Number 5   Ca-Mg-S


By Riley Holly


Now that we have covered the primary plant nutrients (N-P-K), we can take a look at the macro-nutrients;

  calcium (Ca),

  magnesium (Mg), and

  sulfur (S).

These are taken-up as ions in solution in the soil.  They can also be applied through the stomata in the leaves (foliar spray & atmosphere).

These nutrients are required in similar amounts as N, P, & K, but are usually in sufficient amounts in the soil.  All three are mobile in the soil.


Calcium is taken up as the ion Ca++, and is used in the plant to build cell walls and in the formation of new cells.  Ca tends to interfere with Mg absorption. A Ca deficiency is related to a low soil pH. Symptoms of Ca deficiency include:

 *Necrosis of terminal ends of shoots & roots

 * Tip & margin burn

 *Premature shedding of blossoms & buds

 *Discolored areas on fruit 

 *Blossom end rot 

 *Weak stems 

 *Cupping/hooking of leaves, wilting 

 *Pits in some pome fruits

Excess Ca reduces availability of micronutrients, and raises the pH level.


Sulfur is taken-up as the sulfate SO4--. Sulfur combines with nitrogen to form proteins, and soils lacking S tend towards a nitrogen buildup.  S is only slightly mobile in plants.

Sulfur deficiencies are similar to nitrogen deficiencies:

 *Delayed maturity

 *Retarded growth

 *Young leaves light green to yellow, light green veins 

 *Small & spindly plants, brittle stems 

 *Lower leaves turn yellow 

 *Leaves may cup downward

Excess S shows up as reduced leaf size and stunted growth.


Industrial control of SO2 emissions has reduced atmospheric availability


Magnesium is taken up as the ion Mg++.  It is a constituent of chlorophyll and is required for photo-synthesis.  Mg is mobile within plants and moves from older to younger tissue.  It is mobile in soils and is leachable.  Mg is an activator of plant enzymes for growth.

Deficiency symptoms of Mg are:

 *Marginal necrosis

 *Necrosis of lower mature leaves

 *Leaves curl upwards

 *Interveinal chlorosis (Xmas tree)

 *Mostly older leaves affected

 *Purple-red color of leaves

An excess of Mg interferes with Ca uptake.


Organic sources of Mg are dolomite limestone, Epson salts, and seaweed.

Organic sources of S are seaweed and soil sulfur.

Organic sources of Ca are eggshells, seashells, seaweed, wood ashes, limestone, and gypsum.