Essential Nutrients

Magnesium (Mg)

Form used by plants:

Mg²⁺

Important functions:

• Involved in photosynthesis

• Essential for chlorophyll synthesis and nitrogen metabolism

• Activator of enzymes for growth

• Involved in carbohydrate production and transportation (also formation of amino acids, vitamins, sugars, oils, and fats

• Involved in energy transfer and protein synthesis

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Ideal foliage range for apple leaves:

0.35-0.50% 

Should also consider relationship between leaf K and leaf magnesium because as K becomes more available, leaf magnesium concentrations for optimal growth increase.  In general, ratios of about 4:1 (K:Mg) or greater are often associated with the appearance of Mg deficiency symptoms.

Crop requirements:

20-24 pounds per acre per year are typically taken up by the crop (these quantities are generally low relative to the amount of total Mg present in most soils).

Ideal soil range:

• low: < 60 ppm (or < 0.5 meq/100g soil)
• medium: 60-180 ppm (0.5-1.5 meq/100g soil)
• high: > 180 ppm (> 1.5 meq/100g soil)

Magnesium is strongly sorbed by soil components and is thus, not mobile.

Satisfactory ratios of Ca:Mg on an equivalent basis may range from 1:1 to 20:1, provided that adequate Mg is present.  Magnesium is relatively immobile in soil so that maintenance of a healthy root system is critical for magnesium uptake (same is true for potassium). Heavy applications of K can induce Mg deficiency.

Best indicators: 

Soil tests in conjunction with leaf analysis

Mobility in plant:

Remobilized within plant

Deficiency symptoms:

Vegetation symptoms: interveinal chlorosis developing in older leaves first.  In severe cases, area will become necrotic.  Partial defoliation may occur by mid-August (NY pub).

Only a few branches or the entire tree will be uniformly affected (old leaves will abscise first though). In general, ratios of about 4:1 (K:Mg) or greater are often associated with the appearance of Mg deficiency symptoms.

Fruit symptoms: fruit will be small because the leaves of Mg-deficient trees do not produce sorbitol but accumulate starch, and carbohydrates are not transported to the fruit.

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Increased risk of Mg deficiencies:

with heavy applications of K

areas subjected to moisture stress

soils low in pH (increasing soil pH increase availability and efficiency of magnesium uptake)

Excess problems/Interactions with other elements:

High concentration in plant however imbalance with calcium and potassium may reduce growth. Heavy soil applications of K can induce Mg deficiency.

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