1. What is the rating of the opensprinkler program switch sensor input.
I don’t completely get this question: do you mean voltage rating / current rating or something else? Generally the switch should be a dry contact switch, but a digital output that swings between 0V to 5V (or 3.3V) is also acceptable.
2. I also want to control the master zone output of the opensprinkler controller to turn on when the temperature is above a certain value. Is there a way to turn on the master zone when the weather underground temperature is above say 90F? The purpose of this would be to change fertilizer dosing through a bank of relays connected to fertilizer injectors.
The master zone cannot be directly controlled — it’s always associated with other zones. But you can create a dummy zone (a zone that you don’t use, it doesn’t need to be a physical zone, just a dummy zone used to trigger master), and a program that starts that zone. This way you can configure the sensor port as program switch, and when the sensor is activated, it triggers the program to run.
Another way is to use IFTTT: have an IFTTT applet that hooks up current weather temperature to a webhook that sends a command to OpenSprinkler to turn on that dummy zone.