termite: Termidor treating backfill after trenching

Mix some Termidor in a hand pump sprayer and spray the pile of dirt next to the trench.  As one person is slowly pushing the soil back into the trench, the other person with the sprayer is spraying the soil, mixing it in with the soil being pushed back into the trench.  It is not necessary to use another 4 gallons of solution on just the back fill.  You just want some Termidor to be mixed in with the soil.  If you think about it and you were able to push the dirt in to mix with it, you would just be using 4 gallons of solution per 10 feet.  This way the 4 gallons penetrates deeper in the soil, yet you are still mixing the termidor and treating the soil you are pushing in.  Plan for this when estimating how much solution to purchase.