## 4 Discussion
### Soil Lead
#### 4.2.3 Lead Uptake in Radish  
The three amendments produced statistically similar Pb tissue content (mg kg\({}^{-1}\)) inadish relative to the unamended control. However, Pb mass load may be a more useful metric for most home gardeners, as it quantifies Pb on a per plant basis. Guidance from the U.S. Food and Drug Administration on maximum daily intake is also provided on a \(\upmu\)g/day basis, specifying a maximum intake of 3 \(\upmu\)g per day for children and 12.5 \(\upmu\)g for adults [105]. While Pb content in radiishes grown in COMP was, on average, lower than in INF, average biomass was greater in COMP, hence total plant uptake was nearly identical for both. Similarly, average Pb content in radiishes grown in BIOC was lower than in UNAM, but the radiishes had comparable mass loads due to greater biomass production in BIOC. These results suggest that the amendments can have an undesired effect by increasing plant Pb uptake, which is likely attributable to the improved plant growth that often accompanies the incorporation of humic substances [49], along with nutrients in general into low-fertility soils. For instance, in a study on the effects of green waste compost, wheat straw biochar, and a combination thereof applied at 5% and 10% (\(\upnu\)/\(\upnu\)) and planted to five leafy greens, [PERSON] et al. (2020) found that application of compost alone increased the accumulation of Cu, Pb, Cd, and Cr, in _Spinacia oleracea_ L. (spinach) and _Anethum gravelens_ L. (dill) [52]. The study also found that application of biochar often reduced the content of trace elements in tissues, which we observed as well with respect to a modest (22%) difference between the tissue content (mg kg\({}^{-1}\)) of radiishes grown in BIOC versus UNAM, although the difference was not statistically significant. [PERSON] et al. (2020) noted that the most effective amendment was a combination of biochar and compost at 10% (\(\upnu\)/\(\upnu\)) each, which reduced both Pb and Cd uptake in nearly all tested plant species, with reductions of Pb in spinach, dill, and lettuce by 42%, 44%, ad 57%, respectively. The BCFs suggest that radiishes grown in BIOC are significantly less likely than those in COMP and INF to take up Pb relative to exposure. This finding is unsurprising given the tissue mass load findings. [PERSON] et al. (2020) observed a similar trend with plants grown in compost displaying a greater BCF (also referred to \"bioaccumulation factor,\" or \"BAC\") than those grown in biochar. The authors noted that application of organic amendments at high rates (10%) decreased the BCF of the metals they tested in comparison to the control [52]. It is interesting to consider radish BCF and mass load in combination with soil available Pb at the end of the experiment (Figure 9c). Although biomass production in BIOC was similar to COMP and INF, radiishes grown in BIOC took up significantly less Pb. This suggests that the lower BCF and related mass load were a result of successful immobilization of soil available Pb over the course of the experiment, which prevented radish uptake. Compost had a similar BCF to both UNAM and INF, but significantly greater mass load than UNAM in combination with significantly less soil available Pb than both UNAM and INF. Although the addition of compost resulted in increased Pb uptake, the reduction in soil available Pb through plant uptake was sustained over the course of the experiment. This suggests that compost could be an effective amendment in combination with soil phytoremediation efforts. In contrast, radiishes grown in INF had a significantly greater mass load than UNAM and BIOC in addition to greater soil available Pb than all other treatments at the end of the experiment, indicating that despite high plant uptake, the inorganic fertilizer used also continued to contribute to soil Pb availability. For most plant species, the majority of Pb accumulates in the roots [9], as was observed in this study.