Soil pH measures soil acidity, which affects soil factors important for plant health such as bacteria populations, nutrient availability, toxin levels and soil structure. Soil pH is measured on a 14-point scale from 1.0, or extremely acidic, to 14.0, or extremely alkaline. A pH of 7 is the neutral point where soil is neither acidic nor alkaline. Most garden plants prefer pH between 5.5 and 7.0. Soil pH much outside this range adversely affects plants.
Strongly acidic soil reduces the availability of nitrogen, phosphorous, potassium, sulfur, calcium and magnesium. Strongly alkaline soil reduces available iron, manganese, boron, copper and zinc. Soil pH also affects the health of soil bacteria that break down organic matter into nutrients that plant roots can absorb. These organisms thrive in soil pH of 6 to 8. They go dormant or die off when pH falls below 6 or rises above 8.
Inexpensive pH testing kits are available at home, hardware and nursery centers. A typical kit's instructions tell you to collect a soil sample from the bottom of a 4- to 6-inch-deep hole in your garden. Mix a dried-out measured soil sample with the kit's premeasured testing reagent and distilled water in the kit's testing vial. Shake well, allow the soil to settle and read the pH level by comparing the water's color to the kit's indicator chart.
When soil pH tests as too low for the plants you are growing, the soil is said to be acidic or "sour." An amendment such as ground limestone must be spread out evenly and worked into the soil to neutralize the excess acidity. For instance, raising the pH level by one point requires spreading 4.5 pounds of ground limestone per 100 square feet in loam soil. Apply lime in the fall or late winter.
Soil is said to be "sweet" when its pH tests as too alkaline. An acidifying amendment such as ground elemental sulfur or aluminum sulfate should be worked into the soil in the fall to counteract the excess alkalinity. For example, lowering the pH level by one point will take 2 pounds of elemental sulfur or 12 pounds of aluminum sulfate per 100 square feet in loam soil, But too much elemental sulfur at once can burn plant roots, so apply a first application of 1 pound of sulfur per 100 square feet. Wait eight weeks and apply a second sulfur application at rate of the 1 pound per 100 square feet.