I haven't been able to find any info on this, but I have noticed a suspicious correlation between the color-bar on my fields and which element is getting boosted in my crops.
-The fields where I have been growing cotton (which adds to the Blue bar) have crops that are elevated in Sulfur.
-My cabbage fields tend towards salt-rich produce.
-My cereal fields are giving me ears rich in Mercury.
-The pumpkins I just harvested from a field which was previously growing cereal came out high in Mercury as well.
Now, there's a random factor involved, but the overall tendency seems to be towards boosting the element whose bar is the strongest in the field.
Has anybody else noticed anything like this?