An Evaluation of Pulp Sludge as a Component in Manufactured Topsoils

Pulp and paper mill sludges have many attributes which, when utilized as a soil amendment, can improve soil fertility and overall soil quality. The primary objective of this study was to evaluate the use of pulp sludge as the organic matter component in a manufactured topsoil. Seven manufactured topsoils, containing 5.1, 8.8, 9.6, 10.9, or 13.8% pulp sludge and 0, 8.4, or 20.7% flume grit on a dry weight basis, were blended and applied to an abandoned gravel pit. These manufactured topsoils and a control topsoil were evaluated for changes in soil chemistry, impacts on soil solution chemistry and effectiveness as a growing medium for a grass conservation mix and for hybrid poplars. Results from a laboratory incubation study indicated that net N mineralization in the manufactured topsoils ranged from 9.7 to 18.2% of TKN after 105 days. Soil NO3 levels up to 20 times that of the control topsoil within four months of topsoil placement in the field plots confirmed that N mineralization was rapid in the manufactured topsoils. The addition of flume grit to the topsoil blends in some cases reduced the rate of N mineralization, presumably by increasing the C:N ratios of the topsoils. Soil CEC, pH and P availability were all positively correlated to pulp sludge loading rate in the manufactured topsoils. The greatest impacts of the manufactured topsoils on soil solution were in the first collection period following topsoil placement. Nitrate and Ca were the dominant ions in solution during this period. The manufactured topsoils improved both grass and tree yields relative to the control topsoil. Total grass yields from three harvests in the 15 months following topsoil placement ranged from 3.87 to 7.26 Mg ha in the manufactured topsoils. For grass plots, yields were positively correlated to soil NO3 levels, while hybrid poplar growth responses were largely controlled by the growth of herbaceous vegetation within the tree plots. Results from this study demonstrate that the use of pulp sludge in manufactured topsoils can improve soil fertility and create a favorable growing medium for both conservation grass mixes and hybrid poplars.