ARTICLE - What is Alkaline Soil

Alkaline soil, also referred to as "sweet soil" by some gardeners, is identified as having a potential of hydrogen (pH) level above 7.0. The pH scale begins at 0 and ends at 14. A pH of 7.0 is considered neutral, and anything above is considered alkaline, or basic. Alkaline soil usually contains large amounts of calcium, sodium, and magnesium, and is less soluble than acidic soil.

Plant growth is often reduced in alkaline soil, because water and essential nutrients aren't able to easily penetrate the soil. This can lead to nutrient deficiency and stunted growth in plants. Soil that is only slightly alkaline may be beneficial to some plants, such as lilacs and asparagus, but an overabundance of alkalinity is almost always detrimental to growth. In highly alkaline soils, iron, manganese, and phosphorus are not available in quantities high enough to sustain growth in most plants.

Alkaline soil commonly occurs in arid or desert regions with light rainfall, while areas that are heavily forested and have high rainfall tend to be more acidic. Regions that receive less than 20 inches (50 centimeters) of rainfall per year are most likely to have natural alkaline soil. Watering soil with hard water that contains large amounts of lime will increase the alkalinity of soil.

Alkaline soil may be made more acidic by adding sulfur. Approximately 1-3 ounces (30-85 grams) of ground rock sulfur can be added per one square yard, or one square meter, of soil to raise the pH by 1.0 point. Less sulfur should be used for sandy soil, and more for clay soil, and it should be mixed in thoroughly before any planting is done. Adding organic matter such as sawdust, peat moss, compost, and wood chips will also lower soil pH.

If lowering the pH of alkaline soil is not practical, or isn't working, raised beds may be constructed, or topsoil may be used. However, there are many alkaline soil plants that can be grown, and an alkaline soil garden isn't unrealistic. Some plants that like alkaline soil include oregano, oleander, geranium, cucumber, celery, cauliflower, cabbage, soapwort, hens and chicks, mullein, eucalyptus, aloe, gardenia, lavender, clematis, calendula, chrysanthemum, iris, thyme, periwinkle, and zinnia.

Testing for alkaline soil can be performed relatively inexpensively by a commercial soil testing laboratory. The county extension office in your area can provide information on gathering soil samples and submitting them for analysis. Home soil testing kits are also available from hardware stores and home and garden shops. If you suspect that under-performing plants are a result of alkaline soil, test the soil for alkalinity and then work to amend the problem.