FRESENIUS ENVIRONMENTAL BULLETIN, cilt.15, sa.4, ss.295-300, 2006 (SCI-Expanded)
Effects of increasing doses of humic acid (HA), applied in different periods, on the plant growth, nutrient and heavy metal contents of lettuce grown on sewage sludge-applied soils were investigated. A constant dose of 20% sewage sludge (SS) was added to experimental soil placed in pots. Various doses of HA (0, 500, 1000, and 2000 mg kg(-1)) were applied to the pots for different time periods (0, 30, 60, and 90 days before seedling transplantation).