p- keeps the pH value of irrigation water at an ideal level. Contrary to pH-, using pH+ is not dependent on which stage of its growth cycle a crop is in. pH- caters to keep the pH levels at a constant, somewhere between 5.8 and 6.2, and thus provides a stable and ideal environment in which the soil and the nutrients are in perfect balance. Your crop will benefit from it and grow faster.
 
Soil and nutrients are balanced
Hard water is neutralized
Stable environment