Nitrogen (N): The primary macronutrient that is most widely deficient and universally limiting for crop growth and yield across virtually all agro-climatic zones and soil orders in India. (Correct answer)
Carbon: Readily acquired by plants from atmospheric carbon dioxide ($CO_2$) during photosynthesis and is not a soil-supplied mineral limiting nutrient.
Oxygen: Absorbed freely from air and soil pore spaces during root respiration and is not a limiting mineral fertilizer element.
Hydrogen: Supplied in abundant quantities through water ($H_2O$) absorbed by roots during transpirational uptake.
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Calcareous or alkaline soils: Under high pH ($pH \gt 7.5$) and abundant free calcium carbonate ($CaCO_3$), iron precipitates into insoluble ferric hydroxides and carbonates ($Fe(OH)_3$), severely reducing bioavailable $Fe^{2+}$ and triggering characteristic lime-induced iron chlorosis in young leaves. (Correct answer)
Salt affected soils: Primarily induce osmotic drought and specific ion toxicities (excess $Na^+$ and $Cl^-$).
Acidic soils: Low pH increases iron solubility, frequently causing iron toxicity rather than deficiency.
Neutral soils: Offer optimal micronutrient availability without precipitation.
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