium and carbonates in the media. This significantly affects P availability, which drops substantially as the increase in pH, coupled with the increase in Ca concentration, causes significant precipitations of Ca phosphates. His diagram also ignores key developments in the area of heavy metal chelates, where the absorption of heavy metal ions can be unhindered by increases of pH due to the use of strong chelating agents.

The original pH availability chart as published by Truoug in the 1940s. It has been copied without barely any modification for the past 80 years.
Diagram from the 1935 paper by N.A. Pettinger

Reading further into Truog’s paper, I found out that his diagram is actually an extension of a diagram that was created almost 10 years before, in 1935, by N. A. Pettinger, an associate agronomist at the Virginia Agricultural Experiment station. You can read this white paper here. In a similar fashion, Pettinger created a diagram that summed his experiences with different nutrients in soils at different pH values, where the pH was mainly increased or decreased by the presence or absence of lime. You can see big differences between both diagrams, while Truog includes all elements required by plants, Pettinger only includes the most highly used nutrients, leaving Zn, B, Mo, and Cu out of the picture. Pettinger also has substantially different availability profiles for Mg and Fe.

Although these diagrams are both great contributions to the field of agronomy and have been used extensively for the past 80 years, I believe it is time that we incorporate within these diagrams a lot of the knowledge that we have gained since the 1950s. I believe we can create a chart that is specific to nutrient availability in hydroponics, perhaps even charts that show availability profiles as a function of different media. We have a lot of experimental data on the subject, product of research during almost a century, so I believe I will raise up to the challenge and give it my best shot. Together, we can create a great evidence-based chart that reflects a much more current understanding of nutrient availability as a function of pH.

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12 Comments

  • John McGee III
    March 15, 2021 @ 12:46 pm

    Hey Doc
    I have been growing things all my life and even worked in the citrus business for many years. I am now hobbying it and have just switched to floating hydroponics. I have two 4’x8’x 10″ boxes. I have at least eight different vegetables growing in the same box. I am also a UF/IAFS Master Gardener (newbie). I have been toying with PH and EC, although I do not yet have an accurate PH meter, (if you can suggest a moderately priced one) and I am currently using litmus paper, I do have a Blue Lab pen EC meter.
    I have seen the charts you are referring to in fact UF/IFAS suggest them. Are they currently all we have? if not where can we get better charts? Do you have any suggested numbers that you have developed that could be substituted in the charts to improve them now?

    JSM

    • admin
      March 15, 2021 @ 3:27 pm

      Thanks for commenting John! These charts are currently all that is widely available. There is a lot of research in the pH dependence of different nutrients in hydroponics, but this information has not been put together – as far as I know of – and condensed into a chart. This is a project I’m currently working on!

      For an accessible and accurate pH meter, I would recommend Apera’s pH60 meter.

  • John McGee III
    March 17, 2021 @ 1:35 pm

    Thanks for the response.
    I am looking at your Fertilizer Blog now.

  • Drew
    March 18, 2021 @ 11:55 am

    This article is excellent. Thanks for getting the history of this image and explaining some of the inaccuracies. I have been using it or a variation of it to inform what pH I should be setting my DWC reservoir to and while I’ve not encountered major issues, I imagine a better understanding of nutrient uptake at a given PH for a given media would pay dividends.

    I’m curious if there’s more research out there you’re aware of on nutrient uptake at a given pH when using pure salts in water? It seems that would eliminate a lot of media related variables that would make it harder to test.

    • admin
      March 18, 2021 @ 1:31 pm

      Thanks for your comment and kind words Drew! There is a lot of research on nutrient availability at different pH levels in hydroponics, but it is split by individual elements and different tested conditions across probably hundreds of papers. I’m currently trying to put all of this information together to create a diagram of my own, but it will take its time.

  • Taylor king
    September 24, 2021 @ 12:04 am

    Go Daniel go!!! Time to update this “gospel”!

  • Reilly
    December 20, 2021 @ 2:56 pm

    I’ve been wondering where the information for these charts come from as many charts seem to provide conflicting information on what the ideal pH is.

    Then I tend to find information tailored for pH requirements of specific crops.

    Can a chart like this be anything more than a rough guideline?

    Will these charts need to be tailored for acidic loving plants like blueberries? Or can you get a “one size chart fits all”?

    Thank you for all the work you do!

    • admin
      December 21, 2021 @ 11:49 am

      Thanks for commenting Reilly. The main issue I see is that these charts try to combine two things: plant absorption and chemical availability. The chemical availability will not change depending on the plant, for example, Ca will always be unavailable at high pH, because it forms insoluble Ca hydroxides and carbonates. This is a chemistry fact, not dependent on whether a plant is or isn’t there. The plant availability will change depending on which plant we’re talking about. For example, a plant like a blueberry will get all sorts of micro deficiencies at a pH of 6.2, while a tomato plant will happily grow in it. If you want a single chart you do need one for each group of plants that behave differently.

  • Peter
    December 28, 2021 @ 6:34 am
    • admin
      December 28, 2021 @ 9:41 am

      It seems to just be a rehash of the charts shown in this post but in a different format. You will find dozens of versions of these charts, what they all have in common is that no one tells you the science used to come up with them. In the end, these charts come from the intuition of a very experienced soil grower, based mainly on changing pH with lime applications (as described in the post). They were then adapted to hydroponics by someone along the way, no idea when this happened or based on which science this was made, but they all have the same main source.

      Sadly, there is no chart – that I am aware of – depicting nutrient availability as a function of pH that is based on quantitative experiments done in a well controlled manner. For either soil or hydro.

  • Peter
    December 28, 2021 @ 6:08 pm

    Hi Daniel,

    Thank you, I had assumed that these charts were the result of detailed scientific investigation, particularly as they are reproduced a thousand-fold all over the Internet. Never assume right. The charts are very precise for each element but precision of course does not guarantee accuracy. I have to say though that I’ve always had good results at the recommended pH levels of 5.8 for coco and 6.5 for soil. Even though the charts are derived empirically they are also confirmed anecdotally by countless gardeners (likely not to a precise elemental level though). It would be nice to have science provide the elemental precision. Sounds like a job for a hydroponic consultant with ten years in the industry and access to tissue analysis (hint hint)

    Cheers,
    Peter

  • JoshuaE
    February 11, 2022 @ 7:10 pm

    Have you made any progress on this project?

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