Impact of Sewage Sludge Application on Temporal Variation of Plasticity Limits and Aggregate Stability in a Calcareous Soil

Low organic matter in arid and semi-arid soils results in decreasing physical quality and increasing runoff and soil erosion. Application of organic compounds (sewage sludge) in these regions can improve soil physical and chemical properties. In the present research, effect of different levels of sewage sludge (0, 30 and 60 ton ha-1) on organic carbon, aggregate stability (mean weight diameter, MWD and geometric mean diameter, GMD), and plasticity limits (upper plastic limit, UPL, lower plastic limit, LPL, and plasticity index, PI) was studied over three time periods of 30, 60, and 120 days after application of sewage sludge. Soil organic carbon increased with increasing sewage sludge, but decreased significantly over time as compared to that of the control. Aggregate stability increased with sewage sludge application. Aggregate stability decreased 60 days after experiment as compared to that of 30 days, whereas, there was no significant difference between the aggregate stability at 120 and 30 days after the experiment. Upper plastic limit increased with increasing sewage sludge application. This criterion increased over time and decreased after that; whereas, LPL increased with sewage sludge application and did not vary over time. Sewage sludge did not affect PI significantly, while PI increased over time and then decreased. In general, results indicated that application of sewage sludge could increase soil organic matter and water holding capacity. Furthermore, with application of sewage sludge, soils can be tilled at higher moisture contents without any probable compaction. Of course, in addition to considering the positive effects of sewage sludge in improving soil physical properties, economical issues and the probable adverse effects of excessive application of sewage sludge and environmental risks should also be considered.