School Me On Iron

I've got a customer with an IR horse and wants hay with low ESC + starch. That I get. However, they were interested in iron levels too in ppm.

Question is - what drives iron levels in hay? Soil, hay variety, something else? Our Timothy is low in iron. I saw a hay analysys for orchard grass and it was very high - I can't explain.

What is the driver for iron levels in hay?

Thanks,
Bill

School Me On Iron