Exchangeable sodium percentage (ESP) > 40, and pH > 9.8 are characteristic of

Correct option is C

Introduction
· Soil alkalinity and salinity are determined by the concentration of soluble salts and the saturation of the exchange complex with sodium ions.
· The Exchangeable Sodium Percentage (ESP) measures the degree to which the adsorption complex of a soil is occupied by sodium.
· Soil pH is a measure of the acidity or alkalinity of the soil solution, which significantly influences nutrient availability and microbial activity.

Information Booster
Strongly saline soil (often categorized under the broader umbrella of saline-alkali or sodic soils in various classification systems) is characterized by extremely high levels of exchangeable sodium and high pH.

While standard saline soils typically have a pH below 8.5 and an ESP below 15, the specific values mentioned— ESP > 40 and pH > 9.8—indicate a severe accumulation of sodium carbonate and bicarbonate. 
· Exchangeable Sodium Percentage (ESP) is calculated as: $(Exchangeable Na^+ / Cation Exchange Capacity) \times 100$.
· A pH value above 9.8 usually indicates the presence of free sodium carbonates ($$Na_2CO_3$$​), which undergo hydrolysis to release hydroxyl ions $$OH^-$$​.
· High ESP causes soil "dispersion," where soil aggregates break down, leading to poor permeability, aeration, and physical structure.
· Strongly sodic/saline conditions often lead to the formation of a "black alkali" crust on the soil surface due to the transition of organic matter into solution.
· These soils require specific amendments like Gypsum ($$CaSO_4 \cdot 2H_2O$$​ to replace exchangeable sodium with calcium and improve soil drainage.

Additional Information
· Acidic soil is characterized by a pH below 7.0 (often < 5.5 in strong cases) and high concentrations of aluminum ($$Al^{3+}$$​ and hydrogen ($$H^+$$​) ions, which is the opposite of the conditions described.
· Weak saline soil typically has an Electrical Conductivity (EC) slightly above $4 dS/m$ but maintains a pH below 8.5 and an ESP below 15, making it much less harsh than the values given.
· Eroded soil refers to a physical condition where the topsoil is lost due to wind or water; while salinity can lead to erosion, "eroded soil" is not defined by specific chemical parameters like ESP > 40 or pH 9.8.