Abstract
Sewage sludge was treated with some Ni stabilizers including silica gel (Si), cement by- pass (CB) and iron oxide (FO) to reduce its availability in the soil, and absorption by onion plants (direct effect) in the first season and Jew's mallow plants (residual effect) in the second season. A loamy sand soil was amended only before the first season with sewage sludge treatments including 10
and 20% Si, CB and FO treated sludge as well as Si + CB, Si + FO and CB + FO treated ones. Sludge treated with 20% Si was the best one in reducing Ni effects in the soil and plants, it also increased the length, shoot and bulb fresh weights, shoot and bulb dry weight of onion plants by 17.2, 148.5, 6.99, 127.8 and 159.2%, respectively, compared to the control. However, the untreated sludge application increased the soil extractable Ni from 0.30 mg/kg in the control to 2.79 mg/kg
after the harvest of onion plants. Amending the soil with sludge treated with FO + CB, Si+ CB and Si+ FO significantly reduced the soil extractable Ni 100% of that amended with untreated sludge after onion harvest. The extractable Ni significantly decreased after Jew's mallow harvest from 0.30, 2.79, 1.15, 3.47 and 2.52 mg/kg to 0.00, 0.00, 0.00, 0.52 and 1.27 mg/kg with the control, untreated
sludge, 10% CB, 20% CB and 10% FO treatments, respectively. In the first season (direct effect), sludge amended with Si+ FO was the best treatment that reduced Ni content of the shoots and bulbs by 71.6 and 75.6% compared to the control. Moreover, in the second season (residual effect), sludge
amended with 20% Si was the best treatment that reduced the Ni content of Jew's
mallow plants by 71.8% and 66.8% compared to the control.