Urbafarmer has a good point.

Farmer Brown, here's a link, showing color pictures of deficiencies.....
(I think this has been posted before ...?)

http://www.hbci.com/~wenonah/min-def/list.htm

The Diagnosis of Mineral Deficiencies in Plants by Visual Symptoms
by Thomas Wallace, M.C., D.Sc., A.I.C.
University of Bristol Agricultural and Horticulture Research Station, Long Ashton, Bristol