How Alkaline Level Affects Plant Growth

Soil pH Levels

Soil can be either alkaline or acid, or a convenient balance between the two. The term pH refers to a 0-14 point scale that measures whether soil is alkaline or acid (with 0 equalling pure acid soil and 14, pure alkaline). Some plants thrive in alkaline soils, some do not. Placing acid-loving plants in alkaline soil poses a problem and creates the need to adjust the pH levels of the soil. While a plant that endures these circumstances may still grow, it probably won't flourish and will put needless stress on the plant. Most plants typically prefer slightly acidic soil conditions, so checking the pH levels of soil is a necessary step in gardening. Soil experts and do-it-yourself pH testing kits are available for testing pH levels. The alkalinity or acidity of soil does not correspond to how fertile the soil is, but it can interfere with a plant's ability to process soil nutrients and the release of nutrients by bacteria from decaying organic matter.

Alkaline Soil and Alkaline-Loving Plants

Areas with light rainfall or that experience periods of drought often have alkaline soils. Alkaline soil is usually lacking in iron, phosphorus and manganese, but contains more lime. Because plants can only take their nourishment in liquid form, the alkalinity or acidity of the soil has a lot to do with how elements like potassium, nitrogen, phosphorus and iron "liquefy" so plants can absorb them. Plants such as cabbage, celery, geraniums, dusty miller and thyme prefer alkaline soils. Cabbage does better in alkaline soil because the alkalinity inhibits the growth of clubroot fungus that thrives in acid soil and can attack the plant. To neutralize alkalinity for acid-loving plants, add peat moss, sulphur or pine needles to the soil.

"Sweetening" Acidic Soils

Gardeners sometimes refer to alkaline soil as "sweet" and acidic soil as "sour." To sweeten highly acidic soils and to bring them to a more neutral pH, ground limestone is generally added. Applications of wood ash will also sweeten sour soil and raise pH levels. Rhododendrons and azaleas will only thrive in acidic soils; if the soil is too alkaline, these plants may exhibit a loss of green color due to a lack of iron. Decaying organic matter produces various kinds of acids, so soils that have been heavily mulched, composted and fertilized will most likely display acidic levels. When adjusting pH levels in the soil, it's easy to overdo it. Therefore, periodic checking of pH levels is advised after applications of various additives are made to the soil.