Q.How Often Should You Spray Your Tomato Plants With Baking Soda Solution To Prevent Blight
I had blight last year for the first time, Should i spray the leaves or the soil

Certified GKH Gardening Expert
Not very often. If you have nothing else, then no more than once every couple of weeks.
There are much better ways to control fungus and bacteria that won't be toxic to your plant. Anything with sodium should be used very sparingly, since it is toxic to plants in high levels as would be found in baking soda.
Here are some articles that will help you to make a safer fungicide:
https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
https://www.gardeningknowhow.com/special/organic/biofungicide-information.htm
https://www.gardeningknowhow.com/garden-how-to/info/homemade-fungicide.htm
https://www.gardeningknowhow.com/garden-how-to/info/applying-bactericide-to-plants.htm