How do you know when to plow your fields?

We have sandy clay loam soil, when you can squeeze a handful of soil without seeing water, and the ball crumbles easily. Also when the soil turns up with a shine to it, this is a good indicator it is too wet. Stamp your foot in one place several times and if you see water it is too wet to do anything. We plow in Oklahoma to rid our fields of cheat type grasses (downy brome, marshall rye grass etc.) Till they become a problem we don't waste the aditional fuel needed for plowing.