spraying tomatoes and potatoes

I was going to spray my tomatoes and potatoes with a copper mixture I have been using for the last two years and looked on the Internet to check on the best time to apply it (it's been raining a lot here in Belgium and now set to hot up). A few people suggested trying a mixture of 1 tablespoon of bicarbonate of soda with 2 half tablespoons of vegetable oil and half a tea spoon of soap to a gallon of water and using that as a spray. Has anyone ever tried anything similar?

Antlon