For early blight, once the tomato plant is established, I always put a piece of copper wire through the stem, just above the soil. However, this will not stop late blight, as it comes from the air not the soil.
Making Soil Acidic for Tomato Plants?
Give Coffee to a Christmas Cactus
Removing Oil Contamination from Soil?
Taming Tomatoes
Preparing Your Soil to Avoid Tomato Blight
Soil Advice for Avocado Plants