Plant Nutrient Symptoms

This is a great website to view plant nutrient symptoms by plant or by nutrient.  It is of ornamental plants, but hey, once you can see it on one plant you will something very similar on avocado, lemon, apple, almond, lychee or whatever alphabetic fruit you work with. Kudos to University of Florida. Of course once you see what iron deficiency looks like, you'll be able to identify it on most other plants:

http://hort.ufl.edu/database/nutdef/index.shtml

Below is Boron toxicity from a grower who got too excited about correcting B deficiency,