pH: Symbol to express the level of acidity, on a scale of 0 to 15, in which 7 represents neutrality (equilibrium between acid and alkaline). Values higher than 7 represent increasing alkalinity, and below increasing acidity. To have an idea, vinegar has a pH around 3 and sodium bicarbonate a pH around 8. Ornamental plants usually require a pH between 6 and 7.5 (each unit of Ph represents an acidity or alkalinity 10 times greater: a soil with pH 5 is ten times more acid that a soil with pH 6; a soil with pH 8 is 10 times more alkaline that a soil with pH 7).