What sources of calcium can be used by organic gardeners?
Lime - Reducing the acidity of the soil is the primary purpose for using lime in the garden. However, liming materials also provide nutrients for plant use. Calcium and magnesium are the two elements most commonly provided by lime. Gypsum is used where more calcium is needed without raising the pH. In heavy clay soils, the use of gypsum helps increase moisture circulation. Organic gardeners use natural sources of lime, such as limestone, dolomite, shell, and marl. All these forms must be finely ground to provide maximum benefit to the soil and plants. Dolomite is preferred because it contains both calcium and magnesium.
For application amounts, follow the recommendations of a soil test. The test can be performed by your local Cooperative Extension Service. Apply lime well in advance of the planting date, preferably two to three months before the garden is planted. Mix well with the soil and keep moist for best reaction. Application closer to planting time is permissible, but its benefits are delayed.