Jordan Question to Soil Chemists and Fertility Experts About pH and Gypsum

Jordan Question

I have an old data set that shows that at pH 6 there is a positive response to gypsum but at lower pH there can be a negative response (pH 5.6.)  Any explanation as to why calcium sulfate would cause a reduction in yield at the lower pH?  Is there a chance the low pH is further depressed or is there an interaction that is making Al (aluminum) more available?  I did not have many rates but some of the data suggests a rate response (increasing gypsum rate at pH 5.6 has a greater negative impact.)  I've been showing these data for a couple of years and every now and then I get a "why?"  I point to a possible further reduction in pH and possibly greater availability of Al.  But I'm not really sure on the mechanism.  Gypsum is put out around 45 days after planting and it takes about 140 days to reach optimum maturity for harvest.  Thanks for any thoughts you might have.

A summary of the data set can be found at the following link:

https://www.corpuspublishers.com/assets/articles/aart-v1-20-1020.pdf

David Hardy

Yes, your answer is essentially correct.  If pH is low, the Al on the exchange sites is exchanged by Ca, increasing solution Al making acidity worse. It will react with water and further lower pH.

Jordan Follow Up

Also, is there a greater chance we are seeing more aluminum toxicity?  My understanding is that on our mineral soils lower pH and greater aluminum toxicity are related.

David Hardy

Yes. From what I read, on mineral soils a pH of about 5.6 is where  exchangeable trivalent Al is possibly still on exchange sites. It’s kind of a breaking point for soluble Al presence.  This would vary some depending on OM levels since Al can react with it and be complexed…insoluble.

Luke Gatiboni

As David Hardy said, the calcium will displace Al3+ from the CEC, pushing it to the soil solution. Once in the soil solution, it will occur Al hydrolysis, releasing H+ and increasing acidity.  When studying Al hydrolysis in DI (deionized) water, trivalent Al only exists in pH below 5.5. In mineral soils, it is still possible to have Al3+ at pH 5.6 because of the interaction with other ions in the soil solution. It is possible the existence some Al3+ even at pH 5.7 if there is a lot of dissolved organic carbon in the soil solution. However, when under the effect of DOC, Al3+ is less toxic for plants because plants don't uptake the DOC-Al complex.  The interaction DOC-Al is well studied in no-till soils, where there is trivalent Al but less toxic for plants than in conventional tillage. In no-till, DOC is constantly released in the soil solution, while in CT the DOC decomposition occurs all at once when the plant residues are incorporated into the soil.