In my humble opinion, pH is the one very basic tool you need to be successful in hydroculture. But, you ask, “How can a physics 'concept' be a tool? I sort of remember what it is from my school days, but why should it affect my plants?”
It's the most important tool in your hydroponics “toolkit.” Using pH as a tool, aside from the test kit or meter itself, is free! You don't really have to know exactly how it works, just that it can directly influence your plant's health.
Most people use salts-based nutrients in hydroculture. A smaller group use organic nutrients that are fully derived from natural products. Either way, those nutrients are added to a body of water that comes into intimate contact with, and DIRECTLY feeds, the roots. In soil growing, the roots are more dependent on a less-direct nutrient delivery which includes microorganisms. I'm not going to go into that area. There are many books and websites that explain that process better than me. Using pH is important to soil-based growing but in a somewhat different way.
The balance of hydrogen and hydroxyl ions in a solution directly affect the plant by either promoting or limiting nutrient and water uptake in the roots. Hydrogen ions are positively charged and hydroxyl ions are negatively charged. Excessively low or high pH can also throw a pipe wrench into the “gears” of oxygen and carbon exchange. Like most living things, plants need a certain amount of “sugar,” or carbohydrates, to live. Most plants use photosynthesis to convert light, water (consisting of hydrogen and oxygen), carbon (organic materials and carbon dioxide), and nutrients (macro and micro) to a usable food in the form of sucrose (table sugar). For higher-level life, it would be glucose, a longer chain of carbohydrate. By the way, pH is also absolutely critical to ALL life! I'm diabetic; large swings in glucose (blood sugar) and pH can actually kill me!
We've learned that pH simply means “potential of Hydrogen and hydroxyl ions” in water. The water has to have “solutes,” or dissolved particles in it to make it a solution. All this means is that you will have a dozen or so different “salts” of various elements. A few examples of salts in a nutrient solution might be potassium nitrate, calcium nitrate, and magnesium sulfate. Most solutions also contain copper, manganese, boron, iron, calcium, chlorine, and many other micronutrients. A nutrient solution is full of'-ines,' '-ites,' '-ates,' and '-ides!' Does that sound unhealthy or unnatural? Your body -and seawater- is full of almost the same elements and in almost the same ratios. I use a salts-based nutrient solution, and I trust it like I trust my own body! A recent blog explores salts-based and organic nutrients.
If you want a more in-depth or scientific explanation of pH, there are many websites that can do a better job than me. You just need to know that the pH scale runs from highly acidic (1) to highly alkaline (14), sometimes also called “base.” Distilled water runs at 7 (neutral) on the scale. Here is something to keep in mind though: A change of one point down or up on the scale requires ten times the amount of hydrogen or hydroxyl ions! As an example, to get your solution from an unhealthy acidic 4.5 to a better-tolerated 6.5, it is a hundred-fold (10 x 10) difference! I can tell you that it takes gobs of 'pH up' solution to make that 2-point change. And that, my friend, is the secret to using pH. Make your changes S-L-O-W-L-Y and check often with a cheap test kit or more expensive meter. What method you use doesn't matter; just do it!
Why is an optimal pH critical? All plants have a certain pH “sweet spot” where they grow best. It is at that spot where nutrient uptake is unimpeded, and the plant is hydrating and transpiring properly. Most plants will grow well in the 5.5 to 6.5 pH range, but getting the plant as close to the “sweet spot” as possible is best. The chlorophyll cycle then runs like clockwork, sugar is being made, and the plant is eating and growing.
The information is free and widely available. Don't guess! Use it. You can live without an expensive “EC” or “ppm” meter. You can buy a little ten-dollar chemical pH test kit from many outlets near you, or on the internet, and do hundreds of pH measurements. You can spend a little more and get an electronic pH meter. In my humble opinion, the cheapest tool in your hydroculture toolkit!