Essential Plant Nutrients Questions and Answers

Answer: C

Plants take up nitrogen mainly as Nitrate.

Roots absorb nitrogen from the soil water chiefly as nitrate ions (\(NO_{3}^{-}\)), and to a lesser extent as ammonium ions. Nitrifying bacteria in the soil convert ammonium into nitrate, and inside the plant nitrate is reduced again and built into amino acids and proteins.

Chloride is a chlorine ion and carries no nitrogen. Plants cannot use nitrogen oxides from the soil, and nitric acid is not absorbed as such; it would simply dissociate into nitrate in the soil water.

Answer: C

The nutrient is potassium.

Potassium is needed for the formation and movement of carbohydrates, so it matters especially for starch and sugar crops such as potato, sugarcane and fruit. It strengthens cell walls and regulates stomata, which improves resistance to disease and drought, and it improves the size, colour and quality of flowers, fruits, vegetables and grain.

Nitrogen mainly drives leafy growth, phosphorus root growth and energy transfer, and sulphur protein and oil formation.

Answer: C

The three major nutrients are Nitrogen, Phosphorus, Potassium.

Nitrogen (N), phosphorus (P) and potassium (K) are the primary macronutrients. Plants need them in the largest amounts, soils most often run short of them, and they are the three figures printed on a fertiliser bag as its NPK grade.

Calcium and sulphur are secondary macronutrients, needed in smaller amounts, and iron is a micronutrient. Every other option mixes one of these into the primary trio.

Answer: C

The nutrient is Carbon.

Carbon, taken in as carbon dioxide through photosynthesis, forms the backbone of every organic compound in the plant: carbohydrates, proteins, fats and nucleic acids. Together with hydrogen and oxygen it makes up most of a plant's dry weight.

Nitrogen is essential for proteins and nucleic acids but is not part of carbohydrates, and sulphur is found only in certain amino acids and proteins. Sodium is not an essential nutrient for most plants. The word carbohydrates is what points to carbon.

Answer: C

Potassium mainly helps in enzyme activation and stomatal regulation.

Potassium is not built into any organic compound in the plant; it stays as the K⁺ ion. In that form it activates more than 60 enzymes, including those of photosynthesis, respiration and starch and protein synthesis. Its movement into and out of the guard cells changes their turgor and so opens and closes the stomata, which controls transpiration and is why potassium improves drought tolerance.

Energy transfer through ATP is the role of phosphorus. Chlorophyll synthesis depends on magnesium, which sits at the centre of the molecule, and on nitrogen. Cell wall formation needs calcium, as calcium pectate in the middle lamella, and boron.

Answer: A

The nutrient is Phosphorus (P).

In soil, phosphate is quickly fixed: it reacts with calcium in alkaline soils and with iron and aluminium in acid soils to form compounds of low solubility, and it moves only a few millimetres, mostly by diffusion. That is why phosphatic fertiliser is placed near the seed. Inside the plant, however, phosphorus is highly mobile and is moved from old leaves to young tissues, so its deficiency shows first on older leaves.

Nitrogen as nitrate is mobile in soil, which is why it leaches. Calcium is fairly mobile in soil but immobile in the plant, so its deficiency appears in young tissues. Potassium is more mobile in soil than phosphorus and is also mobile in the plant, so it does not fit the 'immobile in soil' half of the description as well.