The acidity of the substrate should be optimized for efficient nutrient absorption. pH- is used to acidify the water.
Profs pH range
Keeping the pH value right is critical for a plant to be able to take up nutrients. If the pH value is too high or too low, it will not take up nutrients properly. It's for this reason that Profs developed its pH range. The range includes pH controllers for use throughout the culture cycle.
Deciding the ideal pH
The ideal pH lies between 5.4 and 6.2, depending on the substrate you use.
To lower the pH of your feed water we have pH- with 59% phosphoric acid.
To increase the pH of your feed water, we have pH+ with 25% potassium hydroxide.