Periodic Table of Crop Nutrients
Provides detailed information about 17 macro and micro nutrients, including images showing the effects of deficiencies in a range of crops; also gives values to nutrient removal for specific crops based on International Plant Nutrition Institute

Soil and Health Library
This is an unusual site – for a small one-off fee you can download from a large range of out-of-print resources. Head to the Agriculture Library and see what gems can be unearthed.