Berks County Cooperative Extension -

SOIL PH

Color spectrum

0 - Acid****7 Neutral**** Alkaline - 14

What is pH?

Often referred to as "master of soil", pH (Are you ready?) is a measure of the hydrogen (acid-forming) ion activity of soil. With the precise definition out of the way, let's consider pH in less technical terms. The pH scale measures soil acidity or alkalinity from 0 (extremely acid) to 14 (extremely alkaline) with a reading of 7 being neutral. Values below 7 fall within the acid range; values above 7 are within the alkaline range. The pH scale is logarithmic, not linear so that a pH reading of 4.5 is ten times more acid than a reading of 5.5.

Importance to the home gardener.

Extremes of pH affect plant growth by making essential soil nutrients more or less available to the plant. For optimum growth, all essential nutrients should be available for uptake by a plant and none should be concentrated in toxic levels. Soil with a pH of from 6.0 to 7.0 (slightly acid) is ideal for most plants. Some plants, such as rhododendron, require a more acidic soil. It is not possible to guess soil pH. A soil test should be conducted. After the pH level has been determined, adjustments, as recommended in the soil test report, should be made. To reduce soil acidity, agricultural limestone is applied. Application of elemental sulfur or aluminum sulfate will reduce soil alkalinity.

Wood and other ashes.

Many home gardeners in Berks County spread wood ashes into their plots. Wood ashes raise soil pH. Twice as much wood ashes as limestone must be applied to attain the same effect. The ashes should be applied as a thin layer in winter before the ground freezes and incorporated into the soil. Some caution should be exercised in applying wood ashes. To avoid damaging germinating seedlings or plant roots, keep ashes away from them. Soil pH should be checked annually if wood ashes are applied to the home garden. Coal ashes and large amounts of wood ashes (more than 20 pounds per 1,000 square feet) should never be applied to the home garden, as toxicity problems may result.

Source: The Penn State Master Gardener Manual
Document created: 15 December 1999; Revised: 3 March 2001