Which of the following substances must be present in the soil for growth of a healthy green plant?

Analyze the question: We need the substance a healthy green plant must obtain from the soil (not make itself or get from air).

Explain the concept: Green plants are autotrophs. They make their own organic compounds (glucose, amino acids) by photosynthesis, using carbon dioxide from the air and water from the soil. From the soil they absorb water and mineral ions (nutrients) through their roots.

Go through the options:

• A. Amino acids: Plants synthesize these themselves from sugars and nitrate ions. They don't need them ready-made from the soil.
• B. Carbon: Plants get carbon from carbon dioxide in the air, not from the soil.
• C. Glucose salts: Glucose is made in the leaves by photosynthesis, and "glucose salts" is not a plant nutrient.
• D. Magnesium ions: Magnesium is a mineral absorbed from the soil as Mg²⁺. It is a key part of the chlorophyll molecule. Without it, leaves turn yellow (chlorosis) and the plant cannot photosynthesize properly, so it does not grow healthily.

Answer: D. Magnesium ions.

Tip: Remember the split: air gives carbon (CO₂), soil gives water and mineral ions. Also recall key mineral roles: Mg for chlorophyll, N for proteins, Fe for chlorophyll formation, and P for roots and energy transfer. If an option is an organic food substance like glucose or amino acids, it's usually a trap, since green plants make those themselves.