“...a single thought can change the polarity of our entire body toward either life or death - and can likewise change its entire chemistry toward increasing alkalinity or acidity to strengthen it or weaken it...”― Walter Russell
In chemistry, pH is the measure of the acidity or alkalinity or basicity of a solution. Solutions with a pH below 7 are considered acidic, while those above 7 are considered basic or alkaline. Solutions of exactly 7 are neutral. Pure water has a pH of approximately 7. Most species that are commonly used for bonsai are tolerant of a wide pH range. Most that have a preference (e.g., azalea, bougainvillea, many juniper and coniferous species such as spruce and larch, for example) tend to do better in slightly acidic soil. If you have hard water, you are probably slowly but constantly increasing the pH in your soil, which can weaken acid-loving trees.
So how do you safely add acidity to your soil? There are plenty of products at your local garden center, but there is also a cheap alternative. Coffee grounds. Acid loving plants do well with coffee grounds mixed into the soil, sprinkled on top, or poured on. You can also make a soil drench by soaking 6 cups of coffee grounds in a 5 gallon bucket of water. After 2 or 3 days use this solution to water. For faster action, add a few tablespoons of vinegar to your next watering. Adding sphagnum or sulfur can also lower the pH of the soil.
Another handful of plants (e.g., boxwoods, maple, weigela and forsythia) prefer alkalinity. The garden center has solutions for this, too - generally, bags of lime (calcium carbonate) that run $10 to $40 and aren't needed very often. The average household has a cheap alternative to this, too. Eggshells. Eggshells are more than 90% calcium carbonate. And every time you buy a dozen eggs, you get a dozen eggshells for free. I pile them up in their box with the uneaten eggs until the whole thing is full. Then the cartoon goes outside to dry out. Once they are completely dry (which is almost immediately in warm weather) I crush them and stick the pieces into a blender. A few seconds later, I have a little pile of 90%+ calcium carbonate to bag up and use as needed. In addition to the calcium, the eggshells contain about 1% nitrogen, about a half-percent phosphoric acid, and other trace elements that make them a practical fertilizer.
There you have it, two nearly free solutions to your pH level problems.