Question Statement:
To reclaim a sodic soil, exchangeable sodium percentage must be reduced by $\Delta ESP = 10\%$ on an exchange complex with cation exchange capacity $CEC = 20\text{ meq}/100\text{ g}$. Sodium to be replaced is $\Delta Na = \frac{10}{100} \times 20 = 2.0\text{ meq}/100\text{ g}$. Since $1\text{ meq of Ca}^{2+}/100\text{ g soil}$ requires $0.86\text{ tonnes of pure gypsum/ha}$ for the top 15 cm soil depth, the required gypsum in tonnes/ha is $2.0 \times 0.86$, which equals: