THE RESIDUAL EFFECT OF SEWAGE-SLUDGE ON HEAVY-METAL CONTENT OF TOBACCO AND PEANUT

Sewage sludge normally would not be applied on land where peanut (Arachis hypogaea L.) or tobacco (Nicotiana tabacum L.) are being grown because of possible contamination of peanut with pathogens and increased Cd concentration in tobacco. These crops, however, might be grown in rotation with other crops to which sludge had been applied. The residual effect of sewage sludge on metal concentrations in tobacco and peanut was evaluated in a greenhouse pot experiment with a Typic Hapludult soil from a field that received aerobically digested municipal sewage sludge at 0, 9, 18, and 27 Mg ha-1yr-1 for 3 yr. Cumulative amount of metals applied at the 27-Mg rate were (kg ha-1) 1.8 Cd, 39 Cu, 48 Cr, 9 Ni, 30 Pb, and 84 Zn. Sulfuric acid or Ca(OH)2 was used to effect three soil pH regimes: 5.2, 5.8, and 6.4 (median value within each regime). Tobacco ('Speight G-28') was grown to flowering and peanut (NC 7) was grown to maturity. At the termination of the experiment, soils were extracted with diethylenetriaminepentaacetic acid (DTPA) and Mehlich 3 extractant (M3) for metal analysis. Tobacco dry weight (averaged over pH) increased from 66 g pot-1 with no sludge to 94 g pot-1 at the highest sludge rate. Peanut kernel yield (averaged over pH) was depressed at the highest rate (35 g pot-1) as compared to the lower rates (mean of 48 g pot-1). At low pH peanut top growth was depressed by sludge (probably a result of Zn toxicity) and no kernels were formed at the medium and high rates. Sludge rates and soil pH had little effect on concentration of Cr and Pb in tobacco or peanut. Concentration of Cd, Ni, and Zn in tobacco and in peanut top growth decreased as soil pH increased to 5.8 to 6.0 but no decrease was noted at higher pH. Sludge rate effect was significant at low pH but diminished as pH increased. Concentration of Cd, Ni, and Zn declined but Cu concentration increased with increasing height of leaf on the tobacco stalk. Results of stepwise multiple regression using linear and quadratic forms of cumulative sludge rate, soil pH, and DTPA-extractable metals and pH X rate and pH X DTPA-extractable metals interactions as independent variables and metal concentration in the lower tobacco leaves or in peanut tops as dependent variables suggested DTPA would predict tissue metal concentrations better than would M3.