Magnesium (Mg)
Content in Plants
Magnesium content in plant leaves will range from 0.20 to 0.50%, although Mg
content can be as high as 1.00% of the dry weight. The frequency of Mg
deficiency in hydroponically-grown crops may equal that of N as the result of
effects due to improper balance among the other major cations, Ca2+, K+, and
NH4
+. In addition, some plant species are more sensitive to Mg than others, that
sensitivity varying with stage of growth and environmental conditions.
48 Hydroponics: A Practical Guide for the Soilless Grower
Function
Magnesium is a major constituent of the chlorophyll molecule (Figure 5.1),
the substance in which photosynthesis takes place (see page 14). Magnesium
is also an enzyme activator for a number of important energy transfer processes.
Therefore, a deficiency will have serious impact on plant growth and
development. Magnesium is related to a specific enzymatic function particularly
associated with C3-type plants that when Mg is deficient, CO2 fixation is
reduced, and therefore the production of carbohydrates required for active
plant growth declines. Magnesium uptake, like Ca, tends to remain fairly
constant with time, but it differs from Ca in that Mg is more mobile in the
plant. While Ca is mobile only in the zylem, Mg is mobile in both the zylem
and phloem.
Deficiency Symptoms
Magnesium deficiency symptoms are quite distinct as an interveinal chlorosis
that appears first on the older leaves. Once a Mg deficiency occurs, it is very
difficult to correct, particularly if the deficiency occurs during the mid-point
of the growing season. In those plant species that have a high Mg requirement,
the deficiency may be triggered by various types of environmental and
physiological stress. Deficiency can result from an imbalance between K+ and
Mg2+, Ca2+ and Mg2+, or NH4
+ and Mg2+ cations. Of these cations, Mg2+ is the
least competitive for root absorption. The Ca2+ and Mn2+ cations show a
competitive effect on Mg2+ uptake and increased Ca2+ uptake ensures a
concentration of divalent ion capacity sufficient to maintain cation/anion
balance and proper functioning of physiological activity. When Mg is deficient,
the increased uptake of Mn2+ prevents total failure of the biochemical processes
of energy transfer, forestalling the collapse and death of plant cells.
The uptake of Mg shows a number of interactive effects, both synergistic
and antagonistic. An interesting side effect of Mg deficiency is a possible
increase in susceptibility to fungus disease infestation as well as the incidence
of blossom-end-rot (BER) of fruit.
Excess Symptoms
Under normal conditions, Mg excess is not likely to occur. However, some
investigators suggest that Mg concentrations in the nutrient solution, as well
as the plant, should not exceed that of Ca in order to maintain the proper
cation balance for best plant growth and development.
Accumulation in the Rooting Medium
With each application of a nutrient solution containing Mg and P to the rooting
medium, whether inorganic (sand, gravel, perlite, rookwool, etc.) or organic
The Essential Elements 49
(pinebark, coir, peat, etc.), a precipitation of Mg with P begins to occur,
forming in the rooting medium an ever increasing accumulation. Being colloidal
in physical form and in eminent contact with plant roots, a portion of
this precipitate is dissolved by root acidification and the released Mg and P
as well as other elements trapped in the precipitate provide a major source
for these elements for uptake and utilization. This partially explains why the
effect of applied nutrient solution on the composition of the plant with time
becomes less a reflection of the nutrient solution composition for most of the
precipitated elements, both major and micronutrient. although the common
recommended practice for control of the nutrient element content of the
growing medium, determined by EC measurements (see page 106), is periodic
water leaching, leaching that will not remove accumulated precipitates.
Concentration in a Nutrient Solution
Most hydroponic formulas call for Mg to be at a concentration around 50 mg/L
(ppm) in the nutrient solution, although that concentration may be too low to
meet the requirement for some crops, such as tomato and cucumber. Magnesium
is present in the nutrient solution as the divalent cation Mg2+.
Nutrient Solution Reagents
The primary reagent source for Mg is magnesium sulfate (MgSO4•7H2O). Natural
waters may contain a substantial quantity of Mg, as much as 50 mg/L (ppm).
Therefore, when preparing a nutrient solution, the quantity of Mg contributed
by the water should be determined so that the Mg concentration in the nutrient
solution does not exceed that recommended.