Okay you have some options the cheap vacume breaker the put on faucets.
And good vacuum breakers.
With a vacuum breaker they need to be I think at least 18 inches higher than your highest emitter.
I dont think you want pay what I paid for mine, it is rated for a chemical plant.
Here it is.
http://www.sprinklerwarehouse.com/Fe.../fe860-100.htm
Here is what I am thinking about but it isn't for your main line just for the water going to you fertilizer system.
http://www.sprinklerwarehouse.com/Aq...-p/avb-075.htm
Any time you connect chemicals to the water supply you should have these.
You are on a well so if it contaminates your well you are screwed.
Look up Plumbing cross connection.
Worth