Soil amendment is the practice of adding material to the soil to improve its physical, chemical, or biological properties to better support plant growth. These materials are not primarily fertilizers, but substances that enhance the soil environment. Among the many materials used, lime stands out as one of the most widely applied soil amendments in both large-scale agriculture and home gardening worldwide. Lime is a naturally occurring mineral compound primarily used to counteract soil acidification, a common problem that develops over time due to rainfall, plant uptake, and certain fertilizers.
The Composition of Soil Lime Amendments
Agricultural lime is derived from naturally occurring limestone rock, which is ground into a fine powder for application. The two major types are calcitic and dolomitic lime. Calcitic lime is composed predominantly of calcium carbonate (\(\text{CaCO}_3\)) and provides calcium, a nutrient essential for plant growth. Dolomitic lime is a mixture of calcium carbonate and magnesium carbonate (\(\text{MgCO}_3\)), containing substantial levels of magnesium. This type is beneficial in soils deficient in magnesium, a nutrient required for photosynthesis, and both types function by introducing carbonate compounds into the soil, which are the active agents in neutralizing acidity.
How Lime Amends Soil Acidity
The primary function of lime is to correct soil acidity by increasing the soil’s pH level. Soil acidity is measured on the pH scale, where values below 7.0 are acidic; pH below 5.5 is detrimental to many plants. Highly acidic soil causes nutrient availability issues and increases the toxicity of elements like aluminum and manganese. Lime neutralizes this acidity through a chemical reaction involving the carbonate component. When calcium carbonate is added, it reacts with water to form bicarbonate ions (\(\text{HCO}_3^-\)), which consume the excess hydrogen ions (\(\text{H}^+\)) causing the acidity, forming neutral water (\(\text{H}_2\text{O}\)) and carbon dioxide (\(\text{CO}_2\)), which raises the soil pH.
The calcium and magnesium ions released from the lime also improve the soil’s ability to hold onto nutrients. These positively charged ions displace the acid-causing hydrogen and aluminum ions from the soil’s Cation Exchange Capacity (CEC) sites. By occupying these negatively charged sites, the calcium and magnesium ions increase the base saturation. This process frees up the hydrogen and aluminum ions to be neutralized by the carbonate, making essential nutrients more available for plant uptake.
Determining the Need for Lime Application
Applying lime without first testing the soil can lead to over-liming, potentially causing new nutrient deficiencies by making the soil too alkaline. Therefore, the first step is to conduct a comprehensive soil test, which measures the current soil pH and determines the lime requirement. While a standard test measures active acidity, the most important factor is the soil’s buffering capacity, often referred to as the buffer pH. This test measures the soil’s resistance to a change in pH, which is determined by the amount of clay, organic matter, and aluminum present; soils with a high buffering capacity require substantially more lime than poorly buffered, sandy soils. Laboratories use the buffer pH results to calculate the specific lime requirement needed to reach a target pH level for the intended crop.
Proper Application Methods and Timing
Since the chemical reaction that raises the soil pH is a slow process, timing the application is a significant consideration. It is recommended to apply lime well in advance of the planting season, ideally six months to one year before the target crop is sown. This extended period allows the material sufficient time to dissolve and neutralize the soil acidity effectively. The physical form of the lime impacts its reactivity and ease of application. Fine-ground, powdered limestone reacts the quickest due to its larger surface area, but pelletized or granular lime is often preferred for home gardens because it is easier to handle and apply evenly.
For maximum effectiveness, the lime should be incorporated into the soil profile rather than left on the surface. Tilling the lime into the top six inches of soil ensures good contact between the material and the acidic soil particles, speeding up the neutralization process. In no-till or perennial systems, a surface application is necessary, but the pH adjustment will occur more slowly and primarily affect the very top layer of soil.