A good example of salt build up would be GH flora series in a drip system. Anybody that has used GH has probably experienced it right on the bottle. As the solution dries out those salts are left behind and it can happen in soil or coco if you let your medium get to dry between waterings, especially if you don't water enough to break those salts back down and flush them out with enough runoff, they will continue to accumulate and if not chased out will spike ppms with light waterings. One way to prevent this is to water with plain water just enough to saturate the soil, no runoff. Wait 30 minutes then water with your normal solution to adequate runoff. jmho