ABSTRACT
Summary The manurial value of liquid, anaerobically digested sewage sludge is under assessment in west Scotland to evaluate the effects of disposal on grassland. The concentrations of plant nutrients in sludges have been measured with sludge total N being considered as two fractions, volatile N and organic N. The herbage yield response to sludge N, relative to fertilizer N, has been measured in a number of field trials evaluating the effects of sludge composition and season of application. The concentration of organic N, P, and K in sludge depends on the dry-matter (DM) concentration, which was quite variable (3-100 g/kg). Volatile N concentration averaged 40%-50% of the total N content but was largely independent of sludge DM concentration. Relative yield response to sludge N at the first harvest after application was, on average, proportional to the volatile N content but varied with season of application and DM concentration in the sludge. Late-winter and late-summer applications gave the best yield response to sludge N relative to fertilizer N. This is possibly explainable in terms of seasonal rainfall pattern, with the rainfall increasing the relative response to sludge N. Residual response to sludge N during the season of application was equivalent to about 20% of the sludge organic N. This basic information on the manurial value of digested sludge and the results of the ongoing work will eventually be used to optimize the benefits of its disposal on agricultural land in Scotland.
