To study the effects of sewage sludge on tomato seedling growth, a series of pot experiments, each pot containing 17 kg soil, were conducted in a randomized complete design with eight replications. Treatments used were: 5 levels of sewage sludge (5, 10, 20, 30 and 40 tons/ha) and two control treatments (T1, without sewage sludge and T2, sewage sludge + complete fertilizers). Analysis of variance for above ground fresh and dry weights showed that they were significantly higher than T2 (sludge + complete fertilizer) when 5 and 10 tons per hectare of sewage sludge were applied to the pot at %1 level of probability. Application of sewage sludge higher than 10 tons per hectare, due to its toxic effect, decreased fresh and dry weight of above ground plant parts drastically. For example, applying 30 tones sewage sludge decreased dry weight by 20 percent and fresh weight by 44 percent as compared with that of T2. Mean comparisons revealed that water content of tomato plants did not differ significantly in comparison with that of T2 when 10-20 tons of sewage were applied, but its difference was significant at 5 percent level of probability when 30 and 40 tons of sewage sludge were applied. Analysis of variance for dry, fresh and water content of fruit also showed that their differences with T2 were non-significant when 5 and 10 tons/ha sewage sludge were applied to the pots. However, applying sewage sludge beyond 10 tons per hectare reduced all these three attributes significantly. Applying 20, 30 and 40 t/ha of sewage sludge delayed growth and development and caused apparent necrosis in the leaf margins of young and mature plant within the first month, while the toxic effect of sludge on necrosis decreased at later growth stages. Higher availability of iron (Fe) at higher sludge application rates along with soluble cation and anion concentrations in soil solution, especially chlorine and probably other side effects caused by sludge application are the factors that may have toxic effects on plants.
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