You use the runoff to control the EC and the pH of the soil. By doing so, you will always know if you need to feed the next time. When pH start to drop, it’s mostly a sign that NaCl start to build up.
Without diving too deep into mineralogy and crystallization to keep it easy to understand, in short sketch, Nitrogen N turns into Sodium Chloride NaCl (sea salt) after a while when it’s not being used.