Soil alkalinity is a chemical condition where the soil’s pH level is above 7.0, the neutral point on the pH scale. This elevated pH results from a high concentration of base cations, which are positively charged ions that neutralize acidity. Common base cations contributing to alkalinity include calcium, magnesium, and sodium. Soil pH is considered a master variable, significantly influencing nutrient availability and the overall health of the soil ecosystem.
Mineral Composition of Parent Material
The fundamental source of soil alkalinity begins with the geological composition of the parent material, which is the underlying bedrock or sediment from which the soil is formed. Rocks rich in base-forming minerals, such as limestone, dolomite, and basalt, weather over time to release calcium, magnesium, and potassium into the soil solution. These released base cations displace acid-forming hydrogen ions from the soil’s exchange sites, effectively raising the pH.
Limestone and dolomite are influential because they contain high amounts of calcium carbonate (\(\text{CaCO}_{3}\)) and magnesium carbonate (\(\text{MgCO}_{3}\)). The presence of these carbonate minerals acts as a buffer, neutralizing any natural acidity that might develop. The hydrolysis of calcium carbonate, where it reacts with water, produces hydroxyl ions (\(\text{OH}^{-}\)), which are the chemical basis for alkalinity.
A soil containing free calcium carbonate is termed calcareous and typically maintains a pH between 7.0 and 8.3. When the more soluble sodium carbonate (\(\text{Na}_{2}\text{CO}_{3}\)) is present, alkalinity becomes more pronounced, potentially pushing the pH to 8.5 or higher.
The Role of Climate and Water Movement
While parent material provides the base cations, climate dictates whether these compounds accumulate or are washed away. The distinction between humid and arid climates controls the movement of water through the soil profile.
In regions with high rainfall and humid climates, leaching occurs as water percolates downward through the soil. This downward flow dissolves soluble base cations, carrying them out of the soil entirely through drainage. This removal allows acid-forming ions to dominate, resulting in the acidic soils typical of humid environments.
Conversely, in arid and semi-arid regions, rainfall is insufficient for significant leaching, so base cations remain near the surface. Here, evaporation exceeds precipitation, introducing capillary action. As water evaporates, it draws up groundwater containing dissolved salts and carbonates, which are then deposited in the topsoil, concentrating alkaline compounds.
Impact of Agricultural Practices
Agricultural methods can accelerate or induce soil alkalinity, often independently of natural geological and climatic factors. A primary contributor is the use of irrigation water naturally high in dissolved salts, particularly bicarbonates and carbonates. When this water is applied, the water evaporates, leaving the dissolved minerals to accumulate in the topsoil layer.
Over time, this continuous deposition of mineral salts causes the soil pH to rise. This effect is pronounced when irrigation water contains a high proportion of sodium bicarbonate, leading to the formation of highly alkaline, sodic soils.
The application of specific soil amendments to correct existing acidity is another deliberate method contributing to alkalinity. For instance, agricultural lime, primarily calcium carbonate, is routinely applied to raise the pH of acidic soils. While these additions neutralize acidity, excessive or long-term use can push the soil into an overly alkaline state.