Application of sewage sludge to an acid tropical soil : heavy metals in maize and correlation between plant uptake and soil

A field experiment was set up from 1999-2001 at the share farm of Universiti Putra Malaysia using a completely randomized design. Soil at this site is a well-drained Oxisol with topsoil having a sandy clay texture. The total number of plots is 30. The plot size is 5m by 8m and the planting distance is 0.75 x 0.25 m. Distance between the plots is 2 m. The treatments carried out are inorganic N (ammonium sulfate) at 140 kg N haֿ¹, control (no N application) and sludge application rates at 140, 280, 420 and 560 kg N haֿ¹. During the third cycle, sewage sludge was topped up with additional 1/3 of original rates due to visible N deficiency symptom. Sewage sludge application has little effect on increasing the heavy metals accumulation in plant during the 3 maize cycles. Also, heavy metals toxicity is not a problem as the concentrations in grain are below the maximum permitted concentrations (MPC) as gazetted in the Malaysia Food Act 1983 and Food Reglation 1985. Although, Zn and Cu need to be monitored because their concentrations are high in sewage sludge and concentration of Zn in grain has almost reached the MPC value. Heavy metals concentrations in other parts of plants (leaves, stem, sheath and cob) are still within the safety level especially for animal consumption. In general, the distribution of heavy metals in maize plants is as follows: - leaves and stem greater than sheath and cob greater than grain. From correlation studies (n=60), there were correlation between total heavy metals in the soil for Pb (r=0.53**), Fe (r=0.56**) and Cr (r=0.28*) and grain uptake. Similarly, significant correlations were obtained between 'available heavy metals' or IM NH₄NO₃ extractable Cu (r=0.25*), Pb (r=0.40**), Ni (r=0.51**), Cr (r=0.31*) and Fe (r=0.60**) in the soil and grain uptake. In leaves and stem, there was a correlation between total heavy metals for all elements (Cd r=0.43**, Pb r=0.43**, Zn r=0.30*, Ni r=0.40**, Mn r=0.34**, Cr r=0.51** and Fe r=0.46**, respectively) except for Cu. There were significant correlations between 'available heavy metals' of IM NH₄NO₃ extractable Zn (r=0.41**), Ni (r=0.55**), Cr (r=0.45**) and Fe (r=0.51**) in soil with leaves and stem uptake. For sheath and cob there were correlations between total heavy metals for Cd (r=0.40**), Ni (r=0.52**), Mn (r=0.34**) and Cr (r=0.51) in soil and sheath and cob uptake. There were significant correlations between 'available heavy metals' or IM NH₄NO₃ extractable Cd (r=0.28**), Pb (r=0.39**), Ni (r=0.86**), Cr (r=0.57**) and Fe (-0.26*) in soil and sheath and cob uptake. From this study, it can be concluded that soil extracted with NH₄NO₃ is suitable for the determination of available heavy metals for sludge-amended soils.