The term pH refers to the alkalinity or acidity of a growing medium water solution. This solution consists of mineral elements dissolved in ionic form in water. The reaction of this solution whether it is acid, neutral or alkaline will have a marked effect on the availability of mineral elements to plant roots.
When there is a greater amount of hydrogen (H+) ions the solution will be acidic (pH<7.0). If there are more hydroxyl (-OH) ions, the solution will be alkaline (pH>7.0). A balance of hydrogen and hydroxyl ions results in a pH neutral soil (pH=7.0). The optimum pH range for most crops is 5.5 to 6.2 or slightly acidic. This creates the greatest average level for availability of all essential plant nutrients. Different mineral elements are available at diffferent pH levels. Extreme variations in pH can cause mineral deficiencies or toxicity by binding up or releasing large amounts of various elements.