The terms “Garden Lime” and “Hydrated Lime” are often confused by homeowners seeking to adjust soil acidity, yet they refer to two chemically distinct products with vastly different applications and safety profiles. Both substances are derived from limestone, a naturally occurring rock, but the processing each undergoes fundamentally changes its chemical composition and reactivity. Substituting one for the other, particularly in gardening, can lead to consequences ranging from ineffective treatment to severe plant damage. Understanding the composition of each is crucial for safe and successful use.
Understanding the Composition of Garden Lime
Garden lime, commonly sold as agricultural lime or pulverized limestone, is a naturally occurring mineral that has simply been crushed into a fine powder. The primary component is calcium carbonate (\(\text{CaCO}_3\)). Dolomitic lime is a variation that also includes magnesium carbonate (\(\text{MgCO}_3\)), providing both calcium and magnesium nutrients to the soil.
This material is sourced directly from quarries and requires no high-heat chemical processing, which contributes to its mild nature. When applied to soil, the calcium carbonate is relatively insoluble in water, meaning it dissolves slowly over time. This slow-release action allows it to neutralize soil acidity, or raise the pH, gradually, minimizing the risk of damaging plant roots.
The main purpose of garden lime is to increase the soil’s pH level, which helps plants better absorb essential nutrients like nitrogen, phosphorus, and potassium. Because of its slow breakdown, it is often applied in the fall to allow sufficient time for the product to react with the soil before the next growing season. It is the preferred option for long-term soil health management in home gardens and large-scale agriculture.
Understanding the Composition of Hydrated Lime
Hydrated lime, also known as slaked lime or builder’s lime, is a manufactured product with the chemical formula calcium hydroxide (\(\text{Ca}(\text{OH})_2\)). Its creation involves a two-step industrial process starting with quarrying limestone. First, the limestone (calcium carbonate) is heated in a kiln to high temperatures, a process called calcination.
This heat drives off carbon dioxide (\(\text{CO}_2\)), leaving behind a highly reactive substance called quicklime, or calcium oxide (\(\text{CaO}\)). In the second step, a controlled amount of water is added to the quicklime in a process known as slaking or hydration. This reaction is exothermic, releasing a significant amount of heat, which results in the fine, white powder known as hydrated lime.
The manufacturing process creates a compound that is significantly more water-soluble and highly alkaline than garden lime, with a typical pH around 12.4. This high alkalinity makes it caustic, meaning it can cause chemical burns to skin and eyes and is harmful if inhaled. Hydrated lime is primarily used in industrial applications such as construction, water treatment, and waste stabilization.
Choosing the Right Lime for the Job
The chemical and physical differences mean these two products are not interchangeable, particularly for general soil amendment. Hydrated lime is highly reactive and dissolves quickly, causing a rapid spike in soil pH. This sudden shift can easily damage or “burn” the roots and foliage of plants, making it a poor choice for routine gardening.
Garden lime is the standard recommendation for correcting acidic soil because its calcium carbonate composition ensures a slow and gradual pH adjustment. This action supports the long-term health of the soil microbes necessary for nutrient cycling. Hydrated lime should generally be avoided in home gardens unless a professional soil test indicates an urgent need for a high-speed pH change, and it is applied with caution and protective gear.
Before applying any type of lime, soil testing is necessary to determine the current pH level and the actual need for amendment. Using the wrong product or applying too much can make the soil too alkaline, which causes nutrient deficiencies and stunted plant growth. For gardeners seeking to raise soil pH, the slow and sustained action of garden lime is the appropriate choice.