I just found a great website, which you can look at different data for soil analysis in your area on a map, very interesting to see the variations in some of the major minerals between different areas. Its only in percentiles but still interesting to see. Apparently our area is low (0 or 10%) for everything that horses need, explains why I've found it more vital to supplement vits & minerals since living here.
http://mapapps2.bgs.ac.uk/ukso/home.html?layer=NSISelenium
http://mapapps2.bgs.ac.uk/ukso/home.html?layer=NSISelenium