The term pondus hydrogenii (weight of hydrogen) is better known by the abbreviation pH. pH is basically (pun intended) a measure of hydronium (H3O) concentration (formed by hydrogen ions and water) in relation to hydroxide (HO) in aqueous (H2O) solutions such as nutrient solutions. It is a logarithmic scale from 0 (most acidic) to 14 (most basic) with 7 being neutral. If there is more hydronium, it is an acid; if there is more hydroxide, it is a base and if they are equal, they form a neutral substance.
The pH scale
The pH scale is logarithmic according to the distance from neutral (7). A pH of 5 is 10 times as more acidic than a pH of 6, and a pH of 4 is 100 times more acidic. On the basic side, a pH of 9 is 10 times more basic than a pH of 8, and a pH of 10 is 100 times more basic. Liquids with a pH value below 7 have "spare" hydronium, are acidic and have a sour taste.
Vinegar and lemon juice are common acids. Water, plus an additional hydrogen ion, forms a hydronium ion: (H2O+H=H3O). A low pH value indicates that there is a relatively high number of hydronium ions present, and the substance will donate protons. Here are a couple of examples:
Water (H2O) + hydrochloric acid (HCl) rearranges to become a hydronium ion (H2O) and chloride (Cl).
Water (H2O) + phosphoric acid (H3PO4) rearranges to become a hydronium ion (H3O) and dihydrogen phosphate ion (H2PO4)
Strong acids (or weak acids in concentration) tend to be corrosive and can cause damage to living tissue or other materials. The lower end of the acceptable pH range for plants tends to favor micronutrient availability, but plants in mineral soils with a pH less than 5.5, or in soilless media with a pH less than 5.0, may experience problems from calcium and magnesium washing away, aluminium and manganese becoming too soluble and overwhelming the plant.
A pH of 7 is the neutral center point, being neither acidic or basic. Hydronium and hydroxide in equal parts will cancel out the effects of the other (effectively forming H2O). This is how a strong base can be canceled out by a strong acid and vice versa. This is also why pH up products are bases (to offset the acid, thereby raising the pH), and pH down products are acids (to add acid, lowering the pH).
Plain water (H2O) has a pH of around 7. While even at neutral, water can be used as a solvent or as a base in some instances, and plain water tends to be less damaging than a strong acid or base. The H2O can rearrange as H3O and HO, but as long as they are in equal amounts, they will neutralize each other. Many beneficial micro-organisms prefer a neutral or near neutral pH.
Liquids with pH values above 7 such as a lye solution are basic (also known as alkaline). A high pH value indicates that there is more hydroxide present, and the substance will accept available protons. Strong bases (or weak bases in concentration) tend to be caustic, which, like the corrosive property of acids, can cause damage to living tissue or other materials. The higher end of the acceptable pH range for plants tends to favor macronutrient availability in mineral soils, but soilless media with a pH over 7 may experience problems from manganese or phosphorous deficiencies.
Bases raise the amount of hydroxide. Potassium hydroxide (KOH), and sodium hydroxide (NaOH) both increase the amount of available hydroxide in a solution. For example, KOH separates to become potassium and hydroxide (K and OH). Furthermore, lime is often used as an amendment to raise soil pH.
pH tester drops and pH meters are two common ways to test pH, but it is also possible to test for pH using anthocyanins extracted from red cabbage leaves (I haven't tested it with blueberries or beets yet, but they are also high in anthocyanins). Take a half of a head of red cabbage and shred it (you can use a whole head, but a half makes plenty). Add to a small pot with one cup of water (preferably distilled). Bring to a boil and then simmer for 20 minutes or until the liquid in the pot is a deep purple color. Strain out the cabbage and retain the fluid. This homemade fluid can be used to test pH.
In a clear or white cup, put in enough of the liquid to cover the bottom of the cup, and add the substance to be tested. Stir, and dilute with water as needed to see the color clearly. The amount of fluid used won't affect the shade of the color, only the intensity. The liquid will change color according to pH.
In order from lowest pH (most acidic) to highest (most basic) the colors are: red, pink, blue, the original shade of purple, green, yellow and clear. Neutral (pH 7) is the purple starting color, and a pH of 6 is that same purple with a pinkish tone. The testing fluid can be applied to strips of absorbent paper and allowed to dry to create homemade testing strips (although the reaction works best if used within the first couple of weeks).
Some common household items that can be used as testing fluids are lye, bleach, baking soda, lemon juice and vinegar. Be careful with lye and bleach as both are strong bases and can be dangerous if mishandled, and never mix bleach with an acid or it will release its chlorine as gas (not safe to breathe at all).
There are other things that can also be used in a similar fashion, although often with different colors indicating the pH values. Litmus is made from lichen, and turns red in low pH, and blue in high pH. This fluid can be applied to strips of paper to become litmus paper, commonly used in testing pH. Many pH test kits use this same principle, using a chemical that changes color depending on pH.
The substance to be tested (such as a nutrient solution sample) is put in a vial, and a few drops of the test fluid is added. The vial is shaken, and the color compared to a chart. Once the closest color is located, the corresponding pH value is read. The ideal pH value for mineral soils is between 6.5 and 7.5, and for hydroponics and other soilless media it is between 5.0 and 6.0.
Electronic pH meters for liquids use two electrodes. One is used as a reference, and the other is influenced by pH. The second electrode is covered in a special glass that allows only hydrogen ions to pass, changing the electrical charge that is compared and converted into a measure of pH. This probe should be kept moist at all times, even when not in use.
pH tips and tricks
Base cations such as calcium (Ca), magnesium (Mg), and potassium (K) can raise pH, so will an abundance of sodium (Na). As these elements are used by the plant (or washed out of the area) the pH of the media may drift down and become more acidic if not replaced. Over-application of any of these elements can cause pH values to drift towards basic.
Buffers are materials in the solution that help it resist change. They help occupy free hydronium and hydroxide to help stabilize the solution. Reverse osmosis (RO) water for example, has very little buffering capacity, which means only a small amount of an acid or a base can have a large impact on the pH of the solution. Having buffers in a nutrient solution helps minimize fluctuations. Organic matter such as compost tends to have a buffering effect.
In hydroponic gardening, pH is a much more important to test often than with soil or soilless gardening. The mineral properties of soil, and the organic material in soilless, tend to help buffer fluctuations in pH, but in hydroponic systems, particularly when using RO water, pH can fluctuate very quickly with the addition of nutrients or additives.
Now that you know a lot more about pH and how it works, you're well on your way to achieving great growth results.