## 3 Results and Discussion
### Acid- and Alkali-Contaminated Soils Treated with XG-eat Combination  
#### 3.2.1 Moisture Content and Germination Ratio  
Figure 9 shows the growth of oats in the acid- and alkali-contaminated soils treated with 0.25% XG at the 7 th day of plant cultivation. The trends of the volumetric moisture contents for the soils exposed to the solutions with different pH values were basically the same (Figure 10a). Due to the strong acid dissolution capacity of sulfuric acid [27; 57; 60], the acid-contaminated soil became much more porous compared with the other conditions (Figure 7), and therefore the moisture within the soil had more paths to evaporate, resulting in the lowest water content (i.e., pH = 2). However, due to the water retention effect of XG [23; 64], the volumetric moisture contents of the soils with different pH values did not diverge remarkably. Figure 10b shows that the germination ratios of oats were the highest (i.e., 95.63%) under the condition of pH = 9, and lowest (i.e., 64.38%) under the condition of pH = 2, respectively. The low germination ratio of oats at pH = 2 may be due to the reduction in the soil fertility caused by the soil organic matter being eroded by the strong acid [27; 57; 60]. On the other hand, the germination ratio of oats became higher as the pH increased from 7 to 9, and decreased when the alkaline pH value further increased to 12. This phenomenon was also observed in the previous studies. According to [65], the root hairs of oats are promoted under low concentration alkali stress, but inhibited under high concentration alkali stress. Numerous studies have focused on the adverse effect of high soil pH value on plant growth, indicating that the deficiencies in essential elements (i.e., P, Fe and Zn) as a result of reduced solubility [66; 67], and NH\({}_{3}\) toxicity resulting from the dissociation of NH\({}^{4+}\) at a pH value above 9, will severely inhibit plant germination and growth [66; 67; 68]. Overall, the addition of XG helped to improve the soil environment [23] and promoted the growth of oats with the germination ratio above 60% for all the pH values. Figure 8: Shear strength, cohesion \(c\), and internal friction angle \(\varphi\) of the acid- and alkali-contaminated soils. Figure 11a further confirms that oats grew better in neutral, weakly acidic, and weakly alkaline soils, and Figure 11b indicates that the volumetric moisture content slightly increased with the increasing pore fluid pH value. In addition, the volumetric moisture contents of the soil were roughly inversely proportional to the germination ratios (Figure 11c-e); that is, the higher the germination ratio of oats, the lower the volumetric moisture content of the soil. This is mainly because the promoted oat growth underwent a higher level of transpiration and consumed more water from the soil through the oat roots [17]. An exception was observed for pH = 2 with both the low germination ratio and low volumetric moisture content due to the adverse impact of the strong acidic solution on reducing soil fertility [27; 57; 60] and increasing water evaporation due to the increased porosity (Figure 7). Figure 12a indicates that XG could promote oat germination as long as an appropriate XG content was used (e.g., 0.25% XG and 0.50% XG). Mixing an excessive content of XG with soil might end up in a hard XG-soil matrix [24], which would plug the pores for the transportation of water, air, and nutrients (Figure 13) and therefore hinder the growth of oats. On the other hand, the addition of XG increased the volumetric moisture content of the soil, i.e., the volumetric moisture content of the soil generally increased with the XG content at any pH value (Figure 12b). In addition, the increment in the volumetric moisture content was more remarkable as the XG content increased to 0.75% and 1.00%. This is partially due to the inhibited germination and growth of oats at 0.75% XG and 1.00% XG (Figure 12c,d), resulting in less water consumption through plant transpiration.