Sep 03, 2004· Acid or alkaline soil: Modifying pH. Amend your soil to suit the kind of plants you want to grow. Sunset – September 3, 2004 The pH scale indicates acidity or alkalinity. A soil with a pH number below 7 is acid, while one with a pH above 7 is alkaline. Garden plants typically grow best in neutral or slightly acid soil (pH 7 or slightly below ...