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Calcium (Ca) and magnesium (Mg) are considered "secondary major nutrients" or "secondary macronutrients". They are required by plants in relatively large amounts, typically less than nitrogen or potassium, but in similar to or greater amounts than phosphorus. Magnesium requirements by plants are usually lower than calcium requirements.
The general approach toward a soil test level that is below the optimum calcium or magnesium concentration is to recommend a one-time application to "build" the soil to the optimum concentration or slightly above. This is expected to keep the soil test between the critical and the optimum concentrations for about three to four years as the crop harvest removes the available fractions. The soil test may then be at or below the critical level, requiring another fertilizer application to "rebuild" the soil test.
Soil deficiencies of calcium and magnesium, when they occur, are usually found in highly acidic soils. Soil calcium is usually managed by applying calcitic (high calcium) ag-lime to maintain proper soil pH. Magnesium deficiency in an acidic soil can often be managed by using a "dolomitic" or high-magnesium limestone source to adjust soil pH.
Some individuals promote a recommendation approach to "balance the cations". That is, to apply calcium or magnesium to reach some sort of optimum percentage of the calculated cation exchange capacity (CEC). This approach has not typically improved yields, unless ag-lime is used to adjust the soil pH to an optimum level. "Over-liming" can occur if ag-lime is used to increase the calcium or magnesium percentage but increases soil pH to an excess level.
Also see the following Crop Files: