In over year of having my small 365 gallon pond in the gorund, I have never checked the PH or ammonia. Why, because I have never seen any signs as to why I should check the PH and ammonia levels. The fish haven't shown any signs of stress and the plants have always been healthy. Based on the overall appearant balance that I see in my pond tells me there is nothing to be concerned about.
One thing that I have learned from 30 plus years of having indoor aquariums is that when the substarte is well or heavely planted with live plants, the plants will balance all of the chemistry in the water. The PH can be altered, but it will always be adjusted by the natural ability of the plants.
My personal conclusion is, don't over stock the pond with fish, and always have adequate plant life in the pond to create the natural balance you need for healthy water quality.