Alkaline soil is soil with a pH higher than 7.0. It is also sometimes called sweet or basic soil. (Alkali soil is soil with high levels of sodium, and is quite different from alkaline soil.)
Soil is made alkaline by the presence of calcium, magnesium, or sodium. All are present in most soils, but exist in higher amounts under certain conditions. Sodium salts are highly soluble, so are present only in very small amounts in any soil that is well-drained and gets appreciable amounts of rainfall. For more information about sodium in soil, see Sodic Soils.
Calcium, or calcium and magnesium together, are only slightly soluble, and are the causes of most soil alkalinity. Alkaline soils are common in areas where the base rock is limestone (Calcium carbonate), or in arid regions, where calcium is not washed out of the soil. Regions that receive less than 20 inches of rain a year usually have alkaline soil.
Alkalinity can also come from irrigation water. Hard water with a high lime content will make soil increasingly alkaline. In regions that have dry periods with seasonally heavy rainfall, as occurs in much of California, alkalinity from irrigation water builds up until the rainy season, then is reduced by heavy rainfall over the winter.
Sometimes alkalinity is very local, due to a local situation. One common cause of local alkalinity is new cement work. Portland cement is lime-based, and lime leaches out of new cement for a while, making the soil close to it alkaline. Anything that spills lime on the soil can cause local alkalinity, such as washing cement tools or an accidental spill of agricultural lime.
Alkalinity is simple to test for. Soil test kits always include a pH test, and the pH is always reported in commercial soil tests. It’s a good idea to check your pH every few years to be sure your gardening practices or irrigation water aren’t changing it.
Calcium carbonate has a pH of 8.2. If the pH of your soil is 8.2, free limestone is probably present, and efforts to lower the pH will be fruitless. The free limestone acts as a buffer to absorb very large amounts of the sulfur uses to lower pH. The most that can usually be achieved is a short-lived lowering that will revert to 8.2 in a few months.
If your pH is lower than 8.2, it can probably be brought down to levels easier to manage. If it is higher than 8.2, the pH is probably due to the presence of sodium, which must be dealt with as a separate problem.
Plant Growth in Alkaline Soil
Although most plants grow well in slightly alkaline soils, with a pH level slightly above the neutral point of 7.0, only a few will thrive in a soil that is very alkaline, with a pH of 7.5 or above. At higher pH levels (8.0 or more), the plant nutrients iron and manganese become unavailable to plants because they are transformed into insoluble forms.
Plants adapted to very acid soils, where iron is plentifully available, suffer in soil that is even slightly alkaline. The most common garden examples of acid-loving plants are azaleas, rhododendrons, and blueberries, all of which require a soil below 7.0 and grow best with a pH under 6.0.
On the other hand, plants adapted to arid regions thrive on alkaline soils. Most of the plants native to the American Southwest fall into this category.
Plants growing in soil that is too alkaline develop symptoms that indicate iron deficiencies: new leaves become chlorotic, with the veins staying green but the tissue between the veins turn yellow because the plant is unable to get iron.
This symptom can be told from the common nitrogen-deficiency chlorosis from the fact that the newest leaves are most affected, and the chlorosis is interveinal—the leaf tissue between the veins looses color, becoming pale green or yellow, while the veins remain dark green. Nitrogen-deficiency chlorosis affects the oldest leaves first, and the leaves turn evenly pale or yellow.
Iron is probably present in the soil, but in an unavailable form. Changing the soil pH makes it once again available to plants.