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Magnesium is the central atom of chlorophyll. Deficiency shows first on older leaves (Mg is mobile, remobilized to new growth) as yellowing with characteristic red-purple margins. Common on very acid soils (pH < 4.5), sandy soils, and blocks with heavy K fertilization (K and Mg compete for uptake).

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Cause

  • Very acid soils (pH < 4.5)
  • Sandy soils with low Mg reserves
  • Heavy K fertilization without Mg balance (K:Mg ratio > 3:1 in soil)
  • Coastal lowland and peat soils
  • Erosion on hilly blocks

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Effect

Field signs:
  • Older leaves in the lower whorl yellow first (Mg is mobile)
  • Yellowing progresses to red-purple leaf margins (classic signature)
  • Interveinal chlorosis: yellow between veins, veins stay green
  • Newer whorls stay normal until deficiency is severe
  • Premature leaf drop in extreme cases

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Leaf-tissue thresholds

Sample Whorl 2 leaves.
This is a generic band for the crop, with no site, soil, clone, age or season qualifier. Deficient and Optimal resolve. Marginal does not — that tier exists because the transition is not sharp. High does not either, because no page states what High means or what to do about it. And where more than one nutrient reads deficient, only the most deficient rating is valid. See Reading a tissue analysis.
Also check K:Mg ratio in soil; should be 2:1 to 3:1.

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Satellite signals

Mg deficiency is patchy across the canopy (lower whorl only) and does not have a strong satellite signature until severe. Rely on ground scouting and tissue analysis.

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Action

1

Confirm with tissue and soil test

Whorl 2 Mg < 0.20% and soil Mg < 0.5 me/100g confirms deficiency.
2

Apply kieserite or magnesium sulphate

Kieserite (27% MgO, 22% S) is the standard fast-acting Mg source:
  • Immature: 100 to 200 g kieserite/tree/year
  • Mature: 200 to 300 g kieserite/tree/year
3

Long-term correction with dolomite

Ground magnesium limestone (dolomite, ~20% MgO, ~30% CaO) raises pH and supplies Mg. Apply 500 kg to 1 t/ha every 3 to 5 years on acid soils.
4

Rebalance K:Mg

If heavy KCl programs are causing induced Mg deficiency, split K applications more finely and interleave with Mg.
5

Follow up

Re-sample Whorl 2 leaves 4 months after correction. New whorls should be normal green; older leaves may not recover.