Field Identification
Magnesium deficiency appears as interveinal chlorosis on older leaves first because magnesium is mobile in the plant and the canopy remobilizes it to new growth under shortage. Corn shows classic streaked leaves; apples show basal leaf drop patterns that differ from nitrogen shortage. Sandy leached soils, heavy potassium fertilization without magnesium balance, and very acid soils contribute. It is common in intensively cropped sandy fields and in container mixes fed one-sided bloom formulas.
Confirm by comparing old versus new leaves -- nitrogen lack is more uniform yellowing including young leaves in many crops. Tissue tests of recently mature leaves clarify. Rule out mite stippling and virus mosaics with a lens and pattern mapping. Check irrigation water and fertilizer grade for long-term K dominance without Mg.
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How to Deal With It
Organic Control Methods
Organic matter and diverse soil biology improve cation exchange capacity so magnesium is held against leaching. Composts derived from plant residues supply magnesium slowly. Mycorrhizae expand root access in low-input systems. These supports work alongside correcting pH and fertilizer balance rather than replacing them.
Apply dolomitic lime only when pH and magnesium tests together justify it -- calcitic lime will not add magnesium. Use sulfate or chelated magnesium fertilizers per soil test recommendations for high-value crops. In containers, switch to complete fertilizers with micronutrient packages during heavy fruiting. Avoid chronic over-application of potassium chloride without monitoring secondary cations.
Mulch to reduce drought stress that makes chlorosis look worse than soil tests imply. Leach containers if electrical conductivity climbs from salt buildup. Rotate heavy feeders with cover crops that recycle cations when appropriate for your rotation goals.
Crush and incorporate coarse rock dusts only when particle size and soil chemistry match extension guidance -- they are slow. Surface applications of Epsom salt (magnesium sulfate) can be foliar or soil per crop guides for quick rescue on acute cases with confirmed deficiency.
Epsom salt foliar sprays (1-2 tablespoons per gallon is a common starting point for many crops) green older leaves within days when deficiency is real -- test on a branch first. Soil drenches of magnesium sulfate last longer than foliar for perennials. Do not apply blindly every week without tissue confirmation or you risk skewing soil chemistry.
Let Nature Handle It
Natural Enemies
- Beneficial Fungi
- Competitive Microbes
- Antagonistic Bacteria