The pH scale is used to describe how basic (alkaline) or acidic a solution is. Solutions with a pH below 7 are acidic. Solutions with a pH above 7 are basic, and solutions with a pH of 7 are neutral.
A water molecule has 2 hydrogen atoms and 1 oxygen atom (H2O). Water molecules are considered to have a neutral pH of 7.
If a water molecule (H2O) loses one of its hydrogen (H) atoms for some reason, it becomes hydroxide (OH). Since it is the final ratio that matters, another way to get the same effect is to use a reaction that creates excess hydroxide. If lye (NaOH) is combined with water (H2O) it will break into sodium (Na), and hydroxide (OH). These additional hydroxide ions make the solution more alkaline. The greater the concentration of hydroxide in the solution, the more basic it is, and the higher it will be on the pH scale.
On the other hand, if a water molecule (H2O) gains a hydrogen (H) atom for some reason, it becomes hydronium (H3O), the solution becomes more acidic, and the pH will lower. If phosphoric acid (H3PO4) is combined with water (H2O) it donates a hydrogen atom to the water creating hydronium ions (H3O) and dihydrogen phosphate (H2PO4). These additional hydronium ions make the solution more acidic. The greater the concentration of hydronium in the solution, the more acidic it is, and the lower it will be on the pH scale.
Acids and bases counteract each other because hydronium is a water molecule with an extra hydrogen, and hydroxide is a water molecule missing a hydrogen. When the two meet they recombine to become two normal water molecules (or at least they cancel each other out as far as the ratio is concerned).
The numbers on the pH scale are logarithmic and increment by a factor of ten. A simple way to get a feel for how this changes the scale imagine if there was a one followed by the number listed on the scale. For example, if a pH of 5 is worth 100,000 then a pH of 6 is worth 1,000,000, or one extra zero (multiply by ten) worth. Compared to a linear scale, numbers that are similar tend to be more similar than they appear, and numbers that are further apart are even further than they appear.
Litmus paper, pH reactive fluids. and pH meters are all different ways to check the pH of a solution. Checking the pH of a nutrient solution is generally one of the last steps before applying.
Understanding what causes pH can help a gardener select an appropriate treatment for nutrient solution that is either too acidic or too basic. By adding the opposite (for example, an acid to lower a pH that is too high or vice versa) the pH solution of a nutrient solution can be adjusted to be within range for proper nutrient uptake and plant development.