Magnesium (Mg) & Cereal Production
Function of Magnesium
Magnesium is an essential component of chlorophyll and is involved in various
enzyme driven physiological processes. Mg improves the utilization and mobility of
phosphorus in the plant. Mg is vital to plant function and is required in the first 5 – 6
wks after plant establishment
Soil Magnesium Index System
Soil Mg Index Soil Mg Range (mg/l)
1
0 - 25
2
26 – 50
3
51 – 100
4
Above 100
Deficiency Risk
Deficiency is most likely on acid, sandy soils with inherently low magnesium status or
compacted soils and under conditions of either drought or high rainfall. Heavy
textured soils usually contain adequate reserves of available magnesium.
High
potash will reduce Mg uptake at the surface of the root hairs.
Poor soil conditions will reduce Mg uptake – Poor soil structure/soil drainage, cold
wet soils and poorly aerated soils
Deficiency Symptoms
At first the older leaves will show a regular pattern of interveinal dark green spots or
pale yellowish green background, giving a ‘beaded’ effect.
More severe deficiency produces an irregular pattern of interveinal necrotic strips.
Symptoms on most soil types are transient, usually coinciding with periods of rapid
crop growth and treatment is unnecessary unless symptoms persist. Magnesium
deficiency is exacerbated by restricted rooting, caused by soil compaction or pest
damage, and by adverse weather conditions which can also reduce magnesium
uptake by plants.
Magnesium Application
Application of Mg is only advisable for cereals if soil Mg is less than 50mg/l. On such
soils, use magnesium limestone when liming is necessary to correct soil acidity, for
example 5t/ha lime can supply 750 -1,000kg Mg O.
An application of 50 - 100kg/ha MgO as kieserite or calcined magnesite, depending
on soil type, may be justified if the soil is very deficient and magnesium is not applied
elsewhere in the rotation, to prevent a deficiency apply every 3 - 4 years. Where a
growing crop is showing deficiency symptoms apply foliar spray of Mg chelates, Mg
oxide (Magflo 3l/ha) or Epsom salts at 5 – 10kg/ha.
Interactions
1. Low soil pH reduces Mg uptake.
2. High K applications reduce Mg uptake by the plant root system.
3. Over application of lime will induce Mg deficiency.
4. High N application will increase crop requirement for Mg. High
5. P application will increase Mg requirement and visa versa.
In Summary
 Mg vital in early stages of plant development
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Seedbed dressing of Mg are only required if the soil is very deficient or if this
nutrient is not applied elsewhere in the rotation which includes root crops that
are susceptible to Mg deficiency
Transient deficiency symptoms may appear during periods of rapid growth or
when root development or weather conditions temporarily limit Mg uptake.
High applications of K will induce Mg deficiency.
Foliar spraying is not usually worthwhile unless symptoms persist.