I wouldn't risk it. The discharge of airguns within the city limits is illegal. Also, the Airsoft gun must have an orange tip.I just need to know if it's illegal to shoot an airsoft gun in your backyard or In a field?
I would not want to be shooting in my back yard and have the police show up to investigate a complaint. They would see the Airsoft and could overreact.
If you want safe practice with recoil, CoolFire in Tulsa makes a simulator that runs on rechargeable CO2. You can add a laser to it to record hits. It's relatively expensive (CoolFire plus laser plus CO2 tank plus consumables), but it is about as real as it's going to get without a projectile.
We have two of the CoolFires (one for 9mm and one for 45ACP). We did have to change out the recoil springs that came with the CoolFire kits for stronger ones.
This is probably not the answer that you were hoping for, but I have found that weapons and other martial arts are expensive to keep up.
Good luck, and be safe.