What nutrient is likely deficient if you notice yellowing leaves but the rest of the plant is light green?

your answer is Magnesium

Explanation:

Magnesium deficiency results in yellowing between the leaf veins, sometimes with reddish brown tints and early leaf fall. Not so stunted as major nutrient deficiencies. Older leaves turn yellow at edge, interveinal chlorosis causes green arrowhead shapes or marbling in the centre of the leaf (speckling in cereals).