The nutrient solution you mix up is made up of cations and anions. A cation is a positively charged ion and an anion is a negatively charged ion. It’s important to understand that the nutrient solution you mix will “always” have an equal balance of cations to anions. If your reservoir has 10000 positively charged ions, there will always be 10000 matching negatively charged ions. There will never be an imbalance.
Things change when you feed this perfectly balanced nutrient solution to the root zone. If the plant absorbs a cation like Potassium (positively charged) they will replace it with hydronium cation (H3O+) which causes the pH of the nutrient solution and root zone to decrease.
If the plant absorbs an anion like Nitrogen (NO3-) they will replace it with a hydroxide ion (OH-) and the pH of the solution and the root zone will increase. By collecting runoff, we can get great insight into how the plants are absorbing these nutrients and make adjustments to our feeding schedules when necessary.