The degree of acidity or alkalinity of a soil, usually expressed as a pH value. Descriptive terms commonly associated with certain ranges in pH are: extremely acid, <4. 5; very strongly acid, 4. 5-5. 0; strongly acid, 5. 1-5. 5; moderately acid, 5. 6-6. 0; slightly acid, 6. 1-6. 5; neutral, 6. 6-7. 3; slightly alkaline, 7. 4-7. 8; moderately alkaline, 7. 9-8. 4; strongly alkaline, 8. 5-9. 0; and very strongly alkaline, >9. 1.