5.3 ǀ Calcium and magnesium
The factors governing Ca and Mg deficiencies, their behaviours following
soil submergence, and the amounts taken up and removed by rice crops are
all similar (Dobermann & Fairhurst, 2000). Likewise the root-induced
changes in the soil under Fe toxicity should affect Ca and Mg similarly.
Yet deficiencies of Mg in association with Fe toxicity are more common
(Fig. 1). This may reflect the relative availabilities of soil
amendments containing Ca and Mg. Calcium is applied in phosphate
fertilizers and in lime (CaCO3) which is more-widely
available than dolomite (CaCO3 + MgCO3)
or other Mg-containing soil amendments, and is widely used to correct
acidity in highly-weathered soils prone to Fe toxicity.
Magnesium deficiency causes a characteristic yellow-orange coloration of
the leaves (Fig. 2b), as opposed to the bronzing and dark brown streaks
of direct Fe toxicity (Fig. 2a). It is induced or exacerbated by high
levels of Fe2+ in the soil solution as a result of the
root-induced change in the rhizosphere described in Sections 4.1 and
4.2, and uptake competition effects. In already-deficient soils,
symptoms of Mg deficiency become apparent as the solution
Fe2+ concentration increases, and large plants are
more seriously affected than small ones, consistent with a greater plant
demand for Mg if other nutrients are less growth-limiting. Magnesium
deficiency disrupts Fe2+ storage in leaf cell vacuoles
(Kobayashi et al., 2018), and Mg is involved in defence against ROS
damage (Hauer-Jákli & Tränkner, 2019). A combination of this indirect
and direct Fe-toxicity produces plants with a wide range of intensities
of oranging symptoms and brown streaks on the leaves.