Oklahoma State University

Secondary Nutrients

Secondary Nutrients

Essential plant nutrients, chemical symbols and sources
Mostly from air and water

From Soil and or Fertilizers
Element Symbol Element Symbol Element Symbol
Carbon C Nitrogen N Iron Fe
Hydrogen H Phosphorous P Mangeanese Mn
Oxygen O Potassium K Zinc Zn
    Calcium Ca Copper Cu
    Magnesium Mg Boron B
    Sulfur S Molybdenum Mo
        Chlorine Cl


The essential plant nutrients may be grouped into three categories.  They are as follows:

                1.             Primary nutrients - nitrogen, phosphorus and potassium

                2.             Secondary nutrients - calcium, magnesium and sulfur

                3.             Micronutrients - iron, manganese, zinc, copper, boron,

molybdenum, and chlorine


Secondary Elements

Calcium (Ca).  Calcium fertilizers are not usually needed in Oklahoma. Common sources of supplemental Ca are lime and gypsum.
Calcium Carbonate (Lime)                                               20-40% Ca
Calcium Sulfate (Gypsum)                                                23% Ca, (18.6% Sulfur)
Normal Superphosphate                                                   22% Ca, (20% P2O5, 12% Sulfur)

Magnesium (Mg).  The most common sources of magnesium are magnesium sulfate and dolomitic lime.
Magnesium Oxide                                                                               52% Mg
Magnesium Sulfate                                                                            16% Mg
Potassium - Magnesium Sulfate                      11% Mg, (22% K2O, 22% Sulfur)
(Sul-Po-Mag, K-Mag)
Dolomitic Limestone (varies)                                            12% Mg

Sulfur (S).  Sulfur is most available when supplied in the highly water soluble sulfate form.  Ag. sulfur (elemental sulfur) can be used, but requires biological oxidation over time to convert the elemental form to available sulfate.
Calcium Sulfate (Gypsum)                                                17% S    (22% Ca)
Potassium Sulfate                                                                               17% S
Sulfate of Potash, Magnesia                                             22% S
Ammonium Sulfate                                                                            24% S
Normal Superphosphate                                                   12% S
Ammonium Thiosulfate                                                     26% S

Boron (B).  A sodium borate (solubor) containing about 20% B is the source of B most commonly used in liquids.  Boric acid and other soluble forms containing between 14 to 20% B are also suitable for liquid mixes.
Borax                                                                                                                                     11.3% B


Exerts from Oklahoma Soil Fertlity Handbook 2006 (Zhang and Raun 2006).






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