Alkalinity of Soil

Soil Alkalinity and Plant Nutrients

• The pH scale is zero to 14, with 7.0 being neutral. Measurements that are numerically higher represent alkaline soil, and lower numbers represent acidic soil. One unit on the pH scale represents a 10-fold change -- meaning that soil with a pH measurement of 8.5 is 10 times more alkaline than soil with a pH of 7.5. For good growth and development most garden and landscape plants need a soil pH of 6.0 to 7.5. Most nutrients -- and especially micronutrients such as iron -- aren't available when soils are too acidic or too alkaline.

Lowering the PH of Alkaline Soils

• Elemental sulfur, aluminum sulfate, iron sulfate and acidifying nitrogen can be added to soil to lower pH. In small beds and garden areas, you can alternatively add sphagnum moss -- the pH of Canadian sphagnum peat moss is typically between 3.0 and 4.5 -- and use acidic organic mulch. Iowa State University Extension says incorporating elemental or granular sulfur into soil is the safest and least expensive approach, but also the slowest. Soil bacteria changes sulfur into sulfuric acid, neutralizing soil alkalinity. But this effect is temporary; pH will start rising once soil bacteria consume the added sulfur. Adding too much sulfur or using it too frequently can damage plants. Aluminum sulfate and iron sulfate are faster acting, but you need to use five or six times more. Again, too much sulfate can injure plants.

Calcium Carbonate Soils

• The University of Utah notes that soils that are high in calcium carbonate -- such as those in western Colorado and many other areas in the West -- are buffered with free lime due to underlying limestone. Overall soil pH of these soils can't be lowered until after the alkalinity of free lime is neutralized. According to Colorado State University Extension, neutralizing soils containing 5 percent calcium carbonate would take 30 tons of elemental sulfur or 100 tons of sulfuric acid per acre -- an unreasonable cost and physical effort.

Alkaline-Tolerant Plants

• Growing alkaline-tolerant plants is more practical and cost effective than temporarily lowering the pH of alkaline soils. Trees that grow well in alkaline soil include Austrian pine, bur oak, chokecherry, green ash, hackberry, honey locust, ironwood and silver maple. Among alkaline-tolerant shrubs are buckeye, cotoneaster, both "hills of snow" and panicle hydrangeas, lilac, smokebush and many viburnum varieties. Perennials that tolerate alkaline soils include coneflower, coral bells, colorful salvias, daylilies, hostas, garden phlox, sweet William, yarrow and both clematis and honeysuckle vines. Most ornamental grasses also tolerate alkaline soils, as do some herbs including borage, chamomile, thyme, parsley and wormwood. Beans, peppers, spinach, carrots and lettuce are vegetables that will grow well in high-pH soils.