What I did was to have two master valves. One for regular watering and another for fertigation. I use the OSPi. I have OpenSprinkler configured to use the regular valve and then when I want to fertigate, I use a cron job to call a Perl script (could have been a shell script) that shuts down OpenSprinkler and takes over to do the fertigation and then fires OpenSprinkler back up. Fairly easy. Don’t really like shutting off OpenSprinkler, but necessary to keep it from messing with the valves.