Humic Acid Addition Enhances B and Pb Phytoextraction by Vetiver Grass (Vetiveria zizanioides (L.) Nash)

Phytoremediation is an attractive, economic alternative to soil removal and burial methods to remediate contaminated soil. However, it is also a slow process. The effect of humic acid in enhancing B and Pb phytoextraction from contaminated soils was studied (pot experiment) using transplanted vetiver grass (Vetiveria zizanioides (L.) Nash). Boron was applied at 0, 45, 90 and 180 kg B ha−1 soil (as H3BO3) in 16 replicates. Of the 64 pots, four pots each were treated with 0, 100, 200 and 400 kg ha−1 humic acid (HA) solution. In a separate experiment, Pb was applied (as Pb(NO3)2) at 0, 45, 90 and 180 kg Pb ha−1 prior to addition of HA solutions at levels identical to the B experiment. Experiments were conducted using a randomized complete block design with four replicates. Vetiver grass was harvested 90 days after planting. Lead addition beyond 45 kg Pb ha−1 decreased Pb uptake mostly due to a yield decline. Humic acid application increased Pb availability in soil and enhanced Pb uptake while maintaining or enhancing yield. An application of 200 kg HA ha−1 was optimal for maintaining yield at elevated Pb levels. Boron application did not impact yield but greatly increased B content of roots and shoot. Boron uptake was greatest upon addition of 400 kg HA ha−1. We conclude that HA addition to vetiver grass can be an effective way to enhance phytoremediation of B and Pb but optimum rates differ depending on soil B and Pb contamination levels.