Plant physiology and fertilizer burn:\nExcessive application of chemical fertilizers can cause root shrivelling and plant wilting primarily because the

Introduction / Context:
“Fertilizer burn” is a common field symptom appearing as scorched leaf margins, wilt, and root injury after over-application of soluble salts. The underlying mechanism is osmotic stress: when salts accumulate around roots, water movement reverses, dehydrating plant tissues and impairing nutrient uptake. Understanding this helps calibrate application rates and irrigation practices to avoid damage.

Given Data / Assumptions:

• Excess readily soluble chemical fertilizers applied to the root zone.
• Limited immediate leaching or rainfall to dilute salts.
• Typical plant sap osmotic potential determined by cellular solutes.

Concept / Approach:
Water moves across semi-permeable root membranes from regions of higher water potential (lower osmotic pressure) to lower water potential (higher osmotic pressure). Heavy fertilizer application increases dissolved ion concentration in the soil solution, raising its osmotic pressure (more negative water potential). If the soil solution's osmotic pressure exceeds that of the plant sap, water flows out of the roots, causing plasmolysis, root shrivelling, and wilting even if the soil is wet.

Step-by-Step Solution:

Verification / Alternative check:
Electrical conductivity (EC) of the soil solution correlates with soluble salt load; high EC zones after heavy fertilization are linked to osmotic stress and burn symptoms, reversed by adequate leaching irrigation.

Why Other Options Are Wrong:

• “Less than plant sap”: would favor water uptake, not wilting.
• “Soil becomes too alkaline/acidic in all cases”: pH shifts can occur but are not the primary, immediate cause of wilting after salt over-application.
• “Temperature drop”: not the mechanism here.

Common Pitfalls:
Assuming drought when wilting is due to osmotic stress; irrigation without leaching can worsen salinity at the root surface unless salts are flushed below the root zone.

Final Answer:
Osmotic pressure of the soil solution becomes higher than that of the plant sap.