The soil in our area is pretty alkaline, meaning it has a high pH.  Most fruit trees and berry bushes don't appreciate this very much.  Although we have high amounts of iron in our soil, it is not available to plants at pH levels at much above a 7.  This make photosynthesis difficult, and plants turn yellow.  A quick, but temporary fix is to add chelated iron, but it is also expensive, especially year after year.  This year I am trying to change the pH of my soil by adding Epsom salts (Magnesium Sulfate) and soil sulfur.  I buy Epsom salts in bulk at Costco, and soil sulfur at IFA.  I have added 10 pounds of soil sulfur and 20 pounds of Epsom salts on dozens of plants in my yard.  This helps to acidify the soil.  It is not as immediate as just adding iron however.  So while I wait for it to take effect, here are some pictures from around my yard of classic iron deficiency.

A peach tree

Another peach tree

 Raspberry bushes

   A blueberry bush