This study compared N HNO3 to other methods to determine plant available heavy metals in heavy-metal polluted soils. Soil samples were obtained from an experimental field treated with industrial waste after 22 years of the amendment and employed to conduct the comparative and correlation study. Soil samples were analyzed for Cu using various methods, planted in a glass house with several plants, and analyzed for soil and plant Cu and Zn. The relative strength of the chemical extractants followed the order of N HNO3 ≈ N HCl > Buffered DTPA ≈ Unbuffered DTPA > M CaCl2 ≈ N NH4OAc pH 7. A high correlation was observed for soil extracted Cu by M CaCl2 or N NH4OAc pH 7 or N HCl vs. Buffered DTPA and N HNO3 or N NH4OAc pH 7 vs. N HCl. High correlations of plant and soil Cu extracted by N HNO3 were shown by caisim, water spinach, land spinach, and corn, while plant and soil Zn were shown by caisim, water spinach, land spinach, and lettuce.

Biochar was suggested to lower the concentrations of heavy metals in contaminated soils and therefore may improve plant growth. This research was to evaluate the growth of corn (Zea mays L.) in a biochar-amended heavy-metal-contaminated tropical soil. Soil samples were collected from well-maintained experimental plots 22 years after amendment with heavy-metal containing industrial waste at 0 – 60 Mg ha-1. Corn plants were grown for 4 weeks in the soil samples amended with biochar at 0 – 10 Mg ha-1. The corn plant height and dry masses (roots, shoots, and the whole plants) were lowered by waste in relation to the increase in the soil Cu and Zn concentrations. The corn plant dry–weight masses (roots, shoots, whole plant) were well and negatively correlated with the soil Cu and Zn concentrations. The corn plant uptake of Cu and Zn decreased with the increase in the soil Cu and Zn concentration. Biochar improved the corn plant height and dry-weight masses, related to the decrease in the soil Cu and Zn concentrations. Biochar also increased the Zn uptake at waste level of ≥ 15 Mg ha-1 and increased the Cu uptake at waste level ≤ 15 Mg ha-1. The corn plant Cu uptake was linearly and positively correlated with plant dry-weight masses of roots, shoot, and whole plant masses.