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Field investigations into the manurial value of lagoon-matured digested sewage sludge

Summary Experiments were carried out during 1978–81 in which lagoon-matured digested sludge (LMDS) was applied in November and in February to four soils which were cropped with ryegrass and the effects compared with ammonium nitrate fertilizer. Over the whole experimental series the nitrogen in spring-applied LMDS was 47·6% as effective in producing dry matter as nitrogen in ammonium nitrate fertilizer, and was 40·2% as effective in nitrogen uptake. Overall, 20·3% of the N available from the LMDS was derived from mineralization of sludge organic nitrogen, equivalent to a 1st-year release from the organic-N in sludge of about 9%. This compares with reported values of 15–16% for 1st-year release of organic-N liquid digested sludge. Overall, autumn-applied LMDS sludge gave poorer results than when applied in spring. This did not apply to a silty loam with a pH of about 5, but poorer results with autumn-applied LMDS were associated with soils with pH values of 7·5 or more. On average an application of 140 m3 LMDS/ha provided 178 kg available-N/ha and 36 kg potassium/ha; this is an N:K ratio of 4·9:1. There were increases of Cd, Cu and Zn in herbage as a result of applications of both fertilizer and of sludge; the effects were additive. Sludge increased herbage Ni concentrations but fertilizer-N reduced them. Three years' applications hardly raised Cu and Zn herbage concentrations to the threshold level for deficiency for ruminant animals, and were not near toxic levels for plant growth or animal nutrition.