The exchangeable sodium ratio (ESR) can be calculated as

A. $${\text{ESR}} = \frac{{{\text{Exchangeable}}\left( {{\text{C}}{{\text{a}}^{2 + }} + {\text{M}}{{\text{g}}^{2 + }}} \right)}}{{{\text{Exchangeable}}\,{\text{N}}{{\text{a}}^ + }}}$$

B. $${\text{ESR}} = \frac{{{\text{Exchangeable}}\,{\text{N}}{{\text{a}}^ + }}}{{\sqrt {{\text{Exchangeable}}\left( {{\text{C}}{{\text{a}}^{2 + }} + {\text{M}}{{\text{g}}^{2 + }}} \right)} }}$$

C. $${\text{ESR}} = \frac{{{\text{Exchangeable}}\,{\text{N}}{{\text{a}}^ + }}}{{{\text{Exchangeable}}\left( {{\text{C}}{{\text{a}}^{2 + }} + {{\text{K}}^ + }} \right)}}$$

D. $${\text{ESR}} = \frac{{{\text{Exchangeable}}\,{\text{N}}{{\text{a}}^ + }}}{{{\text{Exchangeable}}\left( {{\text{C}}{{\text{a}}^{2 + }} + {\text{M}}{{\text{g}}^{2 + }}} \right)}}$$

Answer: Option D

Solution(By Examveda Team)

The exchangeable sodium percentage measures the production of cation exchange sites occupied by sodium.
The exchangeable sodium ratio (ESR) is the relative amount of the sodium ion present on the soil surface, expressed as a percentage of the total cation exchange capacity.
It is calculated as:

$${\text{ESR}} = \frac{{{\text{Exchangeable}}\,{\text{N}}{{\text{a}}^ + }}}{{{\text{Exchangeable}}\left( {{\text{C}}{{\text{a}}^{2 + }} + {\text{M}}{{\text{g}}^{2 + }}} \right)}}$$