Step 1: Understanding the Concept:
This question asks for the defining characteristic of an alkali soil in terms of its Exchangeable Sodium Percentage (ESP). Problem soils (saline, alkali, and saline-alkali) are classified based on specific chemical properties: electrical conductivity (EC) of the soil extract, pH, and ESP.
Step 2: Key Formula or Approach:
The classification of salt-affected soils is as follows:
Saline Soil:
EC>4 dS/m
ESP<15
pH<8.5
Alkali Soil (or Sodic Soil):
EC<4 dS/m
ESP>15
pH>8.5
Saline-Alkali Soil:
EC>4 dS/m
ESP>15
pH is variable, often<8.5
The Exchangeable Sodium Percentage (ESP) is the percentage of the Cation Exchange Capacity (CEC) that is occupied by sodium ions.
Step 3: Detailed Explanation:
An alkali (or sodic) soil is defined by having poor soil structure due to a high concentration of sodium ions (Na$^+$) on the clay exchange sites. This high sodium content causes clay particles to disperse, clogging soil pores and leading to very low permeability to water and air.
The official criterion used to classify a soil as alkali or sodic is an Exchangeable Sodium Percentage (ESP) greater than 15.
This high ESP is also associated with a high pH (typically above 8.5) due to the hydrolysis of sodium carbonate.
Step 4: Final Answer:
The ESP of an alkali soil is, by definition, greater than 15. Therefore, option (D) is the correct answer.