Discrimination help with Coinmaster

I got my first detector, a Coinmaster, four days ago. I am swinging in an old garden of my granddaddy's. He knows there used to be an old shed/building there about 70 years ago. He is always finding nails. He has also always grown peas, which are a member of the legume family (they naturally produce nitrogen from their roots as fertilizer).
I have my discrimination set to max, and I am finding nothing but nails. I do not know why. Could it be that the ground is highly mineralized? My sensitivity is on 3 and the max is 8. Why am I only finding nails?