Answer: Sally, that is an interesting recommendation. I was associated with the northern nut growers a few years back. I have visited with a number of walnut growers but never heard about "granulated copper". Understanding soil pH as I do, I can't think of any reason to use copper of any kind. Also, too much copper can even become toxic to plants. Copper is used in the lining of a new type of nursery container to prevent root growth and the problem of "circling roots" in the container. I am quite sure what they meant to say was granulated sulfur. This will reduce the pH slowly over time.
Soil scientists tell us that when the soil is heavy (clay as opposed to sand) and the pH is very high, it is difficult to do a lot of good with sulfur applications. Granulated sulfur, as opposed to powdered sulfur, is slow acting and may help in the root micro-environment by creating small areas of moderate pH soil. However, this is a bit speculative.
One thing you can do to help such plants is to mix several inches of compost into the soil surface as deeply as is practical. Research has also shown that vertical trenches filled with a mix of compost and sand can significantly help trees growing in a heavy, high pH soil. Four trenches are dug with a trenching machine (available at local rental businesses) about a foot deep and radiating outward from the base of the tree. If done prior to planting, a large "X" with two long trenches can be made and the tree planted in the center.
I realize this is quite a task, but I also know how we fruit and nut growing enthusiasts will do anything to make it work! Thanks for the question and good luck with your walnut trees.
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