## 4 Discussion  
### Impacts of Different Treatments on Soil Properties  
It has been widely reported that amending salinity-prone soil with organic matter is an effective strategy for improving soil health. Organic amendments are easy to procure and reported by many authors to be effective in the restoration of marginally saline soils [50; 51]. The leaching of salts from salt-affected soils requires the replacement of Na\({}^{+}\) with divalent cations, such as Ca\({}^{2+}\), which can be provided by amendments of gypsum [11]. Adding \begin{table}
\begin{tabular}{c c c} \hline \hline
**Conditions and Treatment** & **Uncertainty RMSE (\%)** & **Correlation (R\({}^{2}\))** \\ \hline \multicolumn{3}{c}{**Pot**} \\ \hline G100 & 3.8\% & 0.9989 \\ FYM + G50 & 8.3\% & 0.9993 \\ PM + G50 & 9.3\% & 0.9994 \\ GM + G50 & 6.0\% & 0.9965 \\ \hline \multicolumn{3}{c}{**Field**} \\ \hline G100 & 4.8\% & 0.9998 \\ FYM + G50 & 5.8\% & 0.9994 \\ PM + G50 & 4.6\% & 0.9995 \\ GM + G50 & 6.2\% & 0.9967 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Evaluation of simulations of the impact of gypsum and organic manures on soil carbon in pot and field experiments at Jhang. Note: FYM = farmyard manure, PM = poultry manure, GM = green manure (all applied at 12 t ha\({}^{-1}\)); G100 = 100% soil gypsum requirement; G50 = 50% soil gypsum requirement. Figure 9: Simulated potential carbon (C) sequestration assuming continued application of the treatments used in the field study (left axis) change in C stock to a depth of 30 cm; (right axis) change in %C in the soil. organic matter to the soil provides more exchange sites to capture Ca\({}^{2+}\) ions, which can be held by a strong organo-mineral Ca\({}^{2+}\) bridges rather than single bonds to Na\({}^{+}\) ions. In our study, we found that the addition of gypsum and three other types of organic amendments to the marginally salt-affected soil reduced the pH in all cases (Table 2). This is a benefit in salt-affected soils that tend to have a pH that is above the optimum range for crop growth. In other studies, soil pH was also shown to be reduced by the application of organic amendments [52; 53]. It is likely that the addition of organic amendments causes a decrease in the soil pH by increasing microbial activity, the formation of organic acids and subsequent dissolution of salts that are then leached down the soil horizons. A decrease in the pH of salt-affected soils may also be due to the higher quantity of basic-cationic compounds in the added organic amendments. This was shown in the study by [PERSON], et al. [54], who found that the addition of compost, biochar and peat all decreased the soil pH. The addition of organic amendments helps to bind soil particles into aggregates, mainly by the deprotonation of humic and fulvic acids. The addition of poultry manure to saline-sodic soils has been demonstrated to enhance both the CEC and the quantity of exchangeable K\({}^{+}\) ions in the soil; exchangeable K\({}^{+}\) competes with Na\({}^{+}\) ions and limits its entry into the soil environment [11]. The addition of green manure has been shown to increase microbial decomposition and respiration, and to reduce the soil pH, increasing the solubility of CaCO\({}_{3}\) due to the increased CO\({}_{2}\) partial pressure in the soil [55; 56]. Organic manures have also been observed to stimulate enzymatic activity. When gypsum and organic amendments are applied in combination, they reduce SAR, pH and soil particle dispersion [57]. The concentration of soluble cations also varied depending on the kind of organic amendment used in the experiment. Applying farmyard manure, poultry manure and green manure had less significant impacts on the Ca\({}^{2+}\) levels in the soil than applications of gypsum. Applying gypsum restored soils by replacing Na\({}^{+}\) with Ca\({}^{2+}\) on the soil exchange sites [58]. When organic amendments were also added, they triggered organic acids production, which, in turn, raised the Ca\({}^{2+}\) concentration by dissolving calcium carbonate (CaCO\({}_{3}\)).