but it wont tell you anything about the PH of the soil.
if you want to check the PPM/PH before and after watering to see what kind of changes are taking place then go ahead but if your looking to get a accurate soil PH reading form PHing runoff it wont happen, you need to do a soil test. not saying that the runoff and soil cant be the same PH but im saying you cant measure the soil PH 100% by runoff.