Magnesium (Mg)

  • principle form of uptake: ionic form (Mg2+)
  • One of the 13 essential elements for plant growth and is one of the 6 Major Elements, based on concentration required
  • amount required for healthy plant growth: 2,000 ppm, 0.2%, dry soil matter
  • soil conditions related to deficiencies of magnesium:
    • acidic sandy soils subjected to high rainfall or irrigation
    • soils with pH < 5.4
    • strongly acid peat and muck soils
    • soils over-fertilized with calcium and/or potassium
  • calcium & magnesium determine the soil’s mechanical properties
    • the amount of calcium versus magnesium on the exchange points determines if the soil is open & airy or loose or tight & airless (too much Mg means tight & airless soil)
    • this ratio also determines if the clay portions cling to themselves or open and separate
    • If Ca is in excess, and Mg is deficient, soil is extremely loose; water flows through without sticking; getting Mg levels to target will improve this
    • also, this ratio has much more effect on the soil’s air supply than does organic matter
    • when calcium:magnesium ratio is correct, less compost is needed
      • ideal ratio for non-calcareous soil: Ca:Mg = 68:12
      • for calcareous soil: Ca:Mg = 85:5
  • magnesium ions move through the soil by diffusion
  • soil factors that affect uptake of magnesium in the plant:
    • temperature and decomposition of organic matter
    • soil test levels
    • soil moisture movement of ions
    • low soil pH (acidic) decreases magnesium uptake by the plant
    • over-watering
  • plant factors that affect magnesium uptake by plant:

Roles of Magnesium in Plant Functions:

  • involved in the osmotic potential, balancing anions, controlling membrane permeability & electropotential
  • is a component of chlorophyll
  • is a cofactor in most enzymes that are involved in energy transactions using ATP & ADP
  • stabilizes ribosome particles involved in protein synthesis

Magnesium Deficiency Symptoms:

  • yellowing and cell death around the edges of the leaves, or in between veins (may be confused with potassium deficiency)
  • Over-watering locks up magnesium in the soil, which causes yellowing of lower leaves between veins (veins remain green)
  • Magnesium is very mobile within the plant, so deficiency symptoms will first show in the older leaves, since the plant is moving Mg from older leaves to newer growth to supply growing points

Magnesium Excess Symptoms

  • excess magnesium tightens up the soil (TIG)

Sources

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