As I learned from my daughter's grade 8 science class notes pH stands for potential of Hydrogen, and it's determined by measuring the number of hydrogen ions. Water has a neutral pH with a balanced number of hydronium and hydroxide ions. This pH is measured as 7. Anything less is acidic and anything above is basic (or alkaline). Lemon juice is 2.2 pH, so it is highly acidic. The best range for plants is between 5.5 to 7.5 (so acidic is better than alkaline), although some plants have evolved to survive in soils outside of that range (like Arabian coffee and highbush blueberry).