I collect and use rain water for my bonsai as much as possible but 70% of the time I have to water with our pH 8-9 alkaline city water when I run out of rain water.
Plant roots have difficulties absorbing iron, zinc and magnesium micro-nutrients in alkaline soil. These micro-nutrient deficiencies caused leaf chlorosis especially in azalea and gardenia. When that happened I foliar sprayed my plants with Epsom salt or chelated iron acidifier. They worked well in treating the symptom but did little or nothing to improve soil acidity. Most plants grow best when the soil is slightly acidic. I usually sprinkle a little sulfur powder to my soil and let the microbes convert it into sulfuric acid to lower the soil pH. But this is a very slow process. Some people add vinegar to lower the water pH during watering.
Recently I started doing Bokashi composting with my kitchen wastes. Though called composting, it is actually an anaerobic fermentation. Every few days I drain the fermented liquid so that the composting bucket does not smell. Many people claimed the liquid is full of beneficial microbes and helps plant growth. I do not know how useful those microbes are for bonsai since we use mostly inorganic aggregates. What interested me most is the highly acidic fermented liquid and I could use it to lower my city water pH.
This is the collected Bokashi liquid or tea, it smells like an apple cider and has a pH of about 4-5.
When diluted at 1:100 and 1:150, our city water pH drops to 6 and 6-7, respectively. This is great. I could now turn my kitchen food wastes into something I could use for bonsai. I use a hose sprayer to deliver the diluted Bokashi liquid at 1.5 tablespoon/gallon of water.
Since I supplemented my watering with fish emulsion, liquid kelp, liquid humate, liquid fertilizer, Epsom salt, iron acidifier etc., and I do not plan to do any controlled experiment to test the effectiveness of Bokashi liquid; I guess I will never know how beneficial the Bokashi microbes are to my bonsai, but am contented that I could lower my city water pH with it. In the future, when I produce enough Bokashi compost tea for daily watering, I will experiment with delivering it using a Hozon syphon mixer or an EZ-flo fertilizer injector.
Bonsai Penjing & More 