http://www.oase-uk.co.uk/expert/nahrstoff.html

NH4+ + OH = NH3 . H2O
ammonium = ammonia (toxic)

pH will determine how much of the dissolved ammonia is gas vs ammonium in water,
lower pH results in ammonia being converted to ammonium ion. It is a lot like CO2
which dissolves in water which drives the pH down.
low pH, OTOH makes metals more toxic. so it is a balancing act. Ingrid