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.