Diagnosing magnesium deficiency in wheat
Western Australia agricultural soils particularly acidic sands are inherently low in magnesium, but magnesium deficiency is rare in broadacre crops. As magnesium deficiency has not yet been recorded in wheat, mild deficiency can initially be expected.
What to look for
- Plants have a pale green to yellow appearance.
- Older leaves are affected first and most severely.
- Mildly deficient leaves have green linear beading that progresses to interveinal chlorosis.
- As deficiency becomes more severe the veins become less distinct and interveinal tissue may die turning pale or dark brown.
What else could it be
|Diagnosing molybdenum deficiency in cereals||Pale plants||Magnesium deficient plants have leaf interveinal-chlorosis and beading|
|Diagnosing nitrogen deficiency in wheat||Pale plants with oldest leaves most affected||Magnesium deficient plants have leaf interveinal-chlorosis and beading|
Iron and sulphur deficiency
|Pale plants||New leaves are affected first|
Where does it occur?
- Magnesium deficiency is more common in acidic sandy soils that have been treated with high rates of potassium or calcium in lime or gypsum that displaces magnesium from the exchange complex.
- No yield responses to magnesium presently justify soil application. However dolomite or magnesium fertiliser could be used if magnesium deficiency arose in WA.
How can it be monitored?
- Use whole-top plant test to diagnose suspected calcium deficiency and compare paired good/poor plant samples where possible.
- There is no calibration for WA, as field response to magnesium fertiliser have not been measured, but whole shoot levels greater than 0.13% have sufficient magnesium.
- If testing the youngest emerged blade, levels above 0.13% at early tillering have sufficient magnesium.
- There is no locally calibrated soil test for magnesium.
- Topsoil calcium:magnesium ratios have little value as magnesium mostly increases with depth, with higher proportions of magnesium to calcium on exchange sites in subsoils.