Alkaline soil, also referred to as “sweet soil” by some gardeners, is identified as having a potential of hydrogen (pH) level above 7.0. A pH of 7.0 is considered neutral, and anything above is considered alkaline, or basic. Soil that is alkaline usually contains large amounts of calcium, sodium, and magnesium, and it 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 soil that is naturally alkaline. Watering soil with hard water that contains large amounts of lime will increase its alkalinity.
Adding sulfur can make soil more acidic. Approximately 1 to 3 ounces (30 to 85 grams) of ground rock sulfur can be added per 1 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 the soil's 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. There are many plants that can be grown in soil that's somewhat alkaline, and a garden of such plants isn’t unrealistic. Some plants that do well in this soil include oregano, oleander, cucumber, celery, mullein, and eucalyptus. Aloe, gardenia, lavender, chrysanthemum, iris, and zinnia also do well.
Testing for soil pH 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. Gardeners who suspect that under-performing plants are a result of the soil can test the soil for alkalinity and then work to amend the problem.