Soil alkalinity in waterlogged fields — the topsoil becomes strongly alkaline and infertile when the measured pH reaches approximately which value?

Introduction / Context:
Waterlogging and poor drainage can drive soluble salts and sodium into the root zone, elevating soil pH. From an irrigation and agronomy perspective, knowing the pH threshold at which soils become markedly alkaline and infertile is critical for reclamation planning (e.g., gypsum application, leaching, and drainage improvements).

Given Data / Assumptions:

• Field prone to waterlogging with capillary rise and evaporative concentration of salts.
• Soil pH scale from acidic (<7) to alkaline (>7).
• Question asks for a typical pH value at which the topsoil becomes strongly alkaline and infertile.

Concept / Approach:

Alkali (sodic) soils commonly exhibit pH above 8.5; as pH approaches ~10, exchangeable sodium percentage and carbonate/bicarbonate hazards are usually severe, degrading soil structure, reducing infiltration, and restricting nutrient availability. This threshold reflects conditions where yields are strongly depressed unless reclamation is undertaken.

Step-by-Step Solution:

Verification / Alternative check (if short method exists):

Cross-check with soil classification: saline-sodic/sodic soils at high pH show dispersion, crusting, poor tilth, and micronutrient deficiencies—consistent with infertility at ~10.

Why Other Options Are Wrong:

pH 6 is acidic; pH 8–9 indicates alkalinity but not typically the severe infertility implied; pH 11 is extreme and less common as a practical field threshold. pH 10 is the most defensible indicator of strong alkalinity and infertility.

Common Pitfalls (misconceptions, mistakes):

Equating any pH above 7 with severe infertility; ignoring the role of exchangeable sodium and carbonate equilibria; overlooking drainage as the root cause.

Final Answer:

pH 10