A. The respiration of root cells is severely inhibited and cannot provide energy, which will affect all physiological functions of the plant. It not only shows a lack of iron and calcium, but also a lack of other inorganic salts. A is wrong.

B. The concentration of the soil solution in the saline-alkali land is too high, causing the plant root cells to lose water and affecting the absorption of all inorganic salts. Not only does it show iron and calcium deficiency, but other inorganic salts are also not absorbed and will be deficient. ． B is wrong.

C. Plants need iron and calcium, so the roots will not selectively absorb iron-box calcium. Inorganic salts such as iron-box calcium can be absorbed and utilized by plants when they are dissolved in water. C error.

D. Inorganic salts must be dissolved in water before they can be absorbed and utilized by plants. They show iron deficiency and calcium deficiency. The most likely reason is that calcium and iron elements exist in the form of insoluble compounds. D is correct.

So choose: D