Use of fresh and composted de-inking sludge in cabbage production

Paper mills generate over three hundred thousand tons of de-inking sludge in the province of Quebec, Canada. These residues have C/N and CIP ratios of respectively 200 and 500 and may benefit from mixing with secondary sludge or composting with manure. This would reduce N and P immobilization and related crop deficiencies. The raw de-inking sludge (RDS) corrected for N deficiency was compared with a compost of sludge and dairy manure (SC), a mixture of primary and secondary sludge (MS) and mineral N on winter cabbage (Brassica oleracea var. capitata L.) yields, soil N and biochemical properties of a Tilly loam (loamy-skeletal, frigid Typic Humaquod). Rates of 0, 15, 30 and 45 Mg ha(-1) RDS, MS and SC (wet basis) were applied alone or with 0 to 235 kg N ha(-1) as NH4NO3. Samples were taken from the 0-20 cm soil layer five weeks after planting and at harvest. Soil N was significantly increased by MS five weeks after planting but not affected by RDS or SC. Alkaline phosphatase activities were increased by RDS or MS. The highest petiole sap NO3- contents were found in the MS treatments. The MS resulted in 20 % larger total and marketable cabbage yields than all other treatments. The RDS performed better than SC without mineral N. The compost had a synergic effect with mineral N on cabbage yields at the highest IN rates. The N and P contents of cabbage heads at harvest were linearly increased by MS addition The Cu content was increased by RDS. The Zn and Fe tissue contents were increased and Ni decreased by high rates of RDS. The mixed sludges are of particular interest since they resulted in significant increases in cabbage yields and N and P uptake while increasing soil inorganic N and phosphatase activity. The RDS need supplemental N whereas the compost should be viewed more as a soil conditioner than as a true nutrient source.