Roots from a previous grow are not a good addition to any pot, (they can and will rot) so if you re-use the soil...be sure to remove as much of 'em as possible. Nothing uncomposted should ever go in your pots.

Ph drops are common with re-used soil, so additional buffering (garden lime or something similar) will likely be needed.

Flushing the soil you are about to re-use is a damn good idea, as it will loosten the salts and crap that have built-up over the last grow. Not a good idea to re-use soil without flushing well.

Farmers often rotate crops, plant companion crops, and use crop sequencing to replenish soils, and keep 'em bioactive.

Re-using soils will create a 'crusty' condition (calcium, lime, rust, etc...) rendering it unable to absorb water. A surfactant can be used to break water surface tention, allowing easier absorption in the soil. Dishsoap works well, but make sure it isn't anti-bacterial, or it will kill the bioactivity in the soil. (1/2 tsp:gallon of ph'd water) One application lasts a while.

If you are going to re-use soil...it's best to have a good grasp of cannabis growing and troubleshooting. It's nowhere near as good as fresh, for consistency and results. In the long run, you can save a few dollars, but it may cost shitloads in frustration, extra work and heartache.