Plumbing and Piping - stinky septic

Any ideas why I keep getting a sewer gas smell in the yard which is coming out of the vent stack ?? I feel I maintain the system well, by having it pumped every year. It seems like it only happens when the tank is stirred up by doing several loads of laundry, or the water softener backwashes itself ???

You can't do much about sewer gas coming out of a vent. That's what they're for.
If the septic smell is due to water bubbling to the surface when loading a lot of drain water into the system, you may have a drainfield problem.
Good luck!
Mike

My first question would have to be "what vent stack". I have never seen a vent on the septic tank itself. Could you be referring to the vent stack for the restroom?
Secondly, A properly installed system should not need to be pumped out yearly. But of course you have already explained why you have to do this.
#1 washers draining into septics are not the best thing to do because of the high phosphate levels, and sorry to say, but the water softener is a big NO NO.
The septic system depends on enzimes to form bacteria to break down the waste inside the tank. Phosphates hamper this process and the salt discharge from your softener completely stops the process.
As for the odor? But I would suggest a yeast cake broken in quaters and flushed once every two months. Its a lot cheaper then more exspensive treatments and does the same thing.