Topic 10


The Secondary Nutrients - Ca, Mg and S

As important as the primary nutrients but taken up in smaller quantities.

Not as much attention is given to adding these nutrients to the soil because they are added  incidentally.

  1. When the pH is raised by liming Ca is added to the soil. Mg is added when dolomitic limestone is used.

  2. Soil sulfur is replenished by sulfur dioxide from the air brought down by precipitation. Regulations on air pollution have reduced this in recent years. Added incidentally with other fertilizers ordinary superphosphate.


A. Form used by plants

Absorbed by plants as the Ca2+ from exchange sites or soil solution

B. Soil Calcium

  1. Contained in rocks and minerals from which the soil is formed - dolomite, calcite, apatite, calcium feldspars.
  2. In humid regions even soils formed from limestone are acid in the surface layer because Ca2+ may be removed by excessive leaching. H2O + CO2 ===> H2CO3 Carbonic acid

C. Calcium added to Soil

  1. Limestone used to improve acid soils add Ca
  2. Gypsum is used to supply Ca without changing the pH.
  3. Phosphorus fertilizers

D. Behavior of calcium in the soil

  1. Availability is dependent on the percent of the cation exchange complex occupied by Ca2+ rather than the absolute amount.
  2. Type of Clay

          a. Kaolitic clays (1-1) can supply enough Ca to plants at saturation values of 40-50?

          b. Montmorillonitic clays require Ca situation of 70% to supply plants

E. Calcium Nutrition

1. General

a. Forms compounds that a part of the cell walls -- to strengthen plant structure.

b. Stimulate root growth and leaf development.

c. Calcium is very immoble in the plant. Ca in the leaves is  not translocated to the fruit. The Ca must be supplied by the soil when the fruit is forming.

d. Ca deficiencies can occur in the fruit when soil moisture is low. ex. blossom end rot of tomatoes.


2. Specific Crops

a. Peanuts

    Ca is required for good peg development. Since Ca is immoble in the plant, gypsum is broadcast at the rate of 1000 to 2000 lbs./acre on the soil surface to insure adequate uptake.

Gypsum is applied at the time of blooming so that Ca is available when pegging begins.

Gypsum sources: Mining, Byproduct, recycling

b. Soybeans (legumes)

     Ca is required for the rhizobia to form nodules on the roots.



Form taken up by plants Mg2+

Soil Magnesium

Dolomite, olivine, biotite mica the minerals containing magnesium are easily weathered. The soil is depleted of weatherable Mg minerals relatively sooner than most Ca, Mg and K minerals.

Exchangeable Mg2+ is the largest source of plant available Mg.

Twelve to 18% of exchangables bases are usually Mg2+ ions compared to 75-85% for Ca.

The Mg2+ hydrates to form a larger ion than Ca therefore, it is adsorbed less strongly by the cation exchange complex.

Mg is excessive if it occupies more than 40-60% of CEC deficient if less than 3-8%

Preferential adsorption of these double charged ions results in a lower percentage in soil solution than other cations like K and Na.

High K fertilization can induce Mg deficiency

Magnesium in plants

Vital to photosynthesis -- every chlorophyll molecule contains a Mg ion

Mg deficiency causes grass tetany in ruminants

potatoes and tobacco are particularly sensitive to Mg deficiency

Sand drown of tobacco

Epsom salts MgSO4 is a fertilizer source other than dolomitic limestone


1. Form utilized by the plant : absorbed by plants as SO42- anion small amount through leaves as SO2

2. Soil Sulfur

a. found in soil solution as the SO42-. Negative charge so there is little adsorption on clays. Moves with the soil water and is easily leached.

b. organic matter is the source of soil sulfur. N-S ratio 10-1

3. Sources of sulfur added to the soil.

a. atmosphere - (SO2) sulfur dioxidebrought down by precipitation.

b. Fertilizer S

1. ordinary superphosphate 9-12% S

2. gypsum - soluble - neutral in reaction 17% S copper sulfate

4. elemental Sulfur - when added to soil it is oxidized to H2SO4 by microorganisms very acid. 300 lbs./acre would require 1000 lbs. of CaCO3 to neutralize.

5. Sulfate movement and retention

a. reacts with the same anion exchange sites as H2PO4-, HPO42- is held more strongly so there is little room for SO42- on these sites in surface soils.

b. leaches readily - accumulation in B horizon

6. Sulfur Nutrition - part of three amino acids Cystine, Cysteine, Methionine

a. Deficiencies occur on sandy soils low in organic matter in areas with high rainfall.

b. Deficiency symptoms plants show a pale green color

c. S is part of protein. Plant need a N:S ratio of 15:1

d. application rates

15-20 lbs/acre S where leaching is not a problem; 40-60% lbs / acre an deep sandy soils

e. Vidalia Onions-low sulfur soils are needed to grow sweet onions. Mattamuskeet sweets

Secondary and Micronutrients for Turfgrass

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