My frustration with tissue analysis a decade ago that lead to our use of sap analysis was that tissue analysis results did not correlate to disease and insect pressure, which the literature indicated should be possible. Tissue analysis also did not correlate with field observation of deficiency symptoms. Michael McNeill discusses how accumulated pesticides residues in the soil profile can chelate micronutrients, and continue to hold them in chelated form even after they have been absorbed by the plant. The chelation constants of many pesticides are much stronger than naturally occuring chelation agents like amino acids and organic acids.

From the Regenerative Agriculture Podcast with Michael McNeill:

John: Michael, what is something that you’ve puzzled over for a really long time? What’s really caught your attention in the agriculture space that you’ve been working on?

Michael: Well, something that I’ve finally figured out, I think, was the impact of the lack of availability of micronutrients in our crops. I was doing tissue testing, for example, and I had adequate copper and iron and manganese and magnesium and calcium—everything looked good. What I didn’t realize was that a lot of those minerals were chelated. They were tied up into a form that the plant could not use—yet they showed up on a chemistry test when we tested the tissue. And when I finally figured that out, then everything started to gel for me.

John: I think what you’re saying is that these various minerals and trace minerals were being chelated inside the plant tissue by the herbicides and fungicides that growers were applying.

Michael: Yes. When I tested the plant, it had adequate levels. But when I looked at the plant, it was showing deficiency symptoms. You could look at it and just tell that there was a zinc deficiency or a manganese deficiency; it was obvious. But when I tested it, it was fine. Why was that? And it’s when I learned about this chelation issue and how it can be such a problem.

John: This is something we’ve been monitoring for a number of years. And it seems that, in some cases, sap analysis reports those a bit more accurately. And perhaps that doesn’t take all the chelation into account—but of course it’s still extracting nutrients that are held within the plant sap, and it’s still possible for them to be chelated. We do see the sap analysis correlate more accurately to what the plants are actually showing visually.

Michael: I would agree. I think the sap analysis has been a good step forward. When I figured out this chelation effect, that’s when it really gelled for me and I could understand why I was seeing deficiency symptoms in what, on paper, looked to be an appropriately healthy plant.