2 A farmer noticed that the leaves of cassava plants on one side of the field were smaller and showed signs of yellowing, while those on the opposite side w...

(a) The process by which mineral salts enter root hair cells from the soil solution is active transport. [1] This requires energy from cellular respiration because minerals are moved against their concentration gradient (from a lower concentration in the soil solution to a higher concentration inside the cell).

(b) Two mineral elements that the yellowing plants are most likely lacking:

1. Nitrogen - essential for making amino acids, proteins, and chlorophyll. Deficiency causes general yellowing (chlorosis) of older leaves first, as nitrogen is mobile and redistributed to younger growth. [1]
2. Magnesium - a central atom in the chlorophyll molecule. Deficiency causes yellowing between the leaf veins (interveinal chlorosis). [1]

Other acceptable answers: iron or sulphur, both of which are involved in chlorophyll synthesis or function.

(c) How water moves from the soil into the root hair cell:

1. The soil solution has a higher water concentration (higher water potential) than the cell sap inside the root hair cell's vacuole, which contains dissolved sugars and salts. [1]
2. Water moves by osmosis - the net movement of water molecules from a region of higher water potential to a region of lower water potential. [1]
3. This movement occurs through the partially permeable cell membrane, which allows water molecules to pass but restricts larger solute molecules. [1]

(d) One reason plants on one side of the field may have received fewer minerals: the soil on that side may be more sandy, causing nutrients to leach away more quickly with drainage water. [1] Other acceptable reasons: uneven distribution of fertiliser during application; the previous crop on that side removed more nutrients; or drainage patterns differ across the field.

(e) To correct the yellowing, the farmer should:

1. Apply a nitrogenous fertiliser or top-dress with a compound fertiliser containing nitrogen and magnesium to replace the deficient elements. [1]
2. Apply as a foliar spray for faster uptake through the leaves, or apply to the soil around the affected plants and water in. [1]

Foliar application is faster-acting because nutrients are absorbed directly through the leaf epidermis, bypassing the slower root uptake pathway.