The degree of acidity is very important for the absorption of nutrients for the plant. With PH + the PH level is increased and with PH- the PH level is lowered.

PH- Grow: Using nitric acid is ideal to lower the Ph value, because it is an important nutrient for the plant in the growth phase.
PH- Bloom: Using phosphoric acid is ideal to lower the Ph value, because it is an important nutrient for the plant in the flowering phase.