Municipal anaerobically digested sewage sludge and wood waste materials applied to land pose potential pollution problems. However, if utilized in proper combination and with proper management, they could prove to be valuable resources in governing NO3 supply, controlling NO3 leaching, and enhancing physical and chemical properties of soils.
Various combinations of sewage sludge and wood wastes were added at four rates ranging from the equivalent of 22.4 to 224 metric tons/ha in a greenhouse study. Every mixture except 50% bark-50% sludge caused an increase in wheat (Triticum aestivum L.) growth compared to the control. Greatest wheat growth occurred for the 224 metric tons/ha of 50% wood-50% sludge, 25% bark-75% sludge, and 25% wood and bark-75% sludge treatments. The highest application rates of 100% sludge, 25% wood-75% sludge, and 50% bark-50% sludge resulted in less growth than the next lower application rates, therefore 224 metric tons/ha of these mixtures appear to be excessive for maximum greenhouse wheat growth.
The addition of the equivalent of 134 kg/ha of N and 19.5 kg/ha of P with each organic material mixture and rate generally caused an increase in growth compared to the organic material treatments without fertilizer, but in most cases the increase was not great.
The relationship between wheat growth (G, g/pot) and N content in the plant tissue (mg/pot) is described by the equation G = 0.03742 N − 0.00002 N2 with an R2 value of 0.94. The equation G = 0.58099 (NO3-N) − 0.0079 (NO3-N)2 describes the relationship between wheat growth and NO3-N content of plant tissue (mg/pot) with an R2 value of 0.85. Correlations between wheat growth and N concentration or NO3-N concentration in plant tissue were not as close as the above described relationships.
The amount of total N, NH4-N, and NO3-N remaining in the greenhouse pots after harvest generally increased as the application rates of each treatment increased. Since NO3-N accumulation and possible ground-water pollution is of considerable concern, the data suggest that caution should be exercised with land applications of 112 and 224 metric tons/ha of 100% sludge and 25% wood-75% sludge, as well as with 224 metric tons/ha of 25% wood and bark-75% sludge treatments. Ammonium N did not accumulate excessively.
The equivalent N fertilizer values for the various organic material mixtures were estimated to range from 100 to 245,235 to 575, 355 to 650, and 270 to 1,130 kg/ha for 22.4, 56,112, and 224 metric tons/ha rates, respectively, if the 50% bark-50% sludge is excluded. Several approaches were used to determine the IM use efficiency or the proportion of the initial total N that was or became available and taken up by plants. It is estimated that from 2 to 22.7% of the initial N was taken up by plants during the 50-day growth period used in this study.