Buffering Potting Soil pH

Understanding Buffer pH Values

• The active pH value of a soil sample is gauged by the use of litmus paper, indicator solutions or a pH electrode, according to the Clemson University Cooperative Extension Service Web page, "What is the Buffer pH Value on the Soil Test Report?" Although these tests provide information on the pH of soil at a given moment, they do not predict the sustainability of the pH level. A special test using a weak base solution is required to determine the buffer pH. An alkaline solution of 8.0 is added to a soil sample, and then the solution is tested for its pH level. The pH level after the addition of the base solution is called the soil's buffer pH, representing the number of points pH decreases from 8.0.

Soil Characteristics and Buffer pH Levels

• Organic soil and soil high in clay particles tend to have low buffer pH levels -- with higher levels being close to 8.0. Sandy soil or potting mixes containing non-organic materials tend to maintain a steady pH level more easily than organic or clay soils. As an example, the buffer value of sandy soil may be around 7.80, decreasing the base solution pH .20 points from its starting pH of 8.0. But clay soil may change the base solution to 7.30, decreasing the pH by .70 points due to leaching of acidifying chemicals out of the clay particles.

How to Buffer Soil pH

• Most plants grow best in a slightly acidic to neutral soil with a pH value between 6.0 and 7.0, according to Spectrum Analytic Inc., a soil testing company. Potting mixes high in sphagnum peat moss generally have a low buffer pH score because peat moss increases soil acidity as it breaks down. The addition of dolomite -- lime with magnesium, lime -- calcium carbonate, or calcium hydroxide -- hydrated lime -- raises the active soil pH and the buffer pH. Add 8 pounds of dolomitic limestone per cubic yard of potting mix to raise the active pH level one point. Coarser forms of lime increase your soil's buffer pH more than fine grades of lime because they break down more slowly than fine grades.

Quick Aid for Soil with Low Buffer pH Values

• Hydrated lime is a fine grade of lime that you can mix with water to moisten potting soil, especially mixes that contain acidifying peat moss. Add one teaspoon of hydrated lime to one gallon of water. Apply just enough water to moisten your potting mix, which should raise the pH by slightly more than one-half point, according to Texas A&M AgriLife Extension. Fast-acting lime increases the active pH, but does not provide the residual effect of coarser lime products to offset your soil's buffer pH. Check your soil pH in two weeks, and reapply the lime water solution if needed.