Learning About Soil Types The size and proportion of clay, sand, or silt particles present in your garden soil influence its chemical and physical nature. They make it either heavy (wet and poorly drained), or light (dry and free-draining), and thus determine what plants will thrive on it. It's pH value, a measure of acidity or alkalinity is measured on a scale of 1 to 14. Below neutral (7), soils are progressively acid; above neutral, they are progressively alkaline (limy).