Water, air, and sunlight are the three most basic and plentiful things that a plant needs to grow. Most people remember enough from a high school biology class to know at least vaguely how photosynthesis works. But do you remember enough from your chemistry studies to know what the optimal pH for bonsai nutrient absorption is? Wait, do you even remember what pH is? Let's combine some bonsai and chemistry talk.

pH it Like a Pro

pH is a measure of the balance of Hydrogen ions - H+ which are positively charged in a solution of water; and Hyroxide Ions - OH- which are negatively charged. The two are in perfect balance at a pH of 7.0. The pH scale is logarithmic; a pH of 5.0 is not 1x stronger than 6.0, but 10x stronger. As pH moves from 7.0 downward toward 0, there are relatively more positively charged ions in the solution compared with the number of negative ions. 

The pH of your soil and your water are both important to the health of a bonsai tree or any plant confined to a container. There are charts that show the uptake of macro- and micro-nutrients at various pH readings, but suffice to say that the optimal pH for watering plants is in the 5.5-6.5 range. This balance of ions allows the nutrients to be absorbed by the plants more readily because nutrients, being mostly positively-charged ions, are not "stuck" to the excess of negative hydroxide ions.

A simple pH meter or liquid test kit will give you an idea of the pH of your tap or well water; most municipal water is treated to remain between 8.0 and 9.0 which prevents it from corroding the pipes in the system. If your pH is too high, you can use a few easy tools to reduce the pH of your water.

1. For just a few plants, you can use a watering can and add citric acid, which is conveniently available inside lemons that you may already have.  It can also be purchased in powdered form. Add a little to the water and it will not only reduce the pH but will also eliminate chloramine if your municipality uses it. Be sure to measure the pH again before using the water on your plants as too much acid can also be a problem.
2. For a larger numbers of bonsai, where you're likely using a hose to water, you can use an inexpensive siphon injector which creates a 20:1 ratio of water to injected concentrate. Place a bucket under the siphon filled with concentrate and it will automatically be drawn into the hose when you are watering. You have to figure out the ratio of the acid you are using to the water to get the pH to the point where you want it. Take caution with these types of injectors as you will need to be sure the hose is drawing quickly enough to work properly, and be aware that changing the rate of flow can also affect the ratio of injection. Perform some tests to familiarize yourself with the results before using this method.
3. For larger numbers of plants and more precision, you can install a Dosatron, or similar precision dosing mechanism. These are more expensive, and will potentially require some plumbing (there are free-standing carts also) but once set-up they provide precision injection control. You can use this not only for pH adjustment, but also for injecting liquid fertilizer, or other treatments (they are used in places like car-washes for soap, farms for medication for cattle etc.). You can use different types of acids; the common type from Hydroponics supply are normally Phosphoric acid, which breaks down to provide some nutrient value. You can also use Muriatic acid (aka Hydrochloric acid or HCl) which has no nutritional value to plants. (Take caution when handling any acid! Wear gloves, and follow all other recommended chemical safety procedures.)

Alkalinity

Alkalinity and pH are related, and sometimes confused; when people say their water "is alkaline" they often mean it has a high pH. You can have a high alkalinity but still have a pH reading near 7.0. Alkalinity is the measure of the resistance of the water to change pH, aka the buffering capacity. The more alkaline your water, the more acid you will have to add to lower the pH and the higher your "TDS" readings. "What's TDS?" you say?

The Deal with TDS

TDS stands for "Total Dissolved Solids". It's an all encompassing term; suffice to say for bonsai trees and this chemistry lesson, that it is a measure of the concentration of electrically-conductive ions that are present in our water. If you take pure water, with no salts of any kind, the TDS is 0; you will get no electrical conduction (and you have no alkalinity). The salt you use on your food is one of many types of salts which are "ionic" compounds; the salt you eat is mostly sodium chloride, but many ionic compounds, salts, readily dissolve in water to form ions, which conduct electricity through water. TDS is most easily measured using a simple EC meter; the resulting number will typically be somewhere in the 20-300 ppm range. This number is a combination of the measure of concentrations of Sodium, Chlorine, Magnesium, Calcium, Carbonates and other ions in the water in "parts per million."

Diagnose and Filter or Supplement

The TDS numbers above contain too little information to really tell us what a problem is when we start to see symptoms in sensitive plants, a good "canary-in-a-coal-mine" is a Japanese maple bonsai tree. If you are not getting the growth you think you should, then you should think about the various water quality issues that are potentially causing problems.

Higher TDS are generally, but not always, bad news. The most common problems indicated by high TDS are higher Calcium, Magnesium, and Carbonate levels, which are annoying but not overly problematic. Do you see white crusty scale on your plants and pots? If so, you likely have excess calcium and carbonate ions in your water. High calcium concentrations can reduce magnesium uptake in plants, as they are essentially in competition for absorption. 

Higher concentrations of Sodium (more than 25ppm) are a BIG problem for most plants. If your water exceeds 25ppm you should consider mitigating with Reverse Osmosis filtration and then adding back a nutrient solution. Reverse Osmosis can seem complicated, but after the initial expense and setup, the water will promote much better growth in most plants as long as you add back a baseline of nutrient using something like DynaGro "Grow". Don't fool yourself into thinking you can skip this; plants will begin to exhibit nutrient deficiency symptoms relatively quickly if you use pure R.O. water.

Sodium is also the reason you should never use water that has been treated with a standard household water softener. If you live in an apartment building, you should be absolutely sure there is no water softener pumping sodium into your water prior to using it to water your plants. If there is, and there is no other way around it (e.g. get the water from another source before it is treated), you can ask your landlord to switch from Sodium Chloride to Potassium Chloride, since Potassium (the K in N-P-K) is a plant nutrient and Sodium is basically poison. Water softeners replace the calcium ions with Sodium (or Potassium) in the water, to prevent scale in plumbing and minerals on glassware, but softeners do not reduce the TDS.

Do your plants have foliage that's too yellow or showing the veins of the leaves? Micronutrients, e.g. Iron, Manganese, Copper, Zinc, and Boron are all required by plants in varying amounts. Your water may not provide the amount needed, either because it's not there in high enough concentration or because other minerals are in higher concentrations and preventing efficient uptake. Adding fertilizer will normally correct nutrient deficiencies that are mild, but if you use rainwater or other water that is very low in minerals, you may have to supplement with something like DynaGro Grow as a baseline nutrient solution. 

Chloramine is a commonly used disinfectant for drinking water, it is much more stable than Chlorine in the water. There is no benefit to plant growth to have this in water and empirically, it appears to kill moss and other organisms that could potentially be beneficial to the microbial activity in the soil. Treated municipal water may be improved by removing this additive using a "KDF" filter, which are available as a hose-end installation for relatively little cost. However, if you are injecting additives into your water, its best to filter it first, and then add the nutrients or acid. 

Fluoride, which is added to drinking water to improve dental health, is also easily removed, but requires a different filter- activated carbon. Fluoride toxicity in plants can happen at very low thresholds; 0.75ppm is the maximum recommended concentration, which drinking water may exceed by as much as 30%.

For additional reading, see this article from the University of Massachusetts.