I think it was BBC who did a programmer about young farmers in the UK. One of the dairy farmers, a 17 year old girl was learning from her father and was a very enthusiastic farmer. So the Dad and Mom decided she was competent to manage and had their first holiday in years. Suddenly a cow looked ill and within minutes was dead. When a second one followed the girl realized what was happening - magnesium deficiency. She rushed to the farm sheds and got magnesium licks, which they stored for emergencies and by the time they were available for the cows she had lost 5 in total. Seemingly it is well known in farming circles that the soil is depleted of magnesium.
I asked my brother in law, a farmer in Ireland about this and he agreed. He said government and EU was now encouraging farmers to allow some “weeds” like clover back into grass seeds as these gave magnesium back to the soil.
So sounds like those taking a supplement may be on the right track and a pity proper tests are not available for checking levels.