You will find an empty chemistry flask inside the van at the roadblock.
Prepare the chemical mix into the flask. There are three different chemical ingredients in the van labeled "A", "B" and "C".
Read the sticky note. It says you should make the mix depending on the pH value of the air.
Search around for hints. ?
There's a dropped weather drone at the bridge.
Looking from the drone you get the pH of the air. Use that value to make the correct mix into the chemistry flask.
Now you just need to spread that chemical mix around the farmhouse.