Centre county Pennsylvania, USA | Bill,
Thanks for that link. I had considered tissue test but I believe it's too late for that. In fact, magnesium toxicity discussed in your linked paper seems to say that tissue test may give misleading data, here is quote from that paper:
"Magnesium toxicity's are rare. Crops grown on heavy Montmorillonite clay soils that have been poorly fertilized with potassium may exhibit excesses of Magnesium in their tissue. But, before the tissue level approaches toxicity, Potassium deficiency will occur."
One of the problems with all of the Mg research papers I have found is that they don't say much about dealing with very high Mg levels (without screwing up something else). They all seem to want to talk about how to deal with low levels of soil Mg, particularly for the dairy folks.
We knew that farm was low in P and K, it had been mined by prior alfalfa/grass farmer. Instead of applying the large amount of P required we used a trick that had always worked for us when no-tilling corn following multi-year alfalfa/grass on newly rented ground. We 2x2 banded 100% of crop removal P (as MAP) with planter then counted on decaying alfalfa/grass to supply remaining nutrient shortfall. That worked OK in most areas of that farm but not where follow-up soil tests were showing near toxic levels of Mg.
We have never had any luck with any of the "specialty fertilizer" fixes for nutrient problems so we are not willing to try any of those (again). The plan is to VRT at least 2x crop removal rate of both P and K on the areas that show low yield on this year's corn yield map and normal (crop removal) rate on other areas on that farm. It will be in soybeans next year so that may be another interesting story.......
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Magnesium levels
