## 3 Results and Discussion  
### Characteristics of Plants  
It had been found that heavy metals in soil affected plant growth indexes, such as water content, biomass and plant and root length [22]. As shown in Figures 2-4, the moisture, biomass, plant height and root length of plants under different treatment groups are shown, respectively. \begin{table}
\begin{tabular}{c c c} \hline
**Remediation Method** & \multicolumn{2}{c}{**Experimental Design**} \\ \hline
**Phytoremediation** & **C** & **B** \\ \hline  & C+M1 & B+M1 \\  & C+M2 & B+M2 \\ \hline \end{tabular}
\end{table} 
Table 6: The detailed design of the experimental group. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**Compound** & **CaO** & **Fe2O3** & **SiO2** & **MgO** & **Al2O3** & **MnO** & **SO3** & **P2O5** & **TiO2** \\ \hline Content (\%) & 44.32 & 18.12 & 12.12 & 8.74 & 7.96 & 3.07 & 1.56 & 1.48 & 1.29 \\ \hline  & & & & & & & & \\ \hline \end{tabular}
\end{table} 
Table 4: Main components of steel slag. \begin{table}
\begin{tabular}{c c c} \hline
**Remediation Method** & \multicolumn{2}{c}{**Experimental Design**} \\ \hline
**Phytoremediation** & **C** & **B** \\ \hline  & C+M1 & B+M1 \\  & C+M2 & B+M2 \\ \hline \end{tabular}
\end{table} 
Table 5: Main components of pyrolusite. Figure 4: Root length and plant height of _Cynodon dactylon_ (**A**) and _Bidentis pilosa_ (**B**) in different treatment groups. Figure 3: Biomass of plants in different treatment groups. Figure 2: Moisture content of plants in different treatment groups. As one of the key indicators of plant growth, moisture content is measured and shown in Figure 2. Water is a key factor affecting the survival, growth and distribution of vegetation in terrestrial ecosystems. The water balance of the soil-plant-atmosphere continuum is a key mechanism for photosynthesis and plant growth. Therefore, monitoring the water content of plants is essential for studying plant growth. Comparing group C and group C+M1 in Figure 2, the results showed that the moisture content was 68.13% without adding material but decreased with little change to 66.13% after adding material 1. However, as shown in group C+M2 in Figure 2, the moisture content decreased significantly to 29.4% after the addition of material 2. It can be seen that material 2 had a devastating effect on the growth of _Cynodon dactylon_. For _Bidentis pilosa_, the moisture content was 71.59% without adding any materials, as shown in group B in Figure 2. The moisture content of plants increased slightly to 80.89% and 81.74%, respectively, after the addition of material 1 (group B+M1) and 2 (group B+M2). From the point of view of the moisture content alone, both materials 1 and 2 can promote the growth of _Bidentis pilosa_. In conclusion, adding materials can promote the growth of both plants. Figure 3 shows the biomass of plants under different treatment groups. The results show that the addition of materials can also affect the growth of _Cynodon dactylon_. As could be seen from group C, C+M1 and C+M2 in Figure 3, material 1 would slightly promote the growth of _Cynodon dactylon_ while material 2 would inhibit the accumulation of biomass of _Cynodon dactylon_ to a large extent. This indicated that material 1 was a good soil additive, which was beneficial to the extraction of heavy metals from plants. For _Bidentis pilosa_, the effects of materials 1 and 2 on its growth were similar to those of _Cynodon dactylon_-that is, material 1 promoted the growth of the plant, while material 2 was the other way around, as shown in groups B, B+M1 and B+M2 in Figure 3. Therefore, material 1 works better than material 2. Based on the above discussion, it can be concluded that material 1 is more suitable as a soil additive to promote the effect of phytoremediation. In Figure 4, the aboveground height and underground length of _Cynodon dactylon_ (A) and _Bidentis pilosa_ (B), respectively, were shown when different soil additives were introduced.