Problem Soils & Chemical Reclamation

According to the US Salinity Laboratory Staff (Richards, 1954) classification:

• Saline soil: $\text{EC} \gt 4\text{ dS/m}$, $\text{ESP} \lt 15$, $\text{SAR} \lt 13$, $\text{pH} \lt 8.5$
• Sodic (Alkali) soil: $\text{EC} \lt 4\text{ dS/m}$, $\text{ESP} \gt 15$, $\text{SAR} \gt 13$, $\text{pH} \gt 8.5$
• Saline-Sodic soil: $\text{EC} \gt 4\text{ dS/m}$, $\text{ESP} \gt 15$, $\text{SAR} \gt 13$, with $\text{pH} \approx 8.5$ or $\lt 8.5$ due to excess neutral soluble salts depressing hydroxyl activity.

Both Assertion and Reason are correct, and (R) explains (A):

• Agricultural lime ($\text{CaCO}_3$) is very poorly soluble and remains restricted to the topsoil layer.
• In contrast, Gypsum ($\text{CaSO}_4\cdot 2\text{H}_2\text{O}$) is much more soluble and leaches readily down into the subsoil.
• In acidic subsoil, $\text{Ca}^{2+}$ displaces phytotoxic aluminum ($\text{Al}^{3+}$), while $\text{SO}_4^{2-}$ undergoes ligand exchange with surface hydroxyls (releasing $\text{OH}^-$ which precipitates toxic aluminum as insoluble $\text{Al(OH)}_3$ / basic aluminum sulfate, known as the 'self-liming' effect).

• The progressive sequence of water erosion stages develops as follows:
• Splash erosion (A): First stage; impact of falling raindrops shatters soil aggregates and splashes particles into air.
• Sheet erosion (B): Second stage; uniform removal of a thin layer of topsoil by shallow overland sheet flow (the most insidious 'silent thief' of soil fertility).
• Rill erosion (C): Third stage; concentrated runoff cuts tiny, distinct finger-like micro-channels (< 30 cm deep) that can still be smoothed out by normal tillage implements.
• Gully erosion (D): Advanced fourth stage; severe deepening and widening of rills into massive ravines that cannot be crossed or removed by ordinary farm implements.
• Correct chronological order: (A) -> (B) -> (C) -> (D), confirming Option 1.