PH is the relative acidity or alkalinity of the soil. It is measured on a scale of 0 to 14. 0 is the most acid and 14 the strongest alkaline. 7 is neutral (neither acid nor akaline). Most plants grow best in a slightly acid pH range of 6 to 7. A few prefer a pH in the more acid 5 to 6 range. In high rainfall climates such as the Pacific Northwest, the natural pH is usually more acid than 6. In low rainfall climates the natural pH is alkaline, ranging from 7 to 9.
If pH is higher than 7.5 or lower than 5.5, most plants will not grow as well. Some sensitive plants will develop chlorosis or yellowing of new growth due to a lack of certain nutrient elements. Iron, and to a lesser extent, zinc and manganese become insoluble at low pH. Young growth becomes light green and eventually turns yellow with the area around the leaf veins remaining green.
Regular addition of organic amendments tends to lower the pH. With acid soils, the addition of organic matter may require the application of additional lime, wood ashes, etc.
Commercial fertilizers which contain sulfur also make soil more acid.
Table 3. Limestone Amounts Needed to Adjust Soil pH
Pounds of Limestone per 100 sq. ft.*
Change in pH Desired Sandy Soil Loamy Soil Clay Soil
From 6.2 to 6.5 5 10 10
From 5.9 to 6.5 12 20 20
From 5.5 to 6.5 32 36 40
*A similar amount of wood ashes or oyster shells may be substituted for ground limestone
Areas with heavier rainfall such as the Pacific Northwest have a low pH or acid soil. Forested areas may also have acid soil. These areas should add at least 5 pounds of ground limestone per 100 square feet to raise the pH. Limestone contains calcium and sometimes magnesium. Limestone should be applied before planting or while plants are dormant. Lime and sulfur are available from nurseries, garden stores and agricultural fertilizer dealers.