Biological functions:

Calcium is present in the cytoplasm at levels that would indicate that it is a micronutrient, ~0.1 µM, in order to prevent interference with the high levels of P. Nonetheless, small fluctuations in Ca levels in the cytoplasm are part of signalling mechanisms for environmental stress. Ca pumps are directed out of the cytoplasm, either to vacuoles, where it may be precipitated as calcium oxalate, or across the plasmalemma. The function of the majority of plant Ca is structural, in the cell walls of shoots and roots.

Soil Ca:

• Soluble Ca²⁺
• Exchangeable Ca²⁺
• Mineral Ca²⁺: gypsum, carbonates, feldspars, etc.

(No "organic-Ca", although some Ca may satisfy the charge of the acidic groups of soil organic matter.)

Ca Inputs to soil:

• Aeolian dust and precipitation
• Fertilizers, liming materials, manures, and residues

Ca Outputs from soil:

• Crop removal
• Leaching

Soil Tests for Ca:

The most common soil test for plant-available Ca²⁺ is 1 M ammonium acetate (pH 7), which is capable of displacing exchangeable Ca²⁺ into the extracting solution, where it can be measured by either AAS, ICP, or even titrated with EDTA using a colorimetric endpoint. This method is not appropriate for calcareous soils (containing free CaCO₃) or gypsiferous soils (containing CaSO₄^.2H₂O) because Ca²⁺ will also dissolve from these soil minerals into the extracting solution. Calcium can also be measured in water extracts of soil, particularly for calculation of SAR (sodium adsorption ratio) and PAR (potassium adsorption ratio).

Calcium Fertilizers

• Calcium carbonate, CaCO₃, or calcium magnesium carbonate, CaMg(CO₃)₂, as agricultural limestone are the primary sources of added Ca to soils, although they are used primarily or additionally to neutralize soil acidity.
• Gypsum, CaSO₄^.2H₂O. No remediation of soil acidity should be expected from gypsum, but Ca will become available.
• Calcium nitrate, Ca(NO₃)₂, contains 19% Ca.
• Chelated calcium, usually as CaEDTA or Ca forms of hydrolzyed protein. If applied, often foliar application.
• Single superphosphate (18-21%Ca) and triple superphosphate (12-14% Ca)

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This page was last modified by Phillip Barak, Univ. of Wisconsin, on 5 Jan 1999. All rights reserved.