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Calcium and Magnesium Fertilizers (Foliar and Soil)

Calcium and magnesium are secondary nutrients required by crops in quantities much larger than the micronutrients but smaller than the primary NPK trio. Deficiencies are increasingly common in intensively cultivated soils that have received only NPK for many seasons, and present as blossom-end rot in tomato, tip-burn in chilli, interveinal chlorosis in older leaves, and poor fruit quality.

Composition

Common calcium sources include:

  • Gypsum (CaSO4.2H2O): approximately 21% Ca + 17% S, the cheapest soil source; see Gypsum In Groundnut
  • Calcium nitrate (Ca(NO3)2.4H2O): 19% Ca + 15.5% N, water-soluble fertigation/foliar source
  • Dolomitic lime (CaMg(CO3)2): supplies both Ca and Mg, also raises soil pH
  • Calcium chloride (CaCl2): foliar source for fruit-quality correction

Common magnesium sources include magnesium sulphate (Epsom salt, 9.7% Mg + 13% S), kieserite (MgSO4.H2O, 16% Mg + 22% S) and dolomite.

Mode of action

Calcium is a structural component of cell-wall pectins and is critical for cell division and membrane integrity. Once incorporated into mature tissue it is largely immobile in the phloem, so deficiency shows first in newly forming organs (fruits, growing points). Magnesium sits at the centre of the chlorophyll molecule and activates many phosphate-transfer enzymes; deficiency shows as interveinal chlorosis on older leaves.

Target use and dose

  • Soil basal: gypsum 200-500 kg/acre on light soils; dolomite as per soil-test acidity.
  • Foliar correction: calcium nitrate 0.5-1.0% solution at fruit set; magnesium sulphate 0.5-1% on older crops showing chlorosis.
  • Fertigation: calcium nitrate and magnesium sulphate are standard tank-A and tank-B salts in protected-cultivation nutrient programmes.

Safety and regulatory status

All listed products are FCO-notified secondary-nutrient fertilisers. A critical tank-mixing rule applies: calcium nitrate and magnesium sulphate (or any other sulphate source) must not be combined in the same fertigation stock tank, since they precipitate insoluble calcium sulphate and block emitters. They are mixed only in the diluted main line.

See also: Gypsum In Groundnut, Cms Calcium Magnesium Sulphur, Input Cms Fertilizer, Sulphur Fertilizer, Ssp Single Super Phosphate.

References

  1. Calcium Nitrate Fertilizer. Gardening Know How.
  2. Fertilizer Compatibility. Purdue Vegetable Crops Hotline.